Tube Terminology

Appendix A

Terminology

1. Valve - same thing as a tube. Sometimes called a "thermionic valve." This is where the amplification of the signal occurs.

2. Cap - capacitor. It is two metals or conductors separated by a non-conductor. They pass AC signal easier than DC signal. They will also store a charge and release it rapidly. Said another way, the capacitor will resist change in voltage. These are used in the tonestack and power supply as well as other parts of the circuit.

3. Resistor - something that will impede the flow of electricity. These are used extensively through an amp's circuitry.

4. Inductor - A spiral of copper (or other material) that will resist change in current. It will store energy in a magnetic field. Typically used in the power supply. Sometimes used in the tonestack and other parts of the circuit.

5. Transformer - Two or more inductors with their magnetic fields coupled. A change in one inductor's voltage or current will be reflected onto the other. These are used in the power supply, reverb circuit, output section, and sometimes in the phase inverter.

6. Variable resistor - rheostat. A resistor who's value can be changed.

7. Potentiometer - Pot. A variable resistor that is center tapped. Can be wired as a variable resistor by connecting an outside lug to the center tap.

8. Diode - a device that allows current to flow in only one direction.

9. Triode (tube) - A tube consisting of a grid, anode (plate), and cathode. The 12AX7 is a dual triode, meaning there are two individual triodes within the tube.

10. Tetrode (tube) - A tube consisting of a grid, anode (plate), cathode, and screen.

11. Pentode (tube) - A tube consisting of a grid, anode (plate), cathode, screen, and suppressor. Most tubes considered to be tetrodes are actually pentodes with an internal connection between the suppressor and cathode.

12. Tonestack - EQ or Equalizer. Where frequency can be attenuated or boosted. Most guitar amps use a passive tonestack, which will only allow for frequency attenuation.

13. Bias - Setting idle current flow of a tube to be within the operating conditions of the amp. Can be either fixed-bias or cathode-bias.

14. Fixed-bias - THIS IS NOT self-biasing. This is applying a small negative voltage to the grid of the power tube. This is a more efficient way of biasing than cathode biasing. In a fixed-bias setup, the cathode of the tube is grounded (0v).

15. Cathode-bias - This is bringing the cathode to higher voltage potential than ground with a resistor and a capacitor. This is what is referred to as "Self-Biasing." This term is true to an extent. A cathode-biased amp can still have the bias adjusted, but may not have to.

Tube Amplifier Circuits and Equations

Chapter 6  

Tube Amplifier Circuits and Equations

General

Following is a collection of tube amplifier circuits and equations.  It will be updated regularly as new papers on different tube circuits and configurations are added to the website.

Common-cathode stage, fully-bypassed cathode:

Voltage Gain:

Av = (mu * Rp)/(Rp + r_a) 

Input impedance:

Rin = Rg

Output impedance (plate):

Rout = (r_a * Rp)/(r_a + Rp)

Output impedance (cathode):

Rk' = (Ra+r_a)/(mu+1)
Rout = Rk'||Rk

Input capacitance:

Cin =Cgk + Cgp*(Av + 1)

Frequency response:

f1 = 1/(2*pi*Ci*Rg)               - highpass breakpoint due to Ci/Rg

f2 = 1/(2*pi*Co*(Rout + Rl)) - highpass breakpoint due to Co/Rout/Rl

f3 = 1/(2*pi*Ck*Rk'||Rk)       - highpass breakpoint due to Ck/Rk

f4 = 1/(2*pi*(Rout+Rg)*Cin)  - lowpass breakpoint due to Rout of previous stage and

Cin Where:

Rg = the grid resistor
Rp = the plate resistor
Rl = the load resistance, or the input resistance of the next stage
Ra = the total load resistance, which is Rp in parallel with the input resistance of the next stage, Rl.  If there is no Rl, Ra = Rp.
r_a = the internal plate resistance of the tube
mu = the mu of the tube
Cgk = the grid-to-cathode capacitance
Cgp = the grid-to-plate capacitance
Av = the stage voltage gain

Note that Rl is ignored in the output impedance calcuations for the output, not because it doesn't affect output impedance of the overall circuit, but because output impedance is traditionally the impedance of the output of that gain stage looking back into the output. Rl is the input impedance of the following stage, so it is not included.   Of course, when calculating overall gain in an amplifier, the loading effect of Rl must be taken into account.  In the case of the cathode impedance, Rl must be included because it will affect the impedance seen looking back into the cathode.

Common-cathode stage, unbypassed cathode:

Voltage Gain (Output 1):

Av = (mu * Rp)/(Rp + r_a + (mu + 1)*Rk) 

Input impedance:

Rin = Rg

Output impedance (Output 1):

Rout =  [(r_a + (mu + 1)*Rk) * Rp] / [(r_a + (mu + 1)*Rk) + Rp] 

Output impedance (Output 2):

Rout = [(Ra + r_a)/(mu + 1) * Rk] / [(Ra + r_a)/(mu + 1) + Rk] 

Frequency response (Output 1):

f1 = 1/(2*pi*Ci*Rg)  - highpass breakpoint due to Ci/Rg

f2 = 1/(2*pi*Co*(Rout + Rl)) - highpass breakpoint due to Co/Rout/Rl

Where:

Rg = the grid resistor
Rp = the plate resistor
Rk = the cathode resistor
Rl = the load resistance, or the input resistance of the next stage
Ra = the total load resistance, which is Rp in parallel with the input resistance of the next stage, Rl. If there is no Rl, Ra = Rp.
r_a = the internal plate resistance of the tube
mu = the mu of the tube

Note that Rl is ignored in the output impedance calcuations for output 1, not because it doesn't affect output impedance of the overall circuit, but because output impedance is traditionally the impedance of the output of that gain stage looking back into the output. Rl is the input impedance of the following stage, so it is not included.   Of course, when calculating overall gain in an amplifier, the loading effect of Rl must be taken into account.  In the case of the output taken from the cathode, Rl must be included because it will affect the impedance seen looking back into the cathode.

Single-stage inverting feedback amplifier:

.

 
Gain:

Acl = (Ro + A*Rf) / (Ri + Rf + Ro - Ri*A)

Input impedance:

Rin = (Ri * A - Ri - Rf - Ro)/(A-1)

Output impedance:

Rout = (Ri + Rf) * [Ro / (Ri + Rf + Ro - Ri*A)]

Frequency response:

f1 = 1/(2*pi*Ci*Ri)  - highpass breakpoint due to Ci/Ri

f2 = 1/(2*pi*Co*(Rout + Rl)) - highpass breakpoint due to Co/Rout/Rl

Where:

Acl = closed loop gain
A = open loop gain
Ri = input resistance
Rf = feedback resistance
Ro = internal output resistance of the stage (the plate load resistor in parallel with the internal plate resistance, r_a, of the tube, plus the reactance of Co, if not negligible at the frequency of interest)

Note that for negative feedback, A must be a negative quantity.  If A is positive, the feedback will be positive.   The value of A can be calculated by using the equations for the common cathode amplifier stage. 

Note that Rl is ignored in the output impedance calcuations, not because it doesn't affect output impedance of the overall circuit, but because output impedance is traditionally the impedance of the output of that gain stage - Rl is the input impedance of the following stage, so it is not included.  Of course, when calculating overall gain in an amplifier, the loading effect of Rl must be taken into account.
 

Global negative feedback amplifier:  
 

Gain:

Acl = A*(Ri+Rf) / (Ri + Rf + Ro + Ri*A)

Note: if Ro is assumed zero,  Acl= A / (1 + A* Ri / (Ri + Rf))

Input impedance:

Rin =  Rg / (1V - Acl*Ri/(Ri+Rf))

Note: If closed loop gain is high enough, Rin is essentially infinite

Output impedance:

Rout = ((Ri + Rf) * Ro) / (Ri + Rf + Ro + Ri*A)

Note: if Ro is assumed zero, Rout = 0.

Where:

A = open-loop gain
Acl = closed-loop gain
A = open loop gain
Ri = input resistance
Rf = feedback resistance
Ro = internal output resistance

Note that for negative feedback, A must be a positive quantity.  If A is negative, the feedback will be positive.   The value of A can be determined by opening the loop and measuring the output voltage and dividing by the applied input voltage.
 

Phase-shift oscillator:

Minimum gain required for oscillation:

A(min) = 29

The value of A can be calculated by using the equations for the common cathode amplifier stage.   It must be 29 or greater to sustain oscillations.  A gain of exactly 29, or just enough to sustain oscillations, will produce the lowest distortion sine wave at the plate of the tube.

Oscillation frequency:

f_o = 1/(2*Pi*Sqrt(6)*R*C)

Impedance and frequency scaling

Impedance scaling

Impedance scaling is accomplished by first calculating an impedance scaling factor, ZSF, as follows:

ZSF = Znew/Zold

The impedance-scaled values are then calculated using the following formulas:

R'  = R*ZSF
L'  = L*ZSF
C' = C/ZSF

Frequency scaling

Frequency scaling is accomplished by first calculating a frequency scaling factor, FSF as follows:

FSF = desired frequency/existing frequency

The frequency-scaled values are calculated as follows:

R' = R
L' = L/FSF
C' = C/FSF

Frequency and impedance scaling

Both frequency and impedance scaling can be accomplished in one step with the following equations:

R' = R*ZSF
L' = (L*ZSF)/FSF
C' = C/(ZSF*FSF)

where

R', L', and C' are the resistance, inductance, and capacitance values after impedance scaling

Troubleshooting Amplifier

Chapter 7   

Troubleshooting Amplifier

AC Power Indicator Does Not Glow

This indicates either that the power indicator is burned out (which does happen...) or that there is no AC power reaching the indicator, which is usually wired directly across the power transformer or wired to one of the filament windings of the power transformer. No AC reaching the indicator is more likely than a burned out indicator AND no sound, so the fault may be one of these:

Fuse blown

• Open primary in the power transformer in those amplifiers where the power indicator is on a secondary
• Power cord open - test it with an ohmmeter
• Faulty AC power switch - with the amp unplugged, test it with an ohmmeter.
• Faulty wire or solder joint in the AC power path. Test from AC plug prong to prong with an ohmmeter.
• Silly stuff like power cord not plugged in

All of the major items may be isolated by using an ohmmeter and tracing out the AC path from prong to prong of the AC line cord. No live voltage tests are needed at this level.

It is also possible that the wall socket is not live. Plug some other AC powered unit into the socket and see if that unit works. If it doesn't, the AC power breaker may be open.

Fuses blow quickly for massive overloads, like AC shorts to a grounded chassis. They blow with a some time lag - maybe only a second or two, up to minutes, as the over current gets closer to the fuse's actual rating. Anything that uses enough power to cause the AC line current to exceed the rating of the fuse will eventually cause it to blow.

It is important to remember that a fuse NEVER blows without something else being wrong. It could be that:

• fuse is the wrong rating - replace it with the correct rating
• power tube shorted
• rectifier tube shorted
• power supply filter cap failing
• Carbon trails on the output tube sockets between the plate lug and the other electrodes, especially the heater electrodes.
• power tubes have lost bias or biased incorrectly
• power section of the amp is oscillating at too high a frequency to hear
• there is an ac wiring short or high leakage
• power transformer is faulty
• choke (if present) is shorted/leaking to chassis
• output transformer is faulty
• AC power wiring or B+ power wiring is faulty/shorted

Fuse Blows

If the power tubes are relatively new or very old, or if the amp has been roughly handled when it was hot, a power tube may have an internal short. These shorts are rare in tubes past infant mortality and not yet worn out.

The whole trick here is to separate truly defective shorted output tubes from other flaws that will kill new, good tubes that you put into the amp.

Remove the power tubes, replace the fuse, and see if the fuse still blows when powered up with the power tubes out. If it still blows, the tubes are not your problem.

If the fuse does not blow with the power tubes out, check to see that the screen resistors are not open or burned, then, for fixed bias amps, ensure that there is an adequately negative voltage on the control grid socket contact with a voltmeter. In cathode biased amps, the grids will be held at ground, but for a fixed bias amp you should have -20 to -60 volts here, depending on what kind of output tubes the amp uses. In fixed bias amps, if you can't find a negative voltage on the control grid socket contact, YOU WILL PROBABLY KILL ANY NEW TUBE YOU STICK IN THERE. Check lost bias or incorrect bias to save killing a new pair of tubes, and if no trouble is found there, then suspect that the tube itself shorted.

Replace the output tubes with new ones, and place the amplifier chassis on a stable, uncluttered surface in a room you can darken. Be certain that you can turn the amp and room lights on and off without touching anything inside the amplifier for safety's sake. Turn the amp on and the room lights out, and watch the new output tubes like a hawk for signs of overheating. If the fuse does not blow again and there is no sign of red or orange glow other than the normal filament glow inside the power tubes after the amp heats up, the old tubes were the problem.

If you've had an output tube die, always be suspicious of a bad screen grid resistor and a bad input grid resistor; check the value and appearance of these every time you have a power tube fail. Particularly Fender's carbon comps go high resistance or cracked

Be sure to rebias for the new tubes.

If the fuse now blows again, or the new tubes glow red-orange on the plates, shut it down immediately and inspect all of the components and wiring around the tubes to find the problem that is keeping the tubes from being biased properly.

Rectifier Tube Problems

Unplug the amplifier. Using a voltmeter, measure for the presence of DC voltage on every pin of the rectifier tube. If there is DC voltage on any pin, drain it away with a 10K resistor at the end of a clip lead, being sure you are insulated from the resistor and clip lead.

Pull out the rectifier tube and replace the fuse. Before replacing the rectifier tube, measure the resistance from the rectifier tube cathode pin to chassis ground. This should deflect the meter at first, but then get to a higher and higher resistance as the filter capacitors charge up from the meter. Once you get to over 50K or so, you can be sure that the filter caps are not dead shorted. If the resistance never comes up, check the filter caps.

If you're getting a reasonable resistance on the cathode, fire the amp up without the rectifier tube in place. If the fuse still blows, the rectifier tube was not the problem - keep going down the debug trail. If the fuse does not blow, replace the old rectifier tube with a new, known good rectifier tube. Try it again. If the fuse now blows, you have an overload somewhere on the load side of the rectifier, perhaps in the power filter caps, perhaps in the inductor or decoupling caps in the preamp.

The state of tubes being what it is today, NEVER trust a new Chinese or Russian tube until it has had some burnin time. Once the amp is working, let it idle for a few hours to see whether the new tube is going to go south, too.

Power Supply Filter Capacitor Problems

There are a limited number of ways for the power supply filter capacitors to be bad. All of the tests on power filter capacitors must be considered hazardous since they may store deadly amounts of voltage and charge even with the amplifier unplugged.

Any time you suspect power filter capacitors, do the following: With the amplifier unplugged and the chassis open, connect one end of a clip lead to the metallic chassis. Clip the other end of the lead to a 10K 1/2W or larger resistor. Holding the resistor with an insulating piece of material, touch the free end of the resistor to each section of the power filter capacitors for at least 30 seconds. Then:

• Visually inspect the capacitor(s) for any signs of bulging, leaking, dents and other mechanical damage. If you have any of these, replace the capacitor. Also note the condition of any series dropping resistors connected to the capacitors to see if they have been damaged by heat. Replace them if they have.
• Use an ohmmeter to measure the resistance from the (+) terminal of each capacitor to the (-). This should be over 15K ohms, preferably much over that. If you get less than that on any capacitor, unsolder that capacitor and remeasure just the capacitor. Under 15K indicates a dead or dying capacitor; replace it. If the resistance is now much higher with the cap unsoldered, there is a low resistance load pulling current, not a faulty capacitor. Always check all of the power filter capacitors while you're in there. If one is bad, consider replacing them all (see "Cap Job" in the Tube Amp FAQ)

If there is no obvious mechanical problem and the resistance seems high enough, temporarily solder a new, known good capacitor of at least as high a capacitance and voltage across the suspected capacitor or section., then plug in and try the amplifier again. If this fixes the problem, turn the amplifier off, unplug it, drain the filter capacitors again, and replace at least the bad section if not all of the filter capacitors.

If you are replacing a multisection can, get a replacement can with multiple sections matching the original before you remove the original capacitor. Once you get it, make yourself a note of the symbol on each terminal of the old capacitor, such as square=1uF/450V, triangle=20uF 450V, etc. and then clip the old terminal with the symbol off the old can. Remove the old can, mount the new one, and use the symbol chart and lugs still on the leads to make sure you connect the right sections up in the new capactor.

Output Tube Biasing Problems

Output tubes handle probably 85% of all the power used by your guitar amp. If they are biased incorrectly or if there is a fault in one of the biasing components, it can cause a number of power supply and output section problems. A failed biasing component that lets the grid assume the same voltage as the cathode will cause an output tube to act almost shorted. Tubes which are conducting too much bias current (older tube-techs would say these are "underbiased" or "biased too hot") can cause blowing fuses, excessive power supply ripple and 120Hz hum, burned out rectifier tubes and could in the long run kill an output transformer or power transformer. They overtax in general everything in the electrical path from the AC power plug to the output transformer.

You will need to know whether your amplifier is fixed bias or cathode biased. If you don't have a schematic make sure the amplifier is unplugged. Remove the output tubes and measure the resistance from the cathodes of the output tubes to chassis ground. If this is under 10 ohms, you have a fixed bias amp. If it is 50 ohms or more, you have an amplifier that is cathode biased. Between these two could be a flaw in the amplifier or could be one of the very rare amplifiers that use a combination of cathode and fixed bias.

There are relatively few causes of output tube bias problems. All of them involve the grid not being held at a negative enough voltage with respect to the cathode.

• In any amplifier:
• If the output tubes have just been replaced with new ones, "infant mortality"/early dying of the new tubes.
• Leaky/shorted coupling capacitor from the driver tube plate to the power tube grid. Note that the chances are that only one of the multiple output tubes will have this problem.
• Dirty, corroded or just old tube socket not making good contact to the tube grid pin Note that the chances are that only one of the multiple output tubes will have this problem. Eagle-eye the socket hole contacts with a magnifier and try to see if they are corroded or dirty looking or maybe the contacts have lost their springiness, and don't make good contact. Try removing the tube, squirting a little tuner cleaner into each socket hole, then wriggling the tubes back in. You can also very gently pry the contacts out so they have more pressure on the tube's pins.
• The socket could be broken.
• Rarely, a very poor or inexpert rebiasing attempt by an unskilled technician.
• Rarely, an output tube itself that is shorted grid to cathode Note that the chances are that only one of the multiple output tubes will have this problem.
• Rarely, the resistance from the tube grid to its bias supply will be open. This can lead to "runaway" in power grid tubes. Measure the resistance from grid socket pin to bias supply point in fixed bias amps or to ground in cathode biased amps and make sure that both (all) output tubes have the same nominal resistance - usually 100K to 220K, never more than 470K. If one grid has a lot higher resistance to ground or to the bias supply, there is a problem with the grid leak resistor on that tube.
• Even more rarely, a wiring flaw in the wires getting the bias voltage to the grid. I have seen an amplifier that came from the factory with all of the wires inserted into the socket lugs and wrapped around the contacts, but not soldered. It worked that way for 20 years, and had intermittent troubles the whole time.
• In fixed bias amplifiers:
• If the tubes have just been replaced with new ones, failure of the owner to get rebiasing done, or "infant mortality"/early dying of the new tubes.
• Failure of the bias section of the power supply, especially the adjustment pot or the bias supply filter cap
• In cathode biased amplifiers:
• If the output tubes have just been replaced with new ones, "infant mortality"/early dying of the new tubes.
• Leaky/shorted cathode resistor bypass capacitor
• Rarely, a noninductively wound cathode resistor with an internal short
• Rarely, a broken or incorrect wire.

Power Amplifier is Oscillating

This one's not done yet. Please refer to the Tube AMP FAQ at http://www.eden.com/~keen/tube_amp.htm/

Power Transformer Problems

These come in several flavors.

• Open windings, which cause the amp to simply not operate, no sound at all.
• Internal shorts to the transformer core, which is connected to the chassis. This causes the amplifier to blow fuses or the transformer to overheat, and may cause electrical shocks if the short is from the primary winding.
• Internal shorted turns, which will cause the transformer to overheat with no external load on it.
• Internal shorts from winding to winding, which cause one winding's voltage to "leak' into the other winding, possibly causing overheating or just mis-operation if not much current is involved.

SAFETY

1. Unplug the amplifier.
2. Open the case up and identify where the Power Filter Capacitors are. Connect a cliplead to the chassis, and then clip the free end of the clip lead to the plate lead of any preamp tube to drain any charge from the filter capacitors. The plates of a 12AX7 are on pins 1 and 6.
3. Identify the terminals of the power transformer.
1. Using an ohmmeter, measure the resistance of the primary (usually Black - Black wires) and the resistance of both leads to the chassis. The primary should be under 1K ohms. If it is not, the transformer is dead.
2. Measure the resistance to the chassis from both ends of the primary winding. It should be more than 1M ohm.
3. If it is shorted or less than 100K ohms, the transformer is bad. If it is between 100K and 1M ohm, unsolder the primary leads from the terminals they contact and measure again.
4. If it is now less than 1M ohm, the transformer is failing, and should be replaced.
5. If it is over 1M ohm, there is a component connected to the wiring leading to the power transformer which is leaking to the chassis that needs to be traced down.
6. Measure the high voltage winding (usually red - red/yellow - red if it is center tapped, red - red or red - red/yellow if it is not centertapped) resistance from end to end. It should be less than 1K ohms. If it is over that, the winding is open, and the transformer should be replaced. The centertap, if present, should be tied to the chassis, perhaps through a standby switch.
7. Measure the rectifier heater winding (usually ? - ?) resistance from end to end. It should be less than 10 ohms. If it is more than that, the transformer is bad. Measure the resistance from either end of the winding to chassis. If it is less than 100K ohms, unsolder the leads and measure again. If this reading is less than 100K ohms, the transformer is bad.
8. Measure the heater (filament) winding(s) (usually green - grern or green - green/yellow - green) as you did the primary, with the same conclusions EXCEPT that the heaters are usually tied to the chassis through the centertap (green/yellow) or through a hum balancing pot or fixed resistors of 100 - 500 ohms, so that having it conduct to the chassis is OK.
9. If none of the previous tests indicate a bad transfomer, make sure all leads are soldered correctly back in their original positions if you unsoldered them. 

WARNING
The following procedures involve measuring hazardous voltages and performing operations inside the chassis with the AC power turned on. Do not attempt them if you do not know how to do them safely. See the safety warning at the beginning of the debug page. Note that these procedures may cause the power filter capacitors to become charged and dangerous even with the AC power turned off.

1. Remove all tubes from the amplifier, leaving it in a position where you can probe the transformer terminals.
2. Set your test meter to it's highest AC voltage scale (at least 750 VAC, preferably 1000VAC).
3. Turn the power switch off.
4. Plug the line cord in.
• Clip one meter lead to one side of the power transformer primary winding. Clip the second meter lead to the other side of the power transformer primary.
• Be certain that you have not placed the meter leads where they can cause a short.
• Check to be certain that you are not touching the amplifier except for the power switch.
• Turn the power switch on.
• Watch for the meter reading. It should read apporximately the same as your local AC power source (120VAC in the USA). If it does not, the wiring leading to the power transformer is bad, and you should check that wiring.
• Turn the AC power switch off.
• Disconnect the meter leads. Connect one meter lead to one side of the filament winding (usually green - green or green - green/yellow - green) and the other meter lead to the other end of the winding.
• Using the same cautions as above, turn the amplifier on, then off, noting the voltage reading. It should be the correct value for the amplifier, or slightly higher because of the high current it normally carries. This is usually 6.3VAC nominal, and may be as high as 7.3VAC without unduely alarming you. If it is lower than 6.0V or above 8V, something is wrong with the transformer. Do this for each filament winding if there is more than one.
• As above for the heater (filament) winding(s), measure the voltage on the heater winding for the rectifier tube. This voltage should be 5.0 to 5.8VAC.
• Using the same cautions as above for the primary, read the voltage on one half of the high voltage winding, from center tap to one end and then to the other end if it has a centertap, or from end to end if the high voltage is not centertapped. This voltage should be 250 to 500VAC, perhaps higher, depending on the designed B+ for your amplifier. If it is not reasonably close to the proper AC voltage, the power transformer is bad.
• Turn the AC power switch off If it is not already off.
• Unplug the AC line cord.
• Using a clip lead, connect one end to the chassis. Connect the other end the the socket lug for the plate of a preamp tube - pin 1 or 6 in a 12AX7. Let the clip lead remain for two minutes, then remove it. This drains any DC voltage from the power filter capacitors that may have been placed there during the test. Be certain to remove the clip lead from the amplifier.
5. If none of these tests show improper results, the power transformer may have an internal short.

·         Unplug the amplifer. Remove all the tubes. Open the chassis, and connect one end of a clip lead to the chassis, then touch the free end of the clip lead to each terminal of the power filter capacitors as above to drain away any charge in them. Remove the clip lead.

·         Using an ohmmeter, verify that the resistance to chassis from the power transformer primary windings is over 100K ohms. If it's lower than that, you need to find out why before proceeding.

·         Make certain that no foreign objects or personal parts are contacting the amplifier.

·         Turn on the AC power switch. Leave it on, periodically touching the outer metal shell of the power transformer to see if it gets hot. If it makes a loud hum, gets noticeably hot to the touch, or emits a burning odor, it is defective. Leave the transformer on for ten minutes, or until it gets hot, whichever comes first.

Choke Problems

There are a very limited number of ways the choke can fail. It can be:

Open: use an ohmmeter to measure the series resistance. If it's under 200 ohms or so, it's probably not open. Over 1K is certainly a problem.

Internally shorted: This will mean that it just does not filter very well, and the ripple on the B+ supply will be high, so the amp may hum. There's no good way for the beginner to detect this other than the difference between a good choke and a bad choke in reducing ripple.

Shorted/leaking to the iron core, and therefore to the chassis. Measure the resistance from the wires going into the choke and the chassis. This should be over 50K at least. If you suspect that the choke is leaking, measure the resistance to the chassis and then unbolt the choke and measure again. If the resistance goes up, the choke is leaky. If it's same, it's not. Sometimes there will be a short that happens only when the choke gets hot or vibrated, but that is rare.

Testing Output Transformers

You can fairly quickly determine if a transformer is grossly bad. This does not say anything about the subtleties of it's tone, only that it is functioning. Tests of relative tonal "goodness" are also possible, but require a lot of equipment and experience to do correctly. For the tests described here, you'll need an ohmmeter for the simplest testing, and for more advanced tests, a means of measuring AC voltage and current simultaneously, such as a pair of VOMs or DMMs, and a 110/120 to 6.3VCT filament transformer, and either a variac (variable transformer) or a light bulb socket in series with the primary of the filament transformer to limit the power you put into the transformer under test.

If you do the advanced tests, be aware:
Both the filament transformer and the transformer under test will have at least AC line voltage on them, an may well have much higher voltage, several hundred volts on one or more windings. You are therefore in danger of being KILLED if you are not both knowledgeable and careful about how you do these tests. These tests are clearly hazardous as defined in the warnings section of this page.

Simple Tests

Unplug the amp and drain the capacitors before you start. Do not remove the transformer from the amplifier before you start. Remove the output and rectifier tubes.

Identify which wires are which by circuit connection. The primary will have two (single ended), three (push pull, or very rarely single ended with ultra linear tap), or five (push pull with ultralinear taps) leads. Noting which leads connect to the B+ line, measure the resistance from the B+ lead to each plate of the output tube(s). Write the resistance reading down. Measure the other side if it's puah pull. Plate windings are almost always in the tens to a few hundred ohms of resistance. A reading much over 1K is pretty sure sign that the winding is burned open. If it's open, the transformer is dead. [There are ways to fix "slightly-dead" transformers, but this is extremely risky, and probably doomed to failure. I'm not going to write down how. Don't waste your time.]

While you're at it, measure the resistance of every lead to the chassis. This reading should be very high, hundreds of K or preferably megohms. A low reading here indicates a short to the transformer core - again, dead transformer.

Do a similar test on the secondaries - look for open windings, or shorts to the core. Finally, test for the resistance from primary to secondary. A low value, under a few K indicates an internal short, and a dead transformer.

If the transformer fails any of the simple test, it's dead. Replace it. If it passes all of the simple tests, it may still have an internal short Sometimes a primary will have enough resistance that the transformer does not burn up or burn the wires in two , but will just bog down, have low power and sound bad. If you suspect that is the case and you don't have the skills to do the advanced tests, take it to a competent tech.

Advanced Tests

Do the shorts/opens tests above first.
Then:
Identify a secondary winding, 8 ohm tap if it's available, and hook up one and only one winding to either 1/2 of the 6.3VCT or to the variac. Make D^&%ED sure the variac is all the way down if you're using one of those. Make sure that no other leads are connected (or shorted together, or touching your screwdriver on your bench or... well, you get the idea). Check for safety load resistors on the output jacks or secondary windings. These can cause you to erroneously think it is bad. There must be no loads on any winding. Put your voltmeter on the winding, and the current meter to measure the AC current through it, hook up the 3.15 VAC source, and turn on the AC - not to the amp, but to the AC feeding the winding you're driving. The voltmeter should measure 3.15 (or close) volts AC, the light bulb (if used) should NOT be lit brightly, and nothing should be humming or smoking ;-). There should be only a little current going through the winding. If the voltage is lower than 3 volts, or you are pulling amps of current, then there is a load on the transformer, internally since you have disconnected all the leads, meaning that there is an internal short. (Which means that you ought to be sure that there is no other load on the transformer, because you're going to throw it away if it has an internal short - be sure you're right!)

If all is well, no smoke, flames, loud hum, poultergeists, or lightning, measure the voltage that now appears on the other windings. The voltages will be equal to the ratios of the voltages that will appear on these windings in normal operations. For the primaries of output tubes, this can be up to a couple of hundred volts, so don't think that you can relax your safety precautions - it can still kill you, even driven with 3VAC on the secondary of the output transformer. The half-primaries of the output transformer should have identical voltages on them. Secondaries should have multiples of the voltage you're putting on the tap you chose. If you used an 8 ohm tap, a 4 ohm tap will have about 2.3VAC on it, a 2 ohm tap will have 1.6VAC on it, and a 16 ohm tap will have about 4.5VAC on it.

If the transformer passes all these tests, it's almost certainly good.

Faint hiss and/or hum 

If the speakers have only a modest hiss or hum coming from them, that indicates that the power amplifier section, speakers and speaker wires are OK, and that the fault is probably in the lower-signal sections of the amplifier.

• Preamp tubes bad
• Preamp power bad
• Input cord or connector is open or has dirty contacts; this can include the effects loop jacks, if present.
• Input jack dirty or corroded
• Open volume or tone control
• Open, shorted, or failing resistor or coupling capacitor
• Faulty signal wiring
• Dirty tricks - I actually saw this once. The clever-but-misguided spouse of a guitar buddy painted the tip of a friend's guitar cords with clear nail polish. All of them. That took a while to find...

Preamp Tubes Problems

This topic is still under construction.

If you suspect that a preamp tube is going or has already gone bad, the most expeditious way to find it is to have a new, known good spare tube to swap in until you find the problem. Just swap in the new, good one and see if the problem is corrected. If you put a new known good one in, and the problem does not change, then the tube you replaced was not the problem - or at least not THAT problem.

Easter-egging (hey! I wonder why they call it that...) with a good tube is the highest likelihood practice to hit a fix quickly.

"OK," I can hear you saying, "but I don't have a spare preamp tube!" Shame on you. All right, maybe there's something we can do. There are usually several preamp tubes of the same kind inside most amps. Try swapping a pair of them, noting which tube is which so you don't lose track of which one you suspect. It should be vanishingly rare for TWO preamp tubes to go bad at once. You'll usually be able to tell if the problem moves around when you move one particular tube, or if it stays the same no matter what you do. If it stays the same, you don't have a tube problem, you have trouble with the circuits around a tube.

Preamp Section Power Problems

Failing or open series dropping resistor in the bypass networks leading to preamp stages; these are usually 8K to 10K, usually 2W or so. They form a series string from the highest voltages at the screens of the power tubes to the plate supply for the input pream tubes. At the junction of each two of them, there will be a bypass capacitor, usually 10 to 20uF to ground. These resistors can get hot and drift upwards in value, dropping the plate supply voltage to the earliest stages too low for proper operation. They can also be cracked or broken sometimes.

Failing bypass capacitor; those bypass capacitors get leaky, shorted or open just like power filter caps- test them the same way.

Squeals

If it squeals, it's oscillating at a frequency low enough to hear. The single most common cause of this is a tube going microphonic. Try swapping tubes first; it's most often the first preamp tube if it actually squeals solidly instead of only when you hit a note that excites the resonance.

In most other cases, the squeal starts just after some pivotal event - the amp has just been repaired or modified, or new tubes put in, or have been dropped. That event is a clue. Think about what changed, then un-change it or tinker with whatever was changed.

Some causes of squeal:

• Tube going microphonic - most often first preamp tubes.
• Shorting contact on input jack (esp #1) not making contact; the "squeal" is from sound vibrations vibrating the chassis and the contact making and breaking contact repeatedly, making a little 'click' each time.
• Power tube shorted (this only happens for a short time - the amp squeals and then dies.)
• Lead dress - the leads carrying the signal around inside the amp have been moved around somehow so that the signal is causing internal electrical feedback. You can find this by running the amp with the chassis open and moving the wires around (gently! with a wooden stick) to see if the squeal changes or goes away. Once you locate the critical wire(s) you can figure out where they have to be to keep this from happening and tie them there. Another option can be to substituted shielded wire for the sensitive ones, with the shield connected at one end of the run only.
• If the amp has been modified, the squeal may be caused by poor lead dress in the modification, improper grounding in the modification, parts layout too close, or just that the new (usually higher) gain has pushed things over the edge. Higher gain makes a lot of things more critical, including grounding, bypassing, lead dress, and signal shielding.
• Wrong polarity/ incorrect hookup of a replacement output transformer.

Putt-putting sound or motor boating

In an amplifier that has ever worked correctly once, motor boating is almost always a signal that the decoupling capacitors in the B+ lines of the preamp section are going high impedance, not decoupling properly. Replace the B+ decoupling capacitors at least for the preamp. Since the other capacitors are old, also, consider replacing ALL of the electrolytic capacitors in the amp (doing a cap job - see the Tube Amp FAQ at http://www.eden.com/~keen for info on the what and why of cap jobs.)

Excessive Hum

A good way to divide and conquer is to turn the volume control(s). If the hum changes levels as you do this, then the source of the hum is something that affects the stages of the amp before the volume control. A faulty, humming preamp tube can be isolated this way very quickly. Conversely, if the volume control does not affect the hum, the cause is somewhere after the volume control.

Faulty tube

Tubes sometimes develop internal hum, for reasons known only to themselves. Do some tube swapping to locate the problem. Use the volume control test

Severely unmatched output tubes in a push pull amplifier. Push pull amplifiers get by with less power supply filtering because they're supposed to cancel this ripple in the output transformer. The cancellation can be upset by output tubes that use different amounts of bias current, allowing the hum to be heard.

Faulty power supply filter caps

Faulty bias supply in fixed bias amplifiers

A bias supply with excessive ripple injects hum directly into the grids of the output tubes. Check that the bias supply diode is not shorted or leaky, and then bridge the bias capacitor with another one of equal value to see if the hum goes away.

Unbalanced or not-ground-referenced filament winding

Defective input jack

If the input jack is not making good contact to the guitar cord shield, it'll hum. Likewise, if the jack has a broken or poorly soldered ground wire, or not-very-good connection to the grounded chassis, it will cause hum. If messing with the jack changes the hum, suspect this.

Poor AC grounding

In amps with two wire cords, defects of the "ground reverse" switch and/or capacitor can cause hum. A leaky power transformer can also cause this. It's especially bad when the ground reverse mess is already dicey.

Induced hum

Placement of the amplifier near other equipment can sometimes cause it to pick up radiated hum from other equipment. Suspect this if the hum changes loudness or tone when you move or turn the amp. There is usually nothing you can do about this except move the amp to where the hum is less.

Poor internal wire routing

If the signal leads inside the amp are routed too near the AC power wires or transformer, or alongside the high-current filament supply wires, they can hum. Sometimes using shielded cable for signal runs inside the cabinet can help. It is hazardous to do, but you can open the amp up and use a wooden stick (NOT A PENCIL) to move the wires around inside to see if the hum changes. This is hard to do well and conclusively, since the amp will hum more just because it is open. BE VERY CAREFUL NOT TO SHORT THINGS INSIDE THE AMP.

Poor AC Chassis Ground at Power Transformer

A common problem is the main ground point to the chassis. The green wire (you DO have a three wire line cord, don't you?) ground to the chassis, the "line reverse" cap, the CT on the filament windings, the CT on the high voltage windings, and other things associated with power or RF shield grounding are often tied to lugs held under one of the power transformer mounting bolts. If this bolt becomes loose, or if there is corrosion or dirt under the lugs, you can get an assortment of hum problems.

Defective internal grounding

There are potentially lots of places that must be tied to ground in the internal wiring. This varies a lot from amp to amp. If one is broken loose or has a poor solder joint or poor mechanical connection, it can show up as hum. Note that modified amplifiers are particularly susceptible to this problem, as the grounding scheme that the manufacturer came up with may well have been modified, sometimes unintentionally. With the amp unplugged, open and the filter capacitors drained, carefully examine the wires for signs of breakage or mods.

Relay Coil Hum

If your amp is home-built, you may have used an AC-coil relay for some switching functions. If you used the filament AC for powering this relay, you can get an AC hum in the signal path induced from the coil. The cure is to run this relay from DC by rectifying and filtering the filament supply or changing to another type of relay that's less susceptible to causing hum. Of course, for homebuilt amps, there could be many hum-inducing problems.

Unbalanced Filament Windings

The filament power must be referenced to the DC in the tubes in some way, otherwise you get a lot of hum. The filaments are usually a centertapped 6.3VAC winding, with the CT grounded for the necessary reference. Other methods are low value pot (200-500 ohms) across the whole 6.3V with the wiper grounded, or two low value (100-200 ohm) resistors in series with the center grounded across an otherwise floating 6.3V. If the winding is not grounded and balanced around ground, it will cause hum. Measure the voltage from each side of the 6.3V to ground; it should be pretty much exactly half the AC voltage at either end. If it is unbalanced to ground, tweak the pot or change the resistors to get it to be. If you have grounded centertap style supply that is not centered on ground, this indicates a faulty power transformer.

Excessive Hiss

·         noisy tubes

·         Noisy plate resistors. The carbon composition resistors used on the plates of preamp tubes often go very noisy, especially in older Fenders. If swapping tubes does not fix the trouble, locate the circuit that seems noisiest by tube swapping, and then replace the plate resistors in that circuit with equal-value metal film or metal oxide resistors. You can use carbon comp if you can find them, but the problem is likely to recur.

·         Noisy resistors in the B+ decoupling string, often around 10K in value

·         Unusual: An ultrasonic oscillation can cause an intense insect-like hissing that sounds very much unlike normal hiss.

·         Rare: leaky coupling capacitors or faulty controls

·         Rare: a slight ongoing arc on the output tube socket(s)

·         Rare: a bad solder joint somewhere in the signal chain

·         Rare: internal arcing or noise in almost any part in the preamp section

A good way to divide and conquer is to turn the volume control(s). If the hiss changes levels as you do this, then the source of the hiss is something that affects the stages of the amp before the volume control. A faulty, hissing preamp tube will be turned up this way very quickly. Conversely, if the volume control does not affect the hiss, the cause is somewhere after the volume control. In general, the volume level of the hiss is an indicator of where the hiss is occuring - the louder the hiss, the more likely its source is near the input of the amp where the gain applied to it will be the greatest

The procedure of locating by removing one tube at at time working from the phase inverter/driver back towards the input until removing a tube no longer stops the hiss should then localize the problem to one tube's worth of circuitry.

Popping Sounds

Popping is almost always an arcing problem. The high voltage in the amp has found some path that cannot stand the high voltages and discharges suddenly through that path. The arc current is high, but cannot be sustained by the power supply, so the voltage drops a little, the arc extinguishes, and it takes some time for the power supply voltage to build back up to where the arc will start again.

Popping is often associated with the time when you flip the standby switch. In standby, the current drain from the power supply is less, so the voltage rises, causing more voltage stress. When the standby switch is thrown, the higher-than-normal voltage can break over things that stand the normal stress of operating voltage.

Tubes

The tubes themselves will sometimes develop internal, intermittent arcs. Do some tube swapping. Start at the front (preamp) end of the amp and pull a tube, listen, pull a tube, listen. When you find one where pulling it makes the popping stop and a new tube makes it quit completely, you're done.

Arcing power tube socket

The B+ may be arcing across the surfaces of the output tube sockets themselves. This is often the case when an amplifier has a lot of dust and dirt inside it. In some cases, the arc can be started by a few seconds of playing without a load on the amp, which causes large spikes on the plates of the output tubes. Contamination of the tube socket surface can let an arc get started, and the arc itself burns the surface of a plastic tube socket body. This leaves a carbon residue in the path of the arc, burned remains of the trail of the arc; the carbon residue is itself somewhat conductive, so in the future, there is a ready made path for the next arc. You have to replace the socket if this is the case.

Intermittent switch

A switch, often the standby switch, can develop internal arcs

Intermittent breakdown of coupling cap or a popping resistor

Sometimes a signal coupling capacitor just can't take it anymore, and it starts breaking down intermittently. Resistors, particularly those delightful, brown sound carbon composition ones, sometimes develop internal pops. Proceed as for tubes. In this case, a new replacement tube will NOT make the noise quit permanently, and you have to figure out which component is causing it.

Intermittent breakdown of output transformer or choke

The filter choke, if your amp has one, and output transformer are connected to the highest voltages in the amp. If they are old (can you say "vintage"? I thought you could) and if they get hot, the insulation on the wires inside can start being intermittent. Internal shorts that clear will cause popping. Shorts that don't clear will pop a fuse, usually. Sometimes it'll just cause smoke.

broken resistor or capacitor or R/C lead/ wire

This is one where the problem is not caused from the B+ breaking down insulation. Mechanical damage can break a part and leave enough pressure on the pieces so it mostly makes contact and kind of functions. Vibration will cause it to open momentarily, causing a pop.

Heat from resistor or output tube melting solder

This is a fun one. Some of the power supply dropping resistors, output tube cathode biasing resistors or the connections on the output tube sockets themselves get so hot that they melt the solder that attaches the leads. Even more interesting is when they just soften it so it gets grainy and any vibration (speakers, anyone?) makes a cold, grainy solder joint. This bad joint can pop and arc, sputter, hiss, rectify AM radio, do lots of nead stuff. Once you find the bad joint, you'll also have to find out why it was so hot.

You can sometimes leave the amplifier turned on and turn out or dim the lights in your workroom, and see arcing happining. On pops that happen when the amplifier is touched or jarred, you can (gently, now) tap the chassis with a rubber hammer or wooden stick, being careful not to break anything or touch the amplifier with your hands as you do this. This often makes a mechanically-motivated arc happen, and you can see where it is, and deal with it when the lights are on. BE VERY CAREFUL NOT TO TOUCH THE INSIDES OF THE AMP IN THE DARK - THE HAZARDOUS VOLTAGES ARE JUST AS DANGEROUS IN THE DARK.

Unintentional and ugly distortion even when set for clean operation

Ugly sounding distortion can take several forms.

Harsh grainy sound

• Output tubes biased 'way too cold
• Rubbing or torn speaker cone
• Rarely, the amp can be oscillating ultrasonically and still get some sound through, with a harsh, ugly sound.

Sound cuts out or squawks on loud notes

• Failing coupling capacitor
• Failing plate resistor, cathode bypass cap, cathode resistor or grid resistor
• Intermittent ultrasonic oscillation

Muffled or constricted sound

• Failing preamp tube; find the offending section by tube swapping and see if a good tube fixes the problem
• Low signal tube bias is pushing it into saturation or cutoff. Measure operating voltages on the preamp tubes. The problem section will have tube pin voltages that are 'way off normal.
• Failing coupling capacitors from the preceeding stage.
• Failing plate resistor, cathode bypass capacitor, cathode resistor, or grid resistor
• Power supply problem; a dropping resistor may have drifted far from it's nominal value, changing the power supply voltage enough to cause this.

Faint out of tune sound on every note

Excessive power supply ripple, usually indicating that the power filter capacitors are going bad. This can also be caused by speaker cones and voice coils with problems so they just rub slightly, too.

Coupling Capacitor Problems

The biasing of any tube stage depends on having the signal coupled into the grid of the tube at the proper DC level. A leaky coupling capacitor lets current through from the preceding stage and upsets the DC bias, usually turning the tube on so hard that no signal can pass through it. A power tube with a leaky or shorted coupling capacitor may blow fuses cause low power or excess hum, or kill transformers. A preamp tube will just cause little or no signal to pass through or ugly-sounding distortion of the signal that does get through.

Output Tubes

Output tubes biasing comes in two major flavors - cathode biasing and fixed biasing. Output tubes need a lower grid resistor than preamp tubes, usually 100K to 470K. The grid is tied through this resistance to either ground (in cathode biased amps) or a negative voltage supply (in fixed bias amps). If the capacitor that couples the AC signal to this grid is leaky or shorted, it conducts the DC from the plate of the preceding stage into the grid. This upsets the biasing and causes the tube to conduct 'way too much current.

In all cases, you must determine whether the coupling capacitor is leaky. A quick way to test the capacitor is to unplug the output tubes, connect the (-) lead of your multimeter to chassis ground, and use the (+) lead of your multimeter to measure the voltage on the socket contact that corresponds to the grid. You must know whether your amp is cathode biased, in which case the grid contact must be at zero volts, not positive at all.

If your amp is a fixed bias amp, the grid contact(s) in the socket(s) must be at a negative voltage, -15 to -60Vdc. If they are more positive than -15, you probably have a leaky coupling cap.

If either of these tests indicate a leaky or shorted coupling cap, open the chassis, determine the value of the suspected coupling capacitor, and replace it with another of the same or higher voltage and capacitance rating. Then button the amp back up and see if the voltage on the socket contact for the grid has changed.

It's best to make this substitution with a capacitor you don't mind leaving in if that was NOT the problem so you don't have to go back and pull it back out.

It is possible to unsolder the grid end of the coupling capacitor, turn the amp on and measure the DC voltage from the unconnected end of the capacitor to ground with an analog voltmeter. If you only have a Digital Multi Meter (DMM), connect a 1M ohm resistor from the free end of the capacitor to signal ground, then measure the voltage across the resistor. If this voltage is even 1V, the capacitor should be replaced; the capacitor is leaky. This is a definitive test, but you do have to have the amp open and powered on, and so it is a more hazardous test.

Preamp tubes

For all tubes except cathode follower stages and output tubes in guitar amps, this means that the grid of the tube is tied to ground through a high (over 470K, often 1M) resistor. Cathode follower stages have their grids tied either directly to the plate of the tube driving them or to a fixed positive voltage.

The tests for a leaky coupling cap in a preamp tube are the same as for power tubes, except that you have to know which tubes may be direct coupled cathode followers. A direct coupled cathode follower stage will have it's grid tied directly to the plate of the driver tube ahead of it, no coupling capacitor at all. This is only the case for some older Fender Bassman models and a number of Marshalls. If you find a tube socket with a high (could be 100V or more!) positive voltage on it's grid contact, unplug and open the amp, and drain the B+ capacitors. Then look at the grid circuit in question. If there is a capacitor between it and it's driving circuit, the coupling capacitor is bad. If it is connected by either a wire or a resistor to the driving circuit, it is direct coupled and the high grid voltage is not a defect.

For all stages that are not cathode followers, the voltage on the socket pin for the grid should be dead zero, not positive even a fraction of a volt.

Replace coupling capacitors with at least 600v rated capacitors regardless of their original ratings unless you simply can't get such caps.

If neither of these fixes the problem or materially changes the symptoms, back up a step and make another guess.

Testing Resistors

If you suspect a resistor, examine it visually first. Is it scorched or burned looking? Does it have obvious chips or cracks? Is it soldered firmly in place or does it feel a little loose when you wiggle it a little?
Resistors can fail in the following ways:

• Their resistance may drift from it's nominal value, sometimes dramatically. Measure the resistor with an multimeter set for ohms and see if it's within tolerance. These things make a resistor more likely to have drifted in value: high nominal value, heat stress (near a hot component or gets hot from the current it carries), high voltage across it (plate resistors).
• They may be completely open (ohmmeter test!)
• They may be mechanically damaged: cracked or chipped, or the leads may be loose on the body or the leads may be broken themselves or poorly soldered.

Low Power or Loss of Volume

Faulty Preamp Tube

Faulty Power Tube(s)

Bad preamp cathode resistor

An unbypassed cathode resistor has drifted upwards (to 5K-10K or over)

Faulty phase inverter

If for some reason the phase inverter input side is good but the inverted side is bad, the power amp will still work, but power will be very low. This can be a bad 1/2 tube, a faulty socket contact, a broken or open plate resistor or coupling capacitor to the output tube, or a bad solder joint on any of these.

Open cathode bypass capacitors in preamp

if they go open, the stage they're in loses gain, but does not otherwise fail. If they short, it dramatically shifts the bias point, and may cause distortion as well as low volume.

Faulty vibrato circuit on neon/LDR vibrato Fenders

If there is a dummy plug in the footswitch hole, or a bad footswitch so the vibrato is always active, sometimes the vibrato tube turns on and stays on, not oscillating. This keeps the neon bulb on all the time, shunting lots of signal away. Same thing can happen if there is a shorted vibrato tube (rare) or a bit of wire or solder shorting the vibrato tube. Check the plate voltage on the vibrato tube to be sure it's oscillating.

High voltage isn't high enough for some reason

• Failing rectifier tube - try swapping in another one
• Failing power filter capacitors
• Failing or open series dropping resistor in the bypass networks leading to preamp stages
• Failing bypass capacitor - treat as in power filter caps.

Open screen resistors on power tubes

Amp cuts out or "goes dead" when the volume control is turned up higher than "X" or when you hit a specific note

You have a parasitic oscillation above hearing range. This can overheat an output transformer, and really needs to get fixed fast. It can often be fixed by tube swapping, but you often need an oscilloscope to see what's happening in the electronics.

Smoke or Burning Smell

There's good news and bad news. The good news is that it's easy to find the problem, at least the thing that is burning. Just unplug the amp, open up the chassis and look for what's burned, charred, or overheated looking. That's what's causing the smoke/smell.

The bad news is that in almost all cases, the part that is burning is a power handling component. These are ALL expensive. Worse, in some cases, the part that is burning is not what is causing the problem, and you still have to find what else is faulty.

In some ways, having smoke coming out of the amplifier is kind of a deviant, hard-headed version of having a fuse blow - something is eating too much power, it's just that the fuse for some reason is not blowing. This is especially suspect if the fuse blew, and you didn't have another of the right rating, so you stuck in a higher current rated fuse.

Possible causes are:

• Failing/shorted output tube - this can overheat the output transformer and/or power tranformer. More rarely, it can also overheat the choke, but usually the transformers go first.
• Improperly biased output tube.
• Failing bias supply on fixed bias amplifiers
• Failing cathode resistor bypass capacitor on cathode biased amps.
• Failing/shorted rectifier tube (or solid state diodes - they do fail, if rarely) can overload the power transformer as well as killing the power filter capacitors by letting AC through. A failing filter cap or shorted output tube can pull so much current that it overloads the rectifier tube, too.
• Failing power filter capacitor. These can sometimes get hot enough to literally explode or burn, as well as just quietly overloading the power supply and popping rectifiers and power transformers.
• Failing power transformer.
• Failing power filter capacitor
• Choke with a "soft" short between winding and core
• Failing output transformer

Intermittent Problems

Intermittent problems are some of the most frustrating ones to solve. The amp works fine until - something - happens, and then it acts up. Most of the effort in debugging this one is to make it act up so you can cause it to happen when you want, which will then in turn let you find the problem, and verify that you have really fixed it. Intermittents always mean that something is just at the very edge of failing and it takes the causing event to push it over. When the causing event or condition is not present, it works fine, or maybe will "reset" when the amp is turned off or cooled.
Notice carefully what makes the intermittent happen, if you can. Very common events that institute intermittent problems are:

• Mechanical vibration - it only happen when it's banged or shaken by being on top of speakers
• Heat (thermal stress) - something only edges over into failure when it gets hot
• Voltage stress, perhaps combined with heat

This one is probably only going to yield to the laundry list technique, so here goes:

• Failure only happens after a longish time of playing: this is most often thermal, as something fails when it gets hot enough. Good places to look:
• Tube develops a problem when it gets really hot
• Bad solder joint opens up when it gets hot enough
• Resistor or capacitor goes bad when it's hot - these usually show signs of overheating to a visual inspection. Power tube screen resistors are a common place this happens.
• Mechanically damaged part opens up under thermal stress. Broken resistor bodies can be held together by the lead's springiness and only open when they get hot. Capacitor leads may have the same problem, as can soldered wires and joints.
• Failure only happens when the amp is sitting on top of speakers
• Bad solder joint or broken part
• Failure only happens when the amp is cold
• Bad solder joint or broken part, or badly drifted resistor value.
• Failure only happens when the amp is taken off standby: Since the B+ voltage rises in standby, this often means that the higher voltage is preaking something over. This may take the form of the amp only coming on slowly after a delay when the switch is thrown, or of a squeal or pop after the switch is thrown, or ugly sounding distortion for a while until it "gets better"
• Preamp decoupling capacitors
• Signal coupling capacitors
• Dirty, contaminated, or arcing tube sockets.
• Amp stutters or cuts out when driven really hard: overdrive is causing the output tubes to go into grid blocking after being over driven; this is caused by the signal causing a temporary bias shift.

Depending on how desperate you are, you might want to apply a shotgun technique: Methodically remelt every single solder joint in the amp, adding in a bit of rosin core solder as you do. A milder form of this would only remelt the ones in the circuits you suspect. This sounds horrible, but really doesn't take all that long. The worst part of doing this is that you never really find which one caused it, just that the problem quits.

I have a friend who repairs amps for a living and who said he once fixed an intermittent problem on a Fender where the amp was cutting out intermittently. He found that the wires to the output tube sockets had been put in the correct places but never soldered!

Electrical Shocks from Amp

If your amp gives you electrical shocks, you probably have leakage of AC voltage to the chassis, either accidentally or by design. Either way is dangerous.

Note that if your equipment is properly three wires grounded and working correctly, other equipment that is leaking may shock you when you touch both of them, leading you to think that your amp is the one that is leaking. You'll have to test both pieces to find out which one is leaky.

Your amplifier should have a three wire AC cord fitted for the AC power for safety reasons whether it originally had this or not. If your amplifier HAS a three wire cord and still shocks you, there is more than one fault in operation, possibly including mis-wiring of the building's AC power outlets - it does happen.

---

SAFETY WARNING
If your amp is shocking you, you have to consider that the whole amplifier represents an electrical hazard to you, and might hurt or kill you under certain conditions. If you do not already know how to work on such faulty equipment safely, take the amplifier to a qualified service technician to fix.

---

To determine whether your amp is actually leaking AC, use a multimeter set to a range that will read at or over 125VAC. Plug the amp in and turn it on. Measure the AC voltage between the ground ring on the input jack and a known AC ground point, such as the chassis of a piece of equipment which is properly three wire grounded. This voltage should be zero. If it is over a volt or two, either your amp has a leakage problem, which is why you're getting shocked, or the equipment you're using for a ground reference has a fault in it which leaves the chassis un-grounded (which is unlikely)

If the voltage you read is over 30VAC, you have enough leakage to be dangerous to your health.

Finding the source of leaking AC

1. Unplug the amplifier. Turn the power switch on.
2. Measure the resistance of each flat prong on the AC cord plug to chassis ground. This resistance should be very high, but since you're getting shocked, probably isn't.
3. If the resistance is very high, over 1M ohm, you have either

·         an intermittent short from the power transformer primary which only happens when the AC is on

·         capacitive leakage though the "ground reverse" switch

·         a short between one of the secondary windings and ground or

·         an intermittent short from the AC wiring, fuse, or ground reverse capacitor which only happens when the AC is on

4. If the resistance is lower than 1M ohm, there is a leakage path in the primary circuit.
• unsolder the power transformer primary leads and measure the resistance to chassis from each one. A low resistance means that there is a leakage path from primary to the transformer core, and it must be replaced.
• If the transformer shows high resistance to chassis from all primary leads, measure the resistance to chassis from each AC plug prong. A low reading indicates that the leakage is in the wiring path.
• Measure the resistance of the AC "ground reverse" capacitor if your amp has one. A low reading means it is bad and must be replaced.
• If you get high readings on all these primary AC points, unsolder and measure the resistance to chassis in turn of the high voltage windings, then filament windings, and of the filament windings to each other and the high voltage windings. A reading under 1M ohm on any of these indicates a defective transformer.

Be certain that you re-solder any wires you removed during the debugging.

Reverb Problems

No reverb

• Input or output cable to the reverb pan not plugged in
• Failing reverb drive tube
• Failing reverb recovery tube
• Bad reverb tank: measure the resistance between the center and outside conductors on the RCA jack on the reverb tank. It should be less than 2K ohms for all types of Hammond/Accutronics tanks, at both input and output sides. ManyFender amps may have a resistance of only 2-4 ohms at the input side. This is normal. Other amps may have higher input resistances, from several hundred ohms up to a couple of K. If the resistance is high, the tank is bad. You can look inside the tank to see if one of the little wires have come loose from the RCA jack; some tanks can be saved this way. If the little coils at the ends of the springs are open, get a new tank. Sometimes the delay springs break. You're welcome to try to solder/putty/glue them back together, but don't expect too much from this.

• Very faint or thin reverb This is caused by too little signal getting through the reverb path. Could be:
• Failing drive or recovery tube
• Failing component making one of these tubes be biased incorrectly.

• Hum from reverb
• Open ground/shield on reverb cable
• Dirty/corroded RCA jack/plug on reverb cable
• Broken ground wire inside reverb tank

• Hiss from reverb
• drive or recovery tube going noisy
• Resistor going noisy in drive or recovery tube circuit

Distorted reverb sound; failing drive or recovery tube or failing component making one of these tubes biased improperly

Crackling or popping sound from reverb

Squealing noise from the reverb, affected by the reverb control

Scratchy reverb control

Scratchy Controls

Controls go scratchy because the sliding contact inside the control makes intermittent contact with the resistive element, and you hear a little "gritch" every time it makes or breaks contact. This can be caused by:

• Old, worn controls
• Dust and dirt inside the controls
• Poor quality controls with bits of the resistor coming off inside.

You can sometimes restore a control to non-scratchiness by spraying contact cleaner inside the pot and turning the shaft rapidly to clean the dirt and stuff off the resistive element; however, the problem is likely to recur. Getting a new pot is the best fix. If you decide to try the spray cleaner trick, use one that leaves a lubricant on the surface to lessen the wear and make it longer until you have to do this again.

Scratchiness is a thousand times worse if there is DC across the pot, so you might check for that. DC across any pot except the bias pots is usually an indication that a coupling capacitor is leaking, so check for that.

Vibrato (Tremolo)

No Vibrato

• Vibrato tube going bad, either the oscillator section or the driver section; swap it and see.
• Neon/LDR module going bad (Some Fenders)
• Broken/open resistor, capacitor, or wiring
• Faulty speed or intensity control
• Faulty vibrato footswitch , jack, or wiring

"Ticking" Vibrato

• Fiberboard contamination: Dust, dirt, and junk can let the LFO signal leak into the audio path. Vacuum the dust and dirt away, and if it still persists, remelt the wax top and bottom with a hair dryer.
• Solder blobs from eyelets touching insulating board: Sometimes excess solder drips out the bottom of an eyelet and can intermittetly contact the insulating board, can cause ticking. Remelt the eyelets and examine the board underneath for any blobs dripped down.
• Funny ground on some SF Fenders; On one of the signal tubes, the cathode cap was placed on the tube socket, and wired to a ground lug on the vibrato cancel jack instead of across the resistor on the fiberboard. The vibrato shares this ground line, and can the vibrato current can cause audible ticking in the audio path. Rewire the cap to another ground or relocate it to the board.
• Poor Signal wire layout: signal wires run too close to vibrato leads can pick up the LFO signal. Move them around and see if the ticking goes away.
• Bad repair/replacement foot switch cable: the Fender footswitch cable is not two conductor; it's single conductor shielded, plus single conductor. The reverb wire is shielded, vibrato wire is not. This keeps vibrato out of reverb. If you retrofit with two conductor shielded, you get vibrato ticking onto reverb audio.
• Sharp tick in vibrato oscillator: On neon/LDR Fenders, on the neon bulb side of the module there is a 10M to one side of bulb, 100K to the bulb; from the 10M straight across the board is the gnd point of the LDR. Put a 0.02 cap from 10M/bulb to the ground point; this works by filtering the output of the oscillator.