Sound System: May 2017

Wednesday, May 31, 2017

Tuesday, May 30, 2017

Sunday, May 28, 2017

Capacitor Tips ( for the beginner )

Capacitor Tips ( for the beginner ) :
Tube Radio CAPACITOR Basics

Your vintage tube radio needs both direct current (DC) and alternating currents (AC) to operate. Capacitors act to pass AC while blocking DC. Capacitors are used to block, pass, filter and tune the various currents in your radio.

Don't let terminology confuse you….."condenser" is just an old fashioned name for "capacitor". If you're not the best speller, a condenser, capacitar, capaciter, capacitor, condensar and condensor are also the same.

Capacitors have a capacitance value and a voltage rating. The capacitance value is a measure of how much electric charge a capacitor can store. The voltage rating is the maximum voltage the capacitor can handle without breaking down. This is sometimes expressed as WVDC (Working Voltage Direct Current).

Your old tube radio uses 4 types of capacitors: variable (tuning) capacitors, mica capacitors, paper capacitors and electrolytic (filter) capacitors. When you restore an antique radio you will replace the paper and electrolytic capacitors, but not the variable and mica capacitors.

In radio service parts lists and schematics, paper and electrolytic capacitors are usually expressed in terms "microfarads". Short forms for microfarad include mfd, MFD, MF, UF and uF. The mica capacitors in your tube radio will have lower capacitance values than the paper and electrolytic capacitors. Micas are expressed in terms of micromicrofarads (picofarads). Short forms for micromicrofarads include mmfd, MMFD, MMF, PF and pF. A pF is one-millionth of a uF. For example, a mica capacitor valued at 500 mmfd (pF) would be 0.0005 mfd (uF). When reading schematics and buying capacitors, you sometimes have to be able to convert uF to pF or pF to uF. For your convenience we have a Capacitor uF-nF-pF Conversion Chart that you can refer to. You may want to tape this conversion chart to your workbench.

As a general rule, if a capacitor in your vintage tube radio is less than 0.001 uF, it is probably a mica capacitor. If it is between 0.001 uF and 1.0 uF it is likely a paper capacitor and if it's more than 1 uF it's probably an electrolytic capacitor.

Size wise, the electrolytics are the largest capacitors and most tube radios use 2 or 3 of them. The original electrolytic capacitors are typically the size of a roll of quarters or larger. On the old AC sets they are usually encased in aluminum and mounted on top of the chassis. With the lightweight AC/DC sets of the 1950's they are quite often under the chassis and may have a cardboard case.

The original paper capacitors in your radio will likely be in a brown paper tubular case (sometimes coated with wax). They are usually 1 to 1 1/2 inches long and 1/4 to 1/2 inches in diameter.

Mica capacitors come in different sizes and shapes, but the most common shape is square or rectangular….brown in color with colored dots (sort of look like "dominos").

Capacitors have either "radial" leads or "axial" leads. With "radial" type, both leads exit from the same end of the capacitors. With "axial" type, there is a lead at each end of the capacitors. Both types are equally good. Just be sure the capacitors you order have long leads.

On schematic diagrams the flat side of the capacitor symbol is the positive (+) side and the curved side is the negative (-) side. The positive end must be kept at the higher electrical potential (more positive voltage). Modern film capacitors are non-polar, so you don't have to worry about polarity when replacing old paper caps with new film capacitors.

How about using NOS (New "Old Stock") capacitors? These are not recommended…use at own risk! As paper and electrolytic capacitors age their capacitance values drift, they dry out and they become leaky. Would you drive a 1930's automobile with NOS 70-year-old tires?

Don't waste your money on audiophile, computer grade or tantalum capacitors. Sure they are good capacitors, but your old tube radio does not have the electronic circuitry to take advantage of those expensive capacitors. The only difference you will notice is a lighter wallet.

Plastic / polyester film capacitors are now used in place of paper capacitors due to their smaller size, lower cost and superior performance. There are many variations of plastic / polyester capacitors. Good types of film capacitors for tube radio restorations include metalized polyester, metalized polypropylene, metal-foil polypropylene, polystyrene and mylar. What is a Mylar? Mylar is simply the trade name of the synthetic film registered by duPont

At higher frequencies polypropylene and polystyrene are more stable that polyester, so for film capacitors under 0.01 mfd, you may want to use polypropylene or polystyrene capacitors rather that polyester capacitors.

What should it cost you to replace the capacitors in your radio? To "recap" a typical tube radio you will need two or three electrolytic capacitors, one or two line filter interference suppression safety capacitors and about a dozen film capacitors…..Total cost for these parts should be $15 or less.

Non-Electrolytic CAPACITOR Tips

When replacing old paper/wax capacitors, you can't go wrong using film capacitors that have a higher voltage rating than the paper ones you are replacing. For example, if you are replacing a paper capacitor rated at 400 volts, you can use a 630-volt film capacitor (but not a 200-volt capacitor). A film capacitor with a higher voltage rating will give your tube radio better reliability and longer life.

Why were tube radios manufactured with 200, 400 and 600 volt paper capacitors if 600 volt could have been used for all the capacitors? Two reasons… cost and size. Capacitors used to be expensive so if a manufacture could use lower voltage capacitors in a circuit, it could cut production costs. Also, the higher the voltage the larger the paper capacitor, so it was easier to install lower voltage paper capacitors. Now-a days, film capacitors are inexpensive and compact, so use 630 volt film capacitors and you can't go wrong.

Radio Schematics and parts lists sometimes do not specify the working voltages of non-electrolytic capacitors. To be safe use a film capacitor rated at 630 volts.

Old paper/wax capacitors are one of the most unreliable parts in an old radio. Don't let "molded" paper capacitors fool you. They are just paper capacitors in plastic cases and are just as unreliable as the ones coated in wax. Molded paper caps were sold under trade-names such as Bumble Bee, Black Cats, Black Beauty, Pyamid, Goodall, etc.

Modern non-electrolytic capacitors i.e. mica capacitors, film capacitors, ceramic capacitors, etc are non-polar. This means you don't have to worry about which end to connect when replacing old paper capacitors with new film capacitors.

Although non-polar, old paper capacitors had black bands at one end. The black band indicated which end of the paper capacitor had some metal foil (which acted as a shield). The end with the metal foil was connected to the ground (or lowest voltage). The purpose of the foil shield was to make the paper capacitor last longer. When replacing these old paper caps with new film capacitors, you do not need to worry about which end goes to the lowest voltage side.

When replacing paper capacitors with film capacitors keep in mind that capacitance values are "easy to please". The uF value does not need to be exactly the same. For example; if replacing a 0.05 uF capacitor you can use a 0.047 uF; if replacing a 0.002 uF you can use 0.0022 uF. These replacements are virtually identical. If you are +/- 10% you be well within your radios factory specifications. (Just be sure your replacement capacitor has a working voltage is equal or greater than the original paper capacitor)

Electrolytic CAPACITOR Tips

Electrolytic capacitors are often referred to a "filter capacitors". Electrolytic capacitors help to convert (filter) AC (alternating current) power into the DC (direct current) voltage that your radio tubes need to operate.

Size wise, the electrolytics are the largest capacitors. On older sets they are usually encased in aluminum (can type) and mounted on top of the chassis. If they are not on top of the chassis you will find them under the chassis.

Capacitors used to be much larger and much more expensive than they are today. To save on space and cost "multiple section" electrolytics were used. These are simply two, three or four capacitors in the same case. You will notice just one ground connection/wire (usually a "black" wire) as all the caps share that ground. These "multi-section" caps can be replaced with single electrolytics. Modern electrolytics are compact and will easily fit under the chassis. You should leave the old can capacitor on the chassis for original appearance. Just be sure to disconnect it.

Electrolytic capacitors work hard and are probably the most unreliable part of an antique radio. As they wear out (or simply get old) you get that famous "tube radio hum". Yes, in most cases it is bad filter capacitors that are the cause of that hum. WARNING! If you tube radio hums "turn it off and don't use it". Bad electrolytics are not only hard on your ears; they are hard on the tubes, transformers and other parts in your radio. Capacitors are cheap….tubes and other parts can be expensive and hard to find.

Electrolytic capacitors have a rated "working voltage" (WV) which is the voltage it can handle for a limited amount of time. Never use a Ecap with a working voltage equal or close to the actual voltage in the circuit. This is asking for trouble. Your car has a maximum RPM that the engine can operate at....if the max RPM is 6000RPM....how long will the engine last if you put the car in park and keep the engine reving at 6000RPM...yes, not long. An electrolytic capacitor should be operated no more than 3/4 of its maximum working voltage. This will both prolong capacitor life and allow some margin of safety for unexpected voltage surges. The higher the V the Ecap is worked at relative to the maximum working voltage the shorter will be the useful life of the Ecap. Never replace an electrolytic with one that has a lower voltage rating than the original Ecap.

As with paper capacitors, the capacitance value of an electrolytic capacitor is "easy to please", and an exact uF replacement is not necessary. For example, you can replace a 30 uF with a 33 uF or replace a 20 uF with a 22uF. If you can't find a close replacement …better to go with a higher uF value than a lower uF.

The old "rule of thumb" when replacing electrolytic capacitors is to not use more than 80% higher (or 20% lower) than "the original" uF size. If you replace an E-cap with one that has too high a MFD, the DC voltages will be higher than called for and your tubes and other parts will wear out faster. If you use too low a uF size, your radio will hum.

Warning ! Electrolytics have a negative end and a positive end…..if you install an electrolytic with the polarity mixed up not only will your radio not work…the electrolytic capacitor could explode. All the modern day electrolytic capacitors that JustRadios sells have an arrows (with negative signs) marked on them. This arrow with negative signs, points at the negative end of the electrolytic capacitor.

As a general rule AC (tube radios with power transformers) can use 450 volt electrolytics while lightweight ac/dc tube radios can use 160 volt filter capacitors. However, there are exceptions so always best to refer to a schematic.

Electrolytic capacitors have a shelf life of a couple of years, so make sure you are buying "fresh" stock electrolytics (not new "old stock"). Would you buy stale loaf of bread if a fresh one available?

Electrolytic capacitors should be stored at temperatures of 5 to 35 degrees C (40 to 95 degrees F) and in non-humid conditions (less than 60 relative humidity) to maximize shelf-life.

Don't put your tube radio into storage after you have restored the electric's. Once a month let the radio sing for a half-hour or so. This will prevent the electrolytic capacitors from drying out.

CAPACITOR Installation Tips

When restoring an antique radio it standard practice to replace certain of the radios capacitors. This is known as "recapping" a radio. An old radio may work with it's original caps….but for how long ?? ….and how safely ?? If the radio is going to be sold with a guarantee or is being given to someone as a gift, you should "recap" the radio.

You will want to replace all the paper and electrolytic capacitors. However, "do not replace the mica capacitors" if your radio was made in the USA or Canada. Mica capacitors found in American and Canadian radios very rarely go bad and if you replace them it will throw off the radios tuning. Replacing the mica capacitors will do more harm than good. Only replace a mica if you are sure it is bad (which is rare).

Mica Capacitor Discussion Update: As a member of the AVRS (Australian Vintage Radio Society) I receive the AVRS newsletter. As many of my customers have never had problems with mica capacitors, I was surprised and puzzled to read in the AVRS newsletter the advice "Mica Capacitors Connected to High Voltage Should be Replaced". I asked Warwick Woods (the current President of the AVRS) about this. Warwick was kind enough to reply with the below information.

Hello Dave,
The text was as follows:
"When restoring a valve radio treat all Mica capacitors that are connected to high voltages, such as between anodes and earth, as potential faults and replace them with a new mica component from the AVRS component store".
Many of the 'Simplex' brand Australian made Mica capacitors from the 1940's and 50's suffer from silver migration through the mica and it appears that this is due to porosity in the mica used at the time. If the outside moulding is damaged or lets moisture through then the failure is accelerated. When the silver finds its way through the mica, small 'whiskers' from either side make contact and can be blown away, (if sufficient voltage or current is available), resulting in intermittent crackling noises and other faults if high voltage finds its way into places where it should not be.
The failure mode only occurs when one side of the capacitor is connected to a high voltage and the other to a low potential point or ground.
As a general rule they need to be treated with suspicion and, to be on the safe side, replaced.
I have heard some restorers from the US say that "I have never changed a Mica in my life" and, while this may be an exaggeration, I have found that quite old US made mica caps do not seem to suffer from the same problems as our own ones. Maybe they used a different grade of mica in their construction.
The dipped types that we purchase from you do not cause any problems.
Regards,
Warwick
Nov. 2014

After reading the above the mystery was solved. I had noticed that overseas customers were mush more likely to order mica capacitors than American customers. The need to replace a mica capacitors must depend on the quality of the original mica capacitor. Tube radios made in the USA and Canada which used high quaity mica capacitors rarely go bad whereas the mica capacitors used in Australian, UK and other overseas radios must have been "not so great" these radios are much more likely to have mica capacitors in need of replacement.

Ceramic capacitors also very rarely go bad. Do not replace ceramic disc capacitors unless you are sure one has gone bad.

Some radios use what are known as "line-filter" capacitors. These capacitors connect across your radios power line and/or go from your power line to ground. When replacing these capacitors, you should use special AC Rated Safety Capacitors. These special capacitors will improve the safety, performance and reliability of your radio. If you would like to learn more about these "safety capacitors", there is a link to the ABC's of Safety Capacitors near the bottom of this page.

Get a schematic (and parts list) before you start your recap job. It is often impossible to read the values that are on the original capacitors. Also, if the radio was repaired at some time in the past, there is a good chance someone threw in the wrong size capacitors, just to get the radio working. Without a schematic you'll be guessing.

Before replacing the capacitors, check all the radios' resistors. Since you will be replacing the capacitors, you should snip one lead of each paper and electrolytic capacitor. This will help prevent false resistance readings. All resistors that are off-spec should be replaced. When it come to tube electronics..."all resistors are not created equal", so when you replace a resistor be sure to use a resistor that has both a "voltage rating & wattage" that is equal to or greater than the original resistor. More information on resistors for tube electronics.

Put heat shrink (spaghetti) tubing on the leads of the capacitors and resistors before you solder them into the circuit. This will help prevent dangerous shorts. If you need some heat shrink tubing, just "let us know" and we will be glad to add some to your capacitor order at no charge.

Always check a capacitor before installing it. Although it is very rare, every once in a blue moon, a new capacitor will be defective or off spec. Taking ten seconds to check a capacitor can save you hours of troubleshooting…..only to find out you accidentally installed a brand new "bad" capacitor.

If you need a higher uF than is available from your retailer, you can connect a couple of capacitors in parallel (side-by-side). For example if you need 200 uF at 450 volts you could connect two 100 uF / 450 volt capacitors in parallel and you would end up with 200 uF at 450 volts. You have kept the voltage the same while doubling the uF.

In "theory" connecting capacitors in series (end-to-end) should result in a higher working voltage. For example "in theory" two 100 uF at 450 volts in series should give you 50 uF at 900 volts (double the voltage and half the uF).....however, connecting capacitors in series is not recommended(& voids our guarantee) because with a series connection, one capacitor will usually end up getting more voltage than the other. This is because the leakage resistances of the two capacitors are rarely the same and the capacitor with the higher resistance will get a greater share of the voltage (which will often result in a series connected capacitor breaking down).

Please remember to always work safely. The high voltages stored in large capacitors can kill! If a radio has been turned on in recent weeks some of the capacitors (especially the electrolytic capacitors) may be holding deadly voltage charge. Before working with these capacitors they should be completely discharged. This can be come by (bridging) connecting the two ends the capacitor in question with a high wattage 1000 ohm resistor via insulated clips and leads.

Thursday, May 25, 2017

Rates and Servicces

Services
Amplifier Tune-Up
Retube & Bias Adjustment
Replacement of Electrolytic Caps
Amplifier Modifications
Instrument Pickup Replacement

Typical Rates
Minimum Bench Fee: $60
Amplifier Bias Adjustment: $30
Minor Repairs: $80 Labor Plus Parts
Average Repairs: $120 Labor Plus Parts
Extensive Repairs Priced on Inspection

Amplifier Repairs

Fender 6G15 Reverb Unit (original Mallory caps)

Fender 6G15 Reverb Unit (new Sprague caps)

Fender 6G15 Reverb Unit (original Astron caps)

Fender 6G15 Reverb Unit (new Sprague caps)

Fender Twin (original Mallory caps)

Fender Twin (new F&T and Sprague caps)

Fender Vibro-Champ (original Mallory multi-section cap removed)

Fender Vibro-Champ (new CE Manufacturing multi-section cap)

Fender Princeton 5F2 (original Astron caps)

Fender Princeton 5F2 (new Sprague caps)

Gallien Krueger 400RB (original caps)

Gallien Krueger 400RB (new Sprague caps)

Traynor YBA-3 (burned up)

Traynor YBA-3 (new caps and resistors installed)

Fender Pro Junior (50K 25-turn trimpot installed for fixed bias)

Fender Pro Junior (new F&T caps)

Replacement Capacitors

(compiled from recent amp restorations)

Alamo Capri 2360

Capacitance	Voltage	Qty.	Type
80/40/20uF	450V	1	Can
50000pF	600V	1	Ceramic
5000pF	600V	3	Ceramic
680pF	600V	1	Ceramic

Crate Vintage Club 30

Capacitance	Voltage	Qty.	Type
100uF	450V	1	Elec
47uF	450V	4	Elec
10000µF	16V	1	Elec
1000uF	35V	2	Elec
220uF	50V	1	Elec
47uF	35V	2	Elec
10uF	16V	6	Elec
1uF	50V	1	Elec
1uF BP	50V	2	Elec

Fender Blues Deluxe

Capacitance	Voltage	Qty.	Type
47uF	500V	1	Elec
22uF	500V	3	Elec
1000uF	35V	2	Elec
100uF	100V	3	Elec
22uF	35V	6	Elec
0.47uF	100V	3	Elec
22uF	63V	1	Elec
2.2uF	50V	1	Elec

Fender Champ

Capacitance	Voltage	Qty.	Type
40/20/20uF	500V	1	Can
25uF	25V	4	Elec

Fender Champ 12

Capacitance	Voltage	Qty.	Type
47uF	350V	4	Elec
220uf	35V	2	Elec
22uf	16V	2	Elec

Fender Hot Rod DeVille

Capacitance	Voltage	Qty.	Type
100uF	350V	2	Elec
47uF	500V	2	Elec
22uF	500V	2	Elec
1000uF	35V	2	Elec
100uF	100V	4	Elec
22uF	25V	5	Elec
22uF	63V	2	Elec
0.47uF	100V	4	Elec
1uF	100V	1	Elec
47uF	16V	2	Elec

Fender Pro Reverb

Capacitance	Voltage	Qty.	Type
100uF	350V	2	Elec
20uF	500V	3	Elec
25uF	25V	7	Elec
100uF	100V	1	Elec

Fender Twin Reverb

Capacitance	Voltage	Qty.	Type
100uF	350V	2	Elec
20uF	500V	3	Elec
25µF	25V	7	Elec
100uF	100V	1	Elec (SF)
50uF	50V	1	Elec (BF)

Gallien Krueger 400RB

Capacitance	Voltage	Qty.	Type
470uF	63V	8	Elec
220uF	50V	1	Elec
10uF	25V	7	Elec
1uF	50V	1	Elec

Gibson GA-19 RVT

Capacitance	Voltage	Qty.	Type
20/20/20uF	450V	1	Can
20uF	25V	1	Elec
20uF	6V	3	Elec

Guild Thunder 1

Capacitance	Voltage	Qty.	Type
60/40/40uF	450V	1	Can
40/40/40/40uF	450V	1	Can
50uF	450V	1	Elec

Marshall JCM 800 Model 2210

Capacitance	Voltage	Qty.	Type
100/100uF	500V	1	Can
50/50uF	500V	2	Can
3300uF	6.3V	1	Elec
100uF	25V	1	Elec
22uF	25V	4	Elec
10uF	100V	2	Elec
10uF	25V	2	Elec

Marshall JCM 900 Model 4100

Capacitance	Voltage	Qty.	Type
50/50uF	500V	2	Can
1000uF	16V	2	Elec
470uF	35V	2	Elec
33uF	450V	1	Elec
100uF	25V	1	Elec
10uF	100V	2	Elec
10uF	25V	2	Elec
2.2uF	63V	3	Elec
0.047uF	600V	1	Film

Marshall Studio 15

Capacitance	Voltage	Qty.	Type
50/50uF	500V	1	Can
33uF	450V	1	Elec
10uF	350V	1	Elec
10uF	100V	2	Elec
470uF	25V	1	Elec
22uF	25V	3	Elec

Randall RGT 100 ES

Capacitance	Voltage	Qty.	Type
200uF	250V	2	Elec
22uF	450V	3	Elec
1000uF	35V	1	Elec
22uF	100V	1	Elec
470uF	35V	1	Elec
10uF	50V	3	Elec
3.3uF	50V	1	Elec

Rivera M100

Capacitance	Voltage	Qty.	Type
330uF	250V	4	Radial
100uF	250V	4	Radial
10µF	450V	7	Radial
1000uF	35V	2	Radial
100uF	100V	1	Radial
100uF	35V	2	Radial
47uF	10V	1	Axial
33uF	16V	3	Axial
10uF	100V	1	Radial
3.3uF BP	50V	6	Radial
1uF	50V	2	Axial

Tube Bias

EL84 / 6V6GT - 12W

Plate Voltage	(70%)	(60%)	(50%)
200V	42mA	36mA	30mA
225V	37mA	32mA	27mA
250V	34mA	29mA	24mA
275V	31mA	26mA	22mA
300V	28mA	24mA	20mA
325V	26mA	22mA	18mA
350V	24mA	21mA	17mA
375V	22mA	19mA	16mA
400V	21mA	18mA	15mA

7189A - 13.2W

Plate Voltage	(70%)	(60%)	(50%)
200V	46mA	39mA	33mA
225V	41mA	35mA	29mA
250V	37mA	32mA	26mA
275V	34mA	29mA	24mA
300V	31mA	26mA	22mA
325V	28mA	24mA	20mA
350V	26mA	23mA	19mA
375V	25mA	21mA	18mA
400V	23mA	20mA	17mA

EL34 - 25W

Plate Voltage	(70%)	(60%)	(50%)
300V	58mA	50mA	42mA
325V	54mA	46mA	38mA
350V	50mA	43mA	36mA
375V	47mA	40mA	33mA
400V	44mA	38mA	31mA
425V	41mA	35mA	29mA
450V	39mA	33mA	28mA
475V	37mA	32mA	26mA
500V	35mA	30mA	25mA

6L6GC - 30W

Plate Voltage	(70%)	(60%)	(50%)
300V	70mA	60mA	50mA
325V	65mA	55mA	46mA
350V	60mA	51mA	43mA
375V	56mA	48mA	40mA
400V	53mA	45mA	38mA
425V	49mA	42mA	35mA
450V	47mA	40mA	33mA
475V	44mA	38mA	32mA
500V	42mA	36mA	30mA

6550 - 35W

Plate Voltage	(70%)	(60%)	(50%)
400V	61mA	53mA	44mA
425V	58mA	49mA	41mA
450V	54mA	47mA	39mA
475V	52mA	44mA	37mA
500V	49mA	42mA	35mA
525V	47mA	40mA	33mA
550V	45mA	38mA	32mA
575V	43mA	37mA	30mA
600V	41mA	35mA	29mA

Wednesday, May 3, 2017

Parts List

Parts List

5 Varable Risistors/Potentiometers
10 Watt Power Resistor
1/2 Watts Carbon Composition Resistors (25 Pieces $0.18 each)
4 Diodes
Capacitors (coupling non-polor), Electrolytic capacitors)
3 x Can Capacitor Clamp Large
Electrolytic Capacitors uf (micro Faraid)
Silver Mica pf (Pico Faraid)

2 Terminal Post (for output to Speaker connection)

5 Flat Topped Chicken Head Knob
3 Turret Boards
24 Post on a single Terminal Post
23 2 Post terminal for mounting Preamp risistors
3x lug Terminal strip @ .50 each
8x lug Terminal strips @ .95 each
12 Screws / Bolts / Nuts / Washers / Standoffs / Grommets

1. Chassis
3 Capacitor caps
2 8 pin octal sockets (with Key)
3 9 pins sockets

1 Output Audio transformer center tap in, Out 8 Ohm/30-50 Watts
1 Power transformer (in 115Volts out 6.3 Volts/250Volts)
12' 3 prong power Cord (18AWG)
1 Fuse
1 Fuse holder
1 Single pole ON/OFF Switch
6 o'ring sleeves for Chassis Transformers wires
1 1/4" Jack
1 1/4" plug

Marshall 1986 Circuit Diagram

Marshall 1986

Phase Inverter- The Long Tail Pair

The Long-Tail Pair

General

The Marshall/Fender phase inverter is commonly known as a "long-tail pair", or "Schmitt" type phase inverter, or phase splitter (actually, the original Schmitt inverter was a differential pair with a large "tail" resistor; the "standard" guitar amplifier phase inverter is a self-biased version of this circuit that works better with positive-only power supplies and ground-referenced inputs).

Following is a schematic diagram of a typical phase inverter found in some guitar amplifiers:

The basic circuit is commonly known as a "differential amplifier", which means that it amplifies the voltage difference between the two grid inputs. Technically, it is a differential in, differential out amplifier, because it has differential inputs on the two grids as well as differential outputs on the two plates (the two plate signals produce the same voltage signal, but one is inverted, or 180 degrees out of phase, with respect to the other).

It should be noted that there are actually three inputs used in this type of phase splitter. The first input is the obvious one, the left side of C1. The second input (the lower end of C2) is useful as a feedback input, a reverb or effects return input, or as a second channel input. In the circuit shown above, the second input is used as a feedback return input, taking the signal off the junction of the feedback divider.

The third input is not so obvious; it is the lower end of R6. If a signal is input at this point, the phase splitter will produce an output signal on each output that is in phase with the other, rather than 180 degrees out of phase, and also in phase with the signal input at the lower end of R6. This means that if a signal of equal phase is applied to the first input (C1) and the third input (R6), it will subtract from the out of phase output (R1) and add to the in phase output (R2). Likewise, if an equal phase signal is applied to the second input (C2), and the third input (R6), it will subtract from the in phase output and add to the out of phase output (this is because the out of phase output is actually in phase for the signal applied to the second input, C2, and the in phase output is out of phase). This third input is useful for balancing the feedback signal by subtracting from the in phase output and adding to the out of phase output in order to compensate the unequal gains to each output from the feedback input. The gain is much less than the gain into the first and second inputs.

The two outputs provide (nearly) identical signals, except for a 180 degree phase difference between them. This is exactly the type of signal needed to drive a push-pull amplifier, so this circuit is commonly seen in higher-power guitar amplifiers.

The plate resistors

The output voltage is developed across the plate resistors (R1 and R2), and is proportional to the current changes from the tubes in response to the input signals. The value of these resistors is set using "standard" techniques, such as using the load line to determine the desired amplification and output range. A good value to start with is usually around twice the internal plate resistance of the triode. These resistors have a major effect on gain and output impedance of the phase splitter. The actual output impedance is equal to the plate resistor value in parallel with the impedance seen looking into the plate of the tube. Since there is local feedback in this stage, this is larger than the standard preamp stage output. These resistors also have an effect on frequency response. Higher values will result in less high frequency response. When only one signal input is used (ignoring feedback inputs) R1 is usually made 10% - 20% lower than R2 to compensate the unbalanced gains of the two tube sections and make the two output amplitudes equal.

The grid resistors

These resistors (R3 and R4) provide the grid bias reference voltage. They are the equivalent of the normal "grid-to-ground" resistors in a standard preamp stage, except that they don't go to ground, instead, they go to a different "reference" point, the junction of R5 and R6.

The value of these resistors is not critical, but they should be a moderately large value, somewhere around 100K - 1Meg. Contrary to popular belief, in this type of phase inverter, the input impedance is not equal to the value of this resistor, rather it is around two to five times higher, depending upon the amount of negative feedback from the "tail resistor" and the amount of global negative feedback (around two times higher for the circuit shown above, with no global negative feedback). This is why it is not a good idea to use too large a value of coupling capacitors going into the phase inverter input.

This increase in effective input impedance is known as "bootstrapping". It is similar to the effect you get when you have a self-biased cathode follower. There is an AC signal present at the junction of the grid resistor (R3) and the "tail" resistor (R6), since there is current feedback due to the unbypassed tail resistance. Since this signal is in phase with the input signal, the effective current through the grid resistor is lowered. The signal at the top and the bottom of the grid resistor is subtracted, and that voltage divided by the grid resistance gives the input current drawn by the stage. If you divide the input voltage by the input current, you get the effective input impedance. For example, if you apply a 1V AC signal and the signal at the tail node is 0.5V and in phase, the input impedance is 2 Megohms, not 1 Megohm, because there is 0.5V across the 1Meg grid resistor instead of 1V, which results in a current of 0.5uA for a 1V input, and Rin = 1V/0.5uA = 2 Megohms. If the tail resistor is large enough to be considered a constant current source, and there is no global negative feedback, the input impedance will be twice the value of the grid resistor.

If there is global negative feedback, the signal applied to the second input will be in phase with the signal applied to the first input (this results in a reduction in the output voltage, which means the feedback is negative). This signal will add to the cathode voltage because it is in phase. The impedance seen "looking into" the cathode on each side is (Ra + Rl)/(mu+1). Assuming matched tubes with equal mu's, this means the source and load impedances are equal at the cathode, so the voltage is divided exactly in half. This means that the input impedance is dependent upon the amount of negative feedback applied, and can get very large for large amounts of negative feedback. For example, if 1V is applied to the first input, and 0.5V of feedback is applied to the second input, the cathode voltage would be V=1/2 + 0.5/2 = 0.75V. The resulting input impedance would be 1 Meg/(1-.75) = 4 Meg.

These grid resistors have little or no effect on gain, for normal values. If they are too low in value, they will attenuate the input signal. They do have an effect on the frequency response. Higher values will result in greater low frequency response for a given input coupling capacitor, but this effect is diminished somewhat due to the local negative feedback.

The input coupling capacitors

These capacitors (C1 and C2) are used to block DC levels from previous stages, in order to keep from upsetting the DC bias voltage on the grids of the phase inverter tubes.

These capacitors also determine the lower -3dB point of the frequency response of the phase inverter. If the input impedance is two times the grid resistor value, for instance, or 2Meg, and a -3dB point of 53Hz is desired, a capacitor of C = 1/(2*pi*53Hz*2Meg) = 1500pF, or .0015uF, would be required. Too large a coupling capacitor will increase the tendency for the phase inverter input to generate "blocking" distortion. If C1 is made small (less than .01uF or so, with 1Meg grid resistors), it will improve the low frequency response balance between the two output phases if the second coupling cap, C2, is made at least ten times larger than the first cap, C1.

An interesting thing can happen, though, when the phase inverter hits clipping. This very high input impedance suddenly drops, and can severely clip the input waveform (by "clamping" the top to the cathode voltage level) and raise the lower -3dB point. For this reason, when tapping off the phase inverter input to go to another tube, say, for instance, an effects loop or reverb amplifier, a large value (100k or so) series resistor should be included in front of the grid of the PI, and the signal should be tapped off before this resistor to preserve the original signal. This resistor can also help smooth out the tone of the PI when it clips.

The bias resistor

The bias resistor (R5) is connected to the two cathodes, which are tied together, and sets the bias current for the two tubes. Since it has the cathode current for both tubes flowing through it, the value must be half of what it would normally be for one tube in a "standard" preamp configuration. This value is selected by plotting the load line for the tube in question, and determining the required negative grid bias voltage to give the desired operating point and plate current. The value is then halved, since both tubes will be drawing current through the same bias resistor.

For example, a "normal" 12AX7 preamp stage might have a bias resistor of 820 ohms to 1.5K. If the same bias point is desired for the phase inverter, a value from 410 to 750 ohms would be used (using standard 5% values, pick a resistor from 390 to 820 ohms). The values might be different for a 12AT7, depending upon the desired plate current and bias point.

This resistor will determine both the quiescent DC plate voltage (a smaller resistor equals more current, which results in a lower quiescent DC plate voltage), which determines the symmetry of the clipping, and the "headroom" of the PI. It also determines the headroom of the grid input, which also determines the point at which the PI clips, relative to the input grid voltage. Subjectively, higher currents are usually attributed a "warmer" tone. Too much current results in too much non-linearity, and adds unwanted harmonic distortion even to clean sounds. This resistor is best set to give a fairly decent clip characteristic for the PI, or best linearity, or tone. (Be sure to disconnect any global negative feedback before testing this, as the feedback will tend to correct distortions present in the phase inverter). Since this resistor is the main controller of the current flow, it will drastically affect the quiescent DC levels at the plate, the grids and the cathode. This resistor has a large effect on gain.

The "tail" resistor

The next resistor is the "tail" resistor (R6). It is used as a "pseudo constant current source", providing local negative feedback to the PI. This resistor is necessary because, without it, the differential amplifier would have very unbalanced outputs (the output signal on one plate having a larger peak-to-peak amplitude than the output signal on the other plate), because of the low relative gain of the tubes comprising the differential cathode-coupled amplifier. The larger this resistor is, the better the balance of the PI outputs. There is an upper limit, however, where the tail resistor drops too much voltage and there is no headroom left (or perhaps it should be called "footroom", since it raises the DC level of the cathodes of the tubes). This resistor is best adjusted by careful attention to PI balance and headroom, settling on a good compromise between them.

Making the first tube's plate resistor (R1) 10-20% smaller than the second tube's plate resistor (R2) will compensate the gain difference between the two amplifier sections, and should be done before manipulating the tail resistor. Note that this should done only if one input is used as a signal input, and the second used for a feedback input. If both inputs are used as signal inputs, for channel 1 and channel 2, for instance, the plate resistors should be identical, because compensating the balance of one channel will make the balance of the second channel even worse.

The tail resistor also "bootstraps" the stage, resulting in a higher input impedance, due to the local feedback action, as described in the grid resistor section above. Note that the bias resistor, R5, sets the current through this tail resistor. The amount of current set by the bias resistor, along with the value of the tail resistor, determines the DC voltage dropped across this resistor, which, in turn, partly determines the headroom of the circuit. If no global negative feedback is used, the tail resistor should be made as large as practical, with respect to the amount of current being drawn, and the desired headroom of the amplifier. This will give the best balance to the PI outputs. This resistor has little effect on gain, but a major effect on balance and headroom.

The feedback resistors

Since this type of phase inverter has two main signal inputs (ignoring the third in phase input for a minute), the second one makes a good spot to introduce global negative feedback from the output transformer secondary, to reduce distortion, improve linearity, and lower the effective output impedance of the amplifier (increase damping, for "tighter" bass). The last resistor is usually a small value, such as 5K (Marshall) or 100 ohms (Fender), and is the shunt element of the feedback voltage divider for the global negative feedback loop (pot VR1 in the above schematic). The feedback voltage applied to the phase inverter is the resultant divided-down version of the output voltage. This resistor directly affects the amount of negative feedback, and thus, the overall gain of the output section, as well as the linearity, input range, and distortion. The feedback divider ratio is the ratio between the series feedback resistor (R7) and the shunt feedback resistor (VR1). The amount of feedback also controls the effective bootstrapped input impedance.

The presence control

Potentiometer VR1, in addition to providing the 5K resistance to ground for the feedback attenuation network, is also used as the presence control. Capacitor C3 is used to shunt a portion of the feedback signal high frequencies to ground. By reducing the amount of high frequencies being fed back, there is more gain at these frequencies. This results in a boost of the upper frequencies, adding "presence" to the signal. This is a bit different than just a simple equalization boost, because, in addition to boosting the high frequencies, there is less negative feedback at these frequencies, which means the output stage has less damping, and the effective output impedance is raised, which increases the interaction between the speakers and the amplifier at these frequencies. Increasing the value of the capacitor will lower the corner frequency of the boost.

Conclusions

The long-tail pair phase inverter is generally the best choice for a push-pull guitar amplifier. It provides the very good gain and balance, as well as extra inputs for feedback summing. The best way to get a feel for this circuit is to replace the bias, plate and tail resistors with trimpots, and adjust them interactively while watching both outputs on a dual-channel scope. Alternately, a lot can be learned by simulating the phase inverter with different values in PSpice, or another simulation program.

What does this calculator do?

The long-tailed-pair phase inverter with negative feedback was used by Leo Fender in the 5F6-A Bassman and subsequently became an overwhelming favorite for classic large amp designs, Marshalls in particular. The basic long-tailed-pair without negative feedback is shown below.

As the input signal voltage increases, the plate current through the left tube increases, causing the inverted output voltage to decrease because of the increased voltage drop across the plate resistor R_L1. It also causes the current through the cathode resistor R_Kand the tail resistor R_T to increase, which increases the voltage between the cathodes and ground, thereby making the grid-to-cathode voltage of the right triode more negative, causing its plate current to decrease. This raises the in-phase output voltage. Thus an increase in the phase inverter's input voltage lowers the inverted output voltage and raises the non-inverted output voltage.

Negative feedback from the output transformer secondary can be introduced by splitting the tail resistor into two resistors and driving their connection by a signal taken from the output transformer. This is particular modification is widely used in classic amps.

The calculator takes into account the output load of the power amp's grid resistors R_Gpa. (These are not the phase inverter grid resistors R_G shown in the drawing above.)

To get exactly the same gain in each phase (one positive and the other negative) with identical plate resistors, the tail resistance needs to be infinitely large. (This can be demonstrated, for example, by making the plate resistors equal and setting R_T to an unrealistically high value like 1M.) Having a large tail resistance limits the maximum output voltage swing, which is of particular concern for high-power amps. For this reason a smaller tail is often required, which causes the inverted phase to have substantially more amplification than the non-inverted phase. The usual correction is to reduce the size of the inverting plate resistor R_L1.

Reference

https://www.ampbooks.com/mobile/amplifier-calculators/long-tailed-pair/

https://www.ampbooks.com/mobile/amplifier-calculators/cathode-capacitor/

Chapter 1. Introducton

Chapter 2. Pentodes and Beam Power Tubes

Pentodes 5 Beam Power Tetrodes 7 Plate Characteristic Curves 8 Power Tubes versus Voltage Amplification Tubes 11 Low Plate Voltage Effects 12 Performance Differences Between Pentodes and Beam Power Tetrodes 13 Plotting Curves for a Specific Screen Voltage 16 Variability of Tube Characteristics 18

Chapter 3. Plate and Screen Circuit Design

The Basic Steps of Power Amp Design 19 The Common-Cathode Amplifier 19 The DC Operating Point 20 Using Triode-Connected Curves 21 Vacuum Tube Response to AC signals 23 Cutoff and Saturation 23 Setting the DC Operating Point 25 The AC Load Line 26 Optimum Load Line for Pentodes 30 Screen Dissipation 33 Maximum Power and Headroom 35 DC Grid Bias Voltage 37 Fixed Bias 37 Cathode Bias 38 Practical Aspects of Using Cathode Bias 39 Cathode Degeneration 40 Selecting the Bypass Capacitor Value 42 The Output Transformer 42 The Screen Grid-Stopper Resistor 44 Plate Circuit Design Procedure for Single-Ended Amplifiers 45

Chapter 4. Grid Circuit Design

A Basic Grid Circuit 47 Preamp Output Impedance 48 Equivalent Grid Circuit for Audio Frequencies 49 Middle-Range Frequency Response 51 Low-Frequency Response 51High-Frequency Response 53 Measuring Parasitic Capacitance 54

Chapter 5. Parallel Tubes and Parasitic Oscillation

Parallel Tubes for More Power 57 Parasitic Oscillation 58 The Effect of RF Suppression on Audio-Frequency Distortion 59

Chapter 6. Push-Pull Power Amps

How a Push-Pull Amplifier Works 61 Class A Push-Pull Operation 63 Class B Push-Pull Operation 65 Class AB Push-Pull Operation 66 Guitar Amplifiers - In a Class All Their Own 67 Power Supply Voltage Excursion 68 Estimating Power Supply Voltage Sag Based on Current Load 68 Design Strategies for Dealing with Class AB Power Supply Sag 73 Drawing Composite Characteristic Curves 75 The Load Line 78 Class AB Power Output 79 Plotting the Effective Load Line for One Tube 80 Computing the Current Load and Plate Dissipation 81 Cathode Bias for Push-Pull Power Amps 83 The Screen Grid-Stopper Resistor 84 The Effects of Mismatched Components 84

Chapter 7. Distortion Characteristics at Full Power

Harmonic Distortion 87 Calculating Percent Harmonic Distortion 90 Intermodulation Distortion 95 Controlling Harmonic Content 95 Rectification Effects 98 Single-Ended versus Push-Pull Distortion 99 Class AB Distortion: Fixed Bias versus Cathode Bias 99

Chapter 8. Distortion in an Overdriven Power Amp

An Overview 102 Headroom 103 The Cushioning Effect 104 Bottoming 105 Positive Grid Voltage Effects 105 Driving a Power Tube Grid Positive with a High-Impedance Source 109 Clipping and Clamping 112 A Different Perspective: How the Circuit Responds over Time 114 Bias Excursion and Recovery 118 The Recovery Phase 120 Bias Recovery Time versus Bass Response 121 An Example of Grid Bias Excursion 124 The Grid Bias Excursion Ratio 127 Bias Excursion Time 130 A Summary of Bias Excursion Formulas 131 Grid Bias Supply Considerations 132 Grid Bias Supply Voltage Excursion and Recovery 133 Bias Excursion for Cathode-Bias versus Fixed-Bias Designs 135 Controlling the Dynamics of Bias Excursion 138 Bias Excursion and Recovery for Some Vintage Amplifiers 138 The Tonal Effects of Overdriving a Power Amp 139

Chapter 9. Crossover Distortion, Blocking, and Blackout

Crossover Distortion 143 Blocking Distortion 143 Minimizing the Likelihood of Blocking Distortion 146 Class AB: Fixed Bias versus Cathode Bias 146 Blackout 147

Chapter 10. The Marshall Model 1967 Head

Pentode-Operated Pentodes 149 Triode-Operated Pentodes 150 Ultra-Linear Power Amplifiers 153

Chapter 11. Real-World Output Transformers

Ideal Single-Ended Transformers 155 Ideal Push-Pull Transformers 156 DC Magnetization Current 157 Hysteresis Losses 158 Middle-Range Transformer Losses 160Low-Frequency Transformer Response 163 High-Frequency Transformer Response 164 Total Response 164 Transformer Power Rating and DC Current Effects 166 Output Transformer Distortion 166 How Real-World Characteristics Affect Power Amp Design 167

Chapter 12. Real-World Loudspeaker Impedance

Nominal Impedance 169 Resonant Frequency and Beyond 170 An Example - The Jensen C12R-8 172 Estimating the Nominal Impedance of a Loudspeaker 172 How Loudspeaker Impedance Affects Power Amp Design 173

Chapter 13. Paraphase Inverters

The Common-Cathode Triode Amplifier 177 Computing the Resistor Values 179 Frequency Response 180 The Gibson GA-20T Inverter 180 Overdriving and Distortion 181

Chapter 14: The Concertina Phase Splitter

The Concertina Phase Splitter, an Overview 183 The DC Circuit 184 Maximum Output Voltage Swing 187 The AC Circuit 189 Phase Splitter Output Impedance for Arbitrary Loads 192 Overdriving and Distortion 197 Nonlinear Distortion Effects 199 Summary of Important Concertina Features 200

Chapter 15: The Long-Tailed-Pair Phase Inverter

The DC Circuit 201 The AC Circuit 204 The Common-Grid Circuit 208 The Common-Cathode Circuit 209 Voltage Gain Imbalance 211 Output Impedance 212Overdriving and Distortion 212 Maximum Output Voltage Swing 213 Adding a Second Signal Input 214 Adding Negative Feedback and a Presence Control 214 Voltage Gain and Input Impedance for Negative Feedback 218 Comparing the Concertina to the Long-Tailed Pair 219

Chapter 16: Negative Feedback

A Generalized Negative Feedback System 221 Loop Gain 222 A Second Look at Cathode Degeneration 223 Negative Feedback from the Output Transformer Secondary 224 Frequency Response with Negative Feedback 226 Other Feedback Effects 228 A Handy Formula for the Long-Tailed-Pair Phase Inverter 229 Stability 230Motorboating 234 An Example of Negative Feedback Design 237

Chapter 17: A Step-by-Step Single-Ended Design Example

The Basic Steps of Single-Ended Design 243 Selecting the Tube and the Screen Voltage 244 Selecting the Idle Plate Voltage 244 Estimating the Cutoff Grid Voltage and Plate Current 245 Setting the DC Operating Point 246 Designing the Cathode Bias Circuit 246 Selecting the Output Transformer Primary Impedance 248 Determining the Plate Circuit Operating Conditions 250 Computing the Harmonic Distortion at Full Power 252 Designing the Grid Circuit 254 The Final Power Amp Design 254

Chapter 18: A Step-by-Step Class AB Parallel Push-Pull Design Example

Accounting for Power Supply Voltage Sag 257 Selecting the Output Transformer Impedance 259 Determining Output Power and Voltage Gain 261 Selecting a Screen Resistor 262 Plotting the Composite Characteristic Curves 262 Plotting the Effective Load Line for One Tube 262 Computing the Average Plate and Screen Current at Full Power 264Computing the Plate and Screen Dissipation 268 Computing the Power Supply Voltage Sag at Full Power 268 Determining the Zero-Signal Characteristics 270 Calculating Third Harmonic Distortion at Full Power 272 Selecting the Grid Resistor Value 273 Applying Preamp Constraints 273 A Paraphase Design 274 Selecting the DC Operating Point 276 Computing the Resistor Values 278 A Concertina Design 279 Selecting the DC Operating Point 280 Examining the Concertina's Nonlinearity 280 Determining the Other Resistor Values 283 A Long-Tailed-Pair Design 283 Tail Resistance and the DC Load Line 284 Selecting the DC Operating Point 285 Examining the Long-Tailed-Pair's Nonlinearity 287 Balancing the Voltage Gains 287 The Final Phase Inverter Design 288 Computing the Coupling Capacitor Value 289 Selecting the Grid-Stopper Resistor Value 289 Computing Bias Excursion and Recovery 291 The Final Power Amp Design 291 Some Last Words About Class AB Design 293

Chapter 19: Epi-Log

Appendices A-G: Vintage Power Amps Listed by Tube Type and Operating Class. Tube Data Sheets

EL84/6BQ5 Power Amps 297 GE 6BQ5 Data Sheet 298 6V6/6AQ5 Power Amps 305 GE 6V6GT Data Sheet 307 7027 Power Amps 313 RCA 7027 Data Sheet 3147591 Power Amps 323 Sylvania 7591A Data Sheet 324 EL34/6CA7/KT77 Power Amps 329 Philips EL34 Data Sheet 331 6L6/5881/KT66 Power Amps 339 Marconi KT66 Data Sheet 342 6550/KT88 Power Amps 353 GE 6550A Data Sheet 354