- output should be about 10x bigger than input
- prevent small signal amplified un-distorted
- prevent signal clipping
- bias can set to high .. output gets clip
- bias set to low .. output gets clips
- Use on Push-Pull (one tube push and other tube pull)
- Have tube in Match sets
- Auto Bias/Cathode Bias
- The tube dissipation at the plate is different at the Cathode, so bias at the plate is better.
- preamp tubes do not needs bias, Preamp is the GAIN stage of AMP
Methods of Biasing
1. current at Cathode
2. negative voltage at grid
3. Current at Anode (proper way to bias)
to Bias you need to know:
1. current of Anode of Power Tube
2. Voltage at anode
3. Max dissipation rating on Anode
How to Bias a Tube Amp
Bias is about 70% of your idle Current
Bias Current = [(MAx Watt Dissipation)/(Plate Voltage)] x 70%
Plate Voltage should measure about 460-480Volts
Example For Amplifier
- if plate voltage is 457Volts
- KT77 tubes are reated for 25W
- Then Bias current = (25W/457V) x 70% = 38.29mA
Note: Ohm's LAW V=IR so if you use a 1 Ohm Resistor, then voltage across resistor will equal to current
Tube: 6L6
Bias Voltage: 30 -37 Volts
Method of Change Bias: Trim pot
Power Tube (EL36 or 6L6)
Decrease in power when tube aged
Bias tube via trim pots
How to Bias a Tube Amp
Warning: Tube amp carries high voltage even when its unplugged because it stores in Capicator
Tube: 6L6
Bias Voltage: 30 -37 Volts
Method of Change Bias: Trim pot
- preamp tubes do not needs bias
- Preamp is the GAIN stage of AMP
Power Tube
Decrease in power when tube aged
Bias tube via trim pots
Fender Twin
