Convert to cathode bias

For the tube to work, the grid has to be negative in relation to the cathode. This is what bias does, either by applying a negative voltage to the grid (fixed bias), or by putting a positive voltage on the cathode (cathode bias).

To calculate this resistor, use Ohm's law:

R=-32V / 0.088A

(remember that there are 2 tubes, each drawing 44mA). R=364 ohms. The wattage of the resistor

(power) = 0.088A * 32V = 2.8W.

To be on the safe side, double this and use a 5W resistor. You can either work up a combination of resistors to get 364 ohms, or use the closest value you can find. Check it in the circuit to see that the tubes are biased within a safe range.

A bypass capacitor will have a noticeable effect on the sound. It can be calculated using

C=1 / (2 * pi * R *f)

where f is the frequency that you'd like to start the bypass. If f=20Hz (the lowest part of the hearing range), C = 22µf. You can play with the frequency you select to change the sound of the amp. This cap can be switched in and out by inserting a resistor between it and its ground connection, and adding a switch to short out the resistor. The resistor should be about ten times the size of the cathode resistor. When the switch is open and the resistor in in the circuit, the bypass cap is effectively eliminated. Switching this way eliminates popping.

The cathode resistor will generate a fair amount of heat, so keep it away from its bypass cap and other components.

If you are building from scratch, and can't measure the bias voltage and current of the tubes, resistor values can be found in tube data sheets, or you can use trial and error. Start in the range of about 500 ohms, get a bias current reading and adjust from there.

If your amp has fixed bias and you'd like to try cathode bias

    1. find the voltage supply that applies a negative voltage to the grid(s). On Fender amps it often connects to 2-220K resistors

    2. measure the voltage at this point (say it's -32V)

    3. measure the current the tubes are drawing at idle using one of the ways to measure bias (say it's 44mA per tube)

    4. unhook the negative voltage, and ground the resistors you found in step 1

    5. lift the ground connection on the cathode of the power tube(s), and insert a resistor, possibly bypassed by a capacitor