What is the Bias?
Tube amplifiers which are based on PUSH-PULL design need to be adjusted to operate in a certain way for best performance & life. This adjustment is done by setting the “BIAS”. For a typical Push-Pull amp based on 2 X EL34 tubes, it is designed for maximum 2 X 25W output Power. Generally this is biased to deliver maximum 7—80% of 25W per tube. This can be adjusted by BIAS. BIAS is negative voltage applied to the GRID of the Power Tubes. The more negative it is the lesser is the load on the tube and more will be the life, however, if it is too negative the tone gets muddy and you’ll end up getting crossover distortion. On the other side, if the bias is set less negative then the Tone will be more open and bright but if it exceeds the rated limit then you might burn out the tube.
To read more on BIAS Refer: Practical Fundamentals of Biasing a Tube Amp, How to Bias your Amp
As a simple ballpark calculation, for the Custom 50 Amplifier, the Power Tubes operate at about 480V and each EL34 tube is designed for maximum 25W, so the current at which it can be safely driven Should be, 80% X Power/480 which gives you a current of about 42 mA or 0.004 A.
To measure this current a very close approximate way is to measure the cathode current through a 1 OHM resistor. For the Custom 50 Amplifier, there are three terminals provided at the back panel. The black Terminal indicates the BIAS Voltage and the two red terminals give you the cathode current measured indirectly by taking voltage readings.
How to Measure & Adjust Bias on Custom 50
Connect multimeter ground pin to the chassis and set the voltmeter in voltage position. Measure the voltage at the black terminal it should be in the range of -35V to -50V. Connect the meter to the red terminals and take the reading on both. Both should be close. If it is below 30mV or more than 50mV, you need to adjust the bias. If the reading on red terminal is more than 50mV then you need to increase the negative bias and if it is below 30mV then you should increase and bring it around 40mV levels. This can be done with the help of a small screwdriver and very very slowly turning the trimpot at the back. You need to turn it one side a little bit and check if the voltage at the red terminals are increasing or decreasing and then accordingly you adjust and bring the voltages at around 40mV level.
Make sure you do not touch any metallic part or tube for safety. Keep an eye on the output tubes and make sure during the process they do no turn orange red and become very hot, in case this happens, recall the adjustment you did. You might have decreased the bias voltage leading to higher current/voltage on red terminals. This lead to the tubes running hot. Sometimes tubes can become so hot that they turn white and you can hear crackling sound and they can even melt-down leading to a collateral damage to the amp, especially the Transformers.
For best results do this very slowly and carefully. Even at the slowest possible manner this process only take about 5 minutes and its worth spending time on this for best results.