Setting the Bias
After you’ve installed the basic mods, your final task is to set the bias. Much mystery seems to surround setting the bias, but it’s really quite simple. Bias controls the current flowing through your output tubes, and it’s well known that the Blues Junior’s original designer chose a large current, or hot bias. This causes excessive tube wear, limits headroom, and constrains the tonal range of the amp.
Each model of output transformer has a characteristic resistance and when current flows through it to power the tubes, there is a voltage drop across the resistance. All you have to do is measure across two leads of the transformer and adjust the bias trimmer potentiometer until you get the correct voltage.
This video, showing a Blues Junior with EL84s and a TO20 output transformer, demonstrates how easy it is to set the bias.
SAFETY REMINDER: You’re measuring the difference between two high voltages! Work carefully, with insulated leads, as shown.
Here are the target voltages for different tube and transformer configurations:
* With TP24 power transformer.