Pro Junior Adjustable Bias
I made the bias adjustable because the factory bias is just too hot on Pro Juniors and Blues Juniors. This particular amp was idling at just under 12 watts, which is supposed to be the maximum dissipation for the output tubes. As a result, it went much higher when you actually played. This “High AB” biasing seems to be fashionable among some EL84 designers, supposedly giving the same sonic attributes as a Class A amp. It doesn’t. It’s just more hiss, more heat, and shorter tube life.
I replaced R29 with a 50K 25-turn trimpot, configured as an adjustable resistor. This gives me a huge adjustment range for the bias. I set the idle at 9 watts. It sounds just as ballsy as it ever did, but the temperature on the glass of the EL84s is 70 degrees lower.
The lower leg of the pot is in the lower R29 hole and I drilled holes for the other two legs. The wiper leg is bent and soldered to the lower leg and a jumper wire connects the upper leg to the upper hole for R29. Simple. Note that I bent CR5 just a bit to make room.
You can see here how easy it is to do the bias mod on the back of the circuit board. Note that the bottom edge of the circuit board in the picture above is now the top edge in this picture.
One leg of the trimpot goes into the existing R29 hole.
Drill an offset hole that matches the center leg of the trimpot. The offset hole comes out pretty close to the existing R29 hole, so all you need to to is bend the lead over so it touches the solder pad. I haven’t soldered them yet in this photo, so you can see them clearly.
For the other lead, a simple jumper does the job. Make a loop in one end and place it over the trimpot lead. Bend the other end 90 degrees and place in the top hole of R29. I crimped the loop a little tighter before I soldered this.
After you’ve put everything back together, it’s best to set the bias voltage to around -12V before you insert the output tubes. That way, you know they won’t immediately red-plate.
When you bias the amp, all you have to do is measure the voltage drop from B+ (the + side of the large filter capacitor) or from the red output transformer wire (they’re connected together).
To adjust the bias: Put a jumper clip on the red transformer lead (P2) and another on the brown (P3). Plug in speaker, insert tubes, warm up amp, turn master volume off. Adjust the trimpot until you get a reading of 2.4 to 2.7 volts across these two leads. Remember that the actual voltages may be over 330V and you’re only measuring the difference between them!
Because the output transformer resistance is 100 ohms, a 2.4 volt drop indicates 24 mA of plate current. Multiply that times roughly 330V on the plate, and you’ve got 8 watts of idle dissipation per tube, a good number. The actual plate voltage will change with your AC line voltage, but the difference in plate dissipation is small.
Here is a video of how to adjust the bias: