Fender Pro Junior Cathode Bias Mod

I got this little Pro Junior in trade a while back. These things sound pretty decent even stock. This one was cutting out intermittently though. Turns a few solder joints were cracked where the sockets were mounted on the PC board (pretty common with board mounted sockets). The board itself was OK so the fix was quick and easy.

Here it is with the back off:





When I looked over the main circuit board, I noticed that the wire marked W1 was a jumper to ground for the power tube cathodes. That would make for a quick and easy modification to cathode bias. So I figured I had it open, why not. And if I'm going to bother, why not make it switchable fixed/cathode bias.


THE UBIQUITOUS DISCLAIMER: AKAVALVE ASSUMES NO RESPONSIBILITY FOR THE SAFETY OF ANYONE IMPLEMENTING THESE INSTRUCTIONS. IF YOU ARE NOT FAMILIAR WITH SAFE PRACTICE IN HIGH VOLTAGE CIRCUITS, DO NOT ATTEMPT THIS YOURSELF.

Here's the 22oK resistor assembly. They'll serve as the bias feed resistors in fixed bias mode and as ground reference for the power tube grids in cathode bias.



I futzed a bit with the cathode resistor value. I was planning on selling this amp, so I ran it slightly on the cool side so that whoever ended up with it wouldn't have a tube eater on their hands. Here's the final assembly.



The black wires run off to the right hand switch. You can see I also added a switchable
negative feedback loop. It was a bit inconvenient having the switches under the chassis but I didn't want to change the faceplate and it was a much shorter run for the wiring.




When I was done the amp sounded pretty cool. The pair of switches made it a good bit more versatile. In cathode bias with the feedback loop disengaged it was much looser, browner and more touch responsive - a much better blues sound. Engaging the feedback loop and or the fixed bias would make it tighter, cleaner and brighter.

I sold this particular one awhile back so I can't get more photos. I think my Dad still has a stock one though. If anyone's interested in a more detailed procedure, drop me a line - he might be convinced to let me use it for a step by step demo.