Tuesday, December 9, 2008

Fender Pro Junior Cathode Bias Mod


Fender Pro Junior Stock
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:



Fender Pro Junior Mod Inside

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.

Fender Pro Junior Cathode Bias Switch
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.

Fender Pro Junior Cathode Bias Mod 220K Ground Reference Resistors

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.

Fender Pro Junior Cathode Bias Mod

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.

Fender Pro Junior Cathode Bias Mod Switches


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.

3 comments:

Frederick said...

Do you sell these modz?
I would like to do this to my Pro jr
I need instructionz?
thankx

akavalve said...

I can do a step by step thing when I'm done with the Champion 600. I hadn't thought about selling kits but I'd be glad to send you the parts for a few bucks. Just drop me an email (akavalve@gmail.com) and we'll figure it out.

Kraig said...

Exactly what I was googling for 10 years later. Thx.