Caliber.22+ adjustable bias?

[Mason]

Hi, I'd like to add a bias pot to a caliber.22+.The wiring seems a bit more complicated than in my other Boogies, since it seems to change also the bias of the phase inverter.So any help is much appreciated.
Mason

 

[Mason]

any ideas out there....??????
mason

 

[stomper]

I don't recommend doing this unless you are a very competent amp tech.
The use of 6BQ5 tubes mounted to the circuit board increases the difficulty level over any amp using a chassis mounted tubes socket.

I converted my Studio .22 and it sounds amazing. But if I had to do it over, I would just mess around with a few sets of Groove Tubes and call it a day.

 

[Mason]

I don't recommend doing this unless you are a very competent amp tech.
The use of 6BQ5 tubes mounted to the circuit board increases the difficulty level over any amp using a chassis mounted tubes socket.

I converted my Studio .22 and it sounds amazing. But if I had to do it over, I would just mess around with a few sets of Groove Tubes and call it a day.

Hi Stomper,thanks for reply.What exactly did you change on your .22?I thought of swapping R111(33K) for a 50K pot, but what to do to keep the 2Volts PI bias after R112?
Thanks for suggestions,
Mason

 

[stomper]

Hi Mason, Yes that is the resistor to change. You may need to experiment with resistor #272. I believe I used 115K. It has been a long time since I did the mod. I also installed 1 ohm resistors on each power tube to measure bias. I wasn't concerned about the PI voltage at all. I just made the tubes run hotter. When they sounded good to me, I kept that setting. That is why I would just try the extremes of the Groove Tube range and go with them. It is a lot of work, and you are hacking the power tube sockets. Not for the timid.

I also changed the output section coupling caps to .1uf. A necessary mod in my opinion to open up the low end. It allows you to run the preamp bass control a bit lower, thereby taming the mud.

The .22 cal is my favorite amp. It is very pedal friendly also.