Need to change 22K resistor to 27K when changing from 5U4 to GZ34?

[PeaveyBandit]

AA1164 BFPR fresh build. Question, I'm reading from someone how changing from the indicated stock 5U4 to GZ34 the 22k resistor on the bias board should change to 27K. Can anyone comment on this? I have a new Heyboer PT that will handle anything thrown at it. It's from Allen and is model T-025. If there's any other information I can provide let me know.

To summarize my plan is to build a stock PR with premium parts then add at least the stokes mod, 12" speaker, and deluxe OT. So far I'm beefing up the 1K resistor at the power supply end and ordered extra components to try various small tweaks recommended here on other threads. So the PR will be running a little hot rodded. Hopefully yall wont have to answer too many newbee questions but I do appreciate your expertise.

PB

[Paul P]

I think you can count on tweaking the power supply resistors, as you seem
to be planning to do. I wonder how Fender managed to keep all component
values (and voltages) the same after changing rectifiers, I doubt the 22k vs
27k is enough to make up the difference. It's been my experience that when
you change the rectifier you have to then play with the resistors to get
voltages back to where they were. And the bias will need readjusting.

I believe the transformer you have is a TP25 ?

Paul P

[PeaveyBandit]

Hi Paul,

Thanks for the help. Yes, my typo. It's a TP25... I might be getting over my skill level with changing to a GZ34 and tweaking resistors without some caution. I better do some more research before I just pop a GZ34 in there.

PB

[MWJB]

The 22K on the bias board sets the bias current for the 6V6s, it is not a power supply dropping resistor. It is impossible to know whether you need a 22K or a 27K until you have built the amp and selected a particular brand of power tubes. Make life easy for your self and use a 50KL trim pot then you will be able to bias up any brand of 6V6s.

If you go to www.hoffmanamps.com and check out his library of information he has the Princeton adjustable bias mod there.

[PeaveyBandit]

Thanks very much. I've been looking at the various ways to add this mod on Hoffman. I'll order some more parts and install a bias pot into the chassis.

Now I'm also thinking of using the ground polarity switch hole to install a standby switch too.