Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: J Rindt on July 10, 2011, 08:18:10 pm

Title: BF Fender Bassman PI ?
Post by: J Rindt on July 10, 2011, 08:18:10 pm

I am rebuilding a Bassman for a friend. It is mostly the AB165 circuit without any "suppression" caps on the power tube plates or preamp plate resistors.
I guess, basically, the tone stacks are AB165, and both channels get the extra gain stage per 165, but I am using the NFB of the AA864.
That leads (finally) to my question. With an amp that is mostly AB165, NFB that is AA864, does it matter what plate resistors I use at the Phase Inverter.?
With what I have, should I use Two 100k, or One 100k and One 82k.?
Do you guys think it would matter.?
Thank You

Title: Re: BF Fender Bassman PI ?
Post by: HotBluePlates on July 10, 2011, 09:09:42 pm

I don't think it will make much difference either way. You'll probably get a bit more measurable output power with 82k, 100k, but you probably couldn't hear a difference.

Make sure you swap the OT plate wires or secondary wires if you change the AB165 feedback loop to look like the AA864. There is a different injection point at the phase inverter for the loop, and if you change the injection point, the wires at the OT need to be swapped to avoid positive feedback howl.