Bassman WTF?

[drewl]

I have a blackface AA165 bassman, looks stock with a couple odd variances from the schematic like a bias balance circuit, but the main thing is it has a 220k ohm resistor on each of the output tube plates connected to the outputs (pin 1&6) of the phase inverter, and each PI plate has a 100k voltage feed.

The chassis is dated from '65, and the parts and wiring all look original.
Anybody seen this oddball before?

It sounds awesome, breaks up nicely.

[Enzo]

Like this?

http://www.webphix.com/schematic%20h...b165_schem.pdf

[drewl]

yup...is that from an old version of Schematic heaven?

I looked through all the Bassman schems and didn't see this version!

I think I might have to try this on one of my Fenders.

Edit: Oh geez....it's right there....my eyes must be going.

Anyway, so what do you think are the advantages/disadvantages to this circuit?

[Gtr_tech]

Its just local feedback to improve linearity. You'll see the same thing on the mixer stage. I don't care for it, so I will always remove it unless the owner wants it left in. Sounds icky to me. But as always the case when removing designed in feedback loops you need to make damn sure there's no oscillation after the changes. If there is, its usually a lead dress issue that needs to be tended to.