AB165 phase inveter quest.

[trem]

In the schem below.......

Why are the plates of the PI connected, via 220k, to the plates of the power tunes.?

Thank You


https://el34world.com/charts/Schemat...b165_schem.pdf

[pdf64]

It creates a negative feedback loop around the output valves.
So it may be intended to linearise the response

[Helmholtz]

Yes, that's local voltage feedback from plate to grid (aka Schade NFB).
It lowers distortion and output impedance, extends the frequency range but also lowers power tube gain.

Also unusual, the global NFB is applied to the PI input, which requires the NFB signal to be out-of-phase.

[trem]

OK.....thank you.
Was this fairly unique or did Fender, and others, use this quite a bit.?

[Justin Thomas]

The AB165 Bassman is the ONLY Fender that ever had it. Having tried it with & without, I couldn't tell a difference, but I can say the wiring layout is very susceptible to oscillation if you move stuff around. As far as the rest of the preamp, I think it's magical & wonderful. I can answer questions when I get off work. I love these amps; I'm cloning one now.

Jusrin

[loudthud]

IIRC the speaker connections to the OT were reversed, the green wire was grounded and the black wire connected to the speakers.

[Helmholtz]

IIRC the speaker connections to the OT were reversed, the green wire was grounded and the black wire connected to the speakers.

Yes, something like that would seem necessary for this global NFB wiring.

[Justin Thomas]

IIRC the speaker connections to the OT were reversed, the green wire was grounded and the black wire connected to the speakers.

Yup. And it stayed that way for every (factory-issue) 2x6L6 Bassman head til 1981. A lot of people miss this when they're trying to mod or restore these amps.

Jusrin