Original Circuit topology, advice? Power supply and biasing question...

[eschertron]

I notice the output stage feedback is now connected to V2A grid. Is that correct?

I saw that too. I haven't scrounged for an example of that in prior art. Is that a Dumble thing?

There might need to be a mixing resistor off the plate of the recovery stage to control the ratio of signal-to-feedback. Just thinking out loud.

[frus]

I've seen that in silverface bassmans
I would still rather do it the 'usual' way, like the first posted schematic (although with presence pot, does wonders sometimes)
Also, it's a good idea to put any kind of resistance between the coupling cap of the previous stage to the grid of the next one, else you get bias shift and grid block. I think even 10k will do, in front of the PI input

[eschertron]

I've seen that in silverface bassmans
I would still rather do it the 'usual' way, like the first posted schematic (although with presence pot, does wonders sometimes)
Also, it's a good idea to put any kind of resistance between the coupling cap of the previous stage to the grid of the next one, else you get bias shift and grid block. I think even 10k will do, in front of the PI input

Yes. I see now the AB165 and later Bassmans have this topology. I'm not sure if there's a good engineering reason for the change, but I see it accompanies a change to the way Fender mixed the two input channels together. So I guess they did it 'because they could', but I don't see a savings in component count?

[Justin Thomas]

They also switched the OT secondaey wires at the speaker jack, green to ground... I've know several who mod AB165 & later to AA864 specs & miss this extra step then wonder why they get a thuddy squealy mess...

The AA165 & later Bassmans were Fender's experimental biotches until lthey finall settled down around 1970 or so. But personally, I prefer the AB165 & later amps for the ability to jump the channels & the super-phat tone on tap. Mine is largely stock & I love it.

Justin

[Chuck H]

They also switched the OT secondaey wires at the speaker jack, green to ground... I've know several who mod AB165 & later to AA864 specs & miss this extra step then wonder why they get a thuddy squealy mess...

The AA165 & later Bassmans were Fender's experimental biotches until lthey finall settled down around 1970 or so. But personally, I prefer the AB165 & later amps for the ability to jump the channels & the super-phat tone on tap. Mine is largely stock & I love it.

Justin

I know better than to think you haven't played through a few of these amps. And I've never played a Bassman head I didn't think was a good amp, but, the SF era amps seem widely variable to me. Some are good and some just aren't within the same model. Surely a lead dress thing. Get it wrong in the right way and you get magic. Otherwise you get harsh, borderline instability. Maybe your Bassman just happens to be a great one

Probably the most impressive Bassman head I played was a BF. Those amps had NOTHING going on. Just a BF amp head with no bells and whistles. But this particular head had "it". It sounded deep, sweet and clear in that way that implies a wet tone even though there's no reverb. Sort of the way your ears feel when they fill with water.?. And the attack had that perfect envelope to compliment a near perfect bell like chime. Bar none the very best clean tone I've heard. Unfortunately I wasn't in a position to crank it into hard clipping. Maybe just as well since no amp is a do all and it might have taken the shine off the experience.

[Justin Thomas]

The thing about BF Bassmans is, they can be any one if three circuits... And all pretty different. Once they hit the AA270 they stayed a lot more "consistent" as far as circuit topology.

As to lead dress in SF amps, I'll just say that one time I replaced a pot in my BF 67 AB165. I didn't put the wire back "just so," and yup, insta-nrittle & squealy. I put it back the way it was, all fixed. But just shows, not unique to the SF.

Mine also has a TR OT in it & has tge feedback on the 6L6s removed; other than that it's stock as far as audio cirvuit; I added a bias adjust to complement the bias balance; we don't need no stinking matched tubes!

Justin