Bassman AA864 vs. AB165 gain stage question

[kingink]

Amp Wizards:

Hopefully, the following inquiry will not embarrass me too much.

I've been poring over chassis drawings and schematics of the AA864 and AB165 Bassman circuits, trying to identify the extra gain stage in the AB165. According to Andy Ruhl on the old Machine Gun Amps website, you can mod the AB165 halfway back to the AA864, so that you retain the extra gainstage of the AB165, thus getting more distortion and compression than a true AA864 circuit.

The problem is, the diagrams and schematics of the two different circuits look exactly the same to me. In both circuits, all the pinouts from the preamp tubes are wired to the board. I was under the (perhaps mistaken) impression that in the AA864, half of one of the 7025s/12AX7s would not be wired to anything, since only half of one of those babies is used in the AA864 circuit.

Can somebody tell me how to identify that extra gainstage in the AB165?

I realize that I am a young grasshopper, and have much to learn. Maybe I need to break down and buy a book.

Thank you,

David

[John Phillips]

There is no 'extra' gain stage - just a relocation of one.

The second tube is the relevant one. Only half of it is used. It's set up as a gain stage with heavy negative feedback.

In the AA864, it's part of the Bass channel only - the Normal channel has the same circuit as all the other Fender amps.

In the AB165, it applies to both channels, which makes the Normal channel somewhat different.

The big thing that is 'undesirable' about the AB165 is the biasing arrangement, which inherently causes a power stage mismatch - so this mod will almost certainly just involve modding the power stage back to AA864 specs and leaving the preamp alone.


Actually, having had both these models, I think the AA864 preamp arrangement sounds better anyway.

[kingink]

Thanks John. That clears things up.

So, can that unused half of the second 7025 be used to get more gain out of the bass channel? It seems like there are folks out there who turn the bass channel into a guitar channel by adding more crunch and removing/replacing a capacitor or two.

Thanks again,

David