why is BF Bandmaster cleaner than BF Bassman?

[Groover]

It seems to be generally acknowledged that BF Bassman is a good amp for guitar and not great for bass because it is not very clean, and it seems to be similarly acknowledged that BF Bandmaster has a lot of clean headroom.

Why does the Bassman break up more? It has a similar output section and B+ voltage, and a significantly larger output transformer. The AA864, AA165, and AB165 all have an additional preamp stage, but only in the AB165 does the Normal channel pass through this 3rd stage.

So let me refine my question: Why does the normal channel of a AA864 or AA165 BF Bassman distort more than a BF Bandmaster, or does it?

[jmaf]

It seems to be generally acknowledged that BF Bassman is a good amp for guitar and not great for bass because it is not very clean, and it seems to be similarly acknowledged that BF Bandmaster has a lot of clean headroom.

Why does the Bassman break up more? It has a similar output section and B+ voltage, and a significantly larger output transformer. The AA864, AA165, and AB165 all have an additional preamp stage, but only in the AB165 does the Normal channel pass through this 3rd stage.

So let me refine my question: Why does the normal channel of a AA864 or AA165 BF Bassman distort more than a BF Bandmaster, or does it?

Wild guess: Perhaps the vibrato circuit robs some of the gain. It's a tens of K OHM pot to ground that no Bassman has before the inverter.

[Sweetfinger]

I would never characterize a Bandmaster as having a LOT of clean headroom(at least, when compared to the Bassman). The two amps are about the same in my book. The vibrato circuit on the Bandmaster does rob some gain on the vibrato channel, but for the most part, they're very similar, I've used them interchangeably in the past. I usually used the normal channel on the Bandmaster.

[Groover]

Maybe I have misinterpreted comments about the Bandmaster.

OK - here's another question - is the PI and output section of the Bassman capable of more clean power than that of the Bandmaster? (considering it's larger output transformer)

[Groover]

bump

[pdf64]

Checking the schematics, the Bandmaster has a B+ of 440V, whereas the Bassman aa165 is 425V. From the perpective of the normal channel, everything else is the same, so therefore the Bandmaster should have a little more power output (=clean headroom) on tap.
The larger output transformer on the Bassman will be needed avoid magnetic saturation at the lower frequencies of the intended application. It shouldn't make any difference when used as a guitar amp.

[Groover]

So putting a Bassman OT in a Bandmaster wouldn't increase the low end power or headroom?

[pdf64]

Which OT is in the Bandmaster - the small vibrolux type with 2 mounting screws, or the larger Super Reverb type with 4 mounting screws?
If it's the 2 screw type, they are a bit undersized, you might notice an improvement if you upgrade it to the larger type (I have done on 1 of my projects).

[Groover]

It is the 2 screw type

[pdf64]

Go for it then, in my experience the bigger OT was a clear improvement - even the drummer noticed and asked what I'd done to the amp!