Modifying a vintage Ampeg B25 to a vintage Ampeg V3

[seveneves]

Hi all,

I recently learned that the early 1970s Ampeg V3 and the B25 were very similar amps. The story is that the V3 was modified/voiced for guitar (the B25 was a bass amp).

I looked at the schematics to see the differences and I'd like to learn more about them, specifically the output/phase inverter area as it seems that it has the most significant changes between the two.

* cathode and plate on the V3: 14k + 1k and 15k, respectively (cathode and plate on the B25: 47k)
* control grid resistors on the V3: 47k (on the B25: 100k)
* cathode bypass cap on 12AX7 channel 1 doesn't exist on the V3
* no NFB on the V3
* 3M9 missing from PI in V3





There are other differences in the power supply but I don't suspect that they contribute significantly in the difference in tone between the two.

I was wondering if I were to convert my B25 head to a V3 if it would then "lose the magic" when used with bass guitar? It is a nice round tone when playing bass through it.

Thoughts?

[didit]

Hello --

Unsure why it's so quiet around here today? Not even a mouse.

You've answered most already, at least semi-implicitly. Suggestion would be to make the few feasibly switchable tweaks and listen. Disconnecting power stage negative feedback and add in bypass cap on V4a, and perhaps also adding the larger initial filter capacitance will get you quite close. These are all changes that will in some degree "pop" so won't be silent to toggle. Most of the rest maybe even nearly indiscernible except when operating it flat out.

Best ..

[Stevem]

Not sure if you have any interest in seeing the schematic for the PI, output and power supply of my Oliver B200.
Jessy Oliver was the man at Ampeg who had a hand in designing the B25.