Modifying a vintage Ampeg B25 to a vintage Ampeg V3

[fazeka]

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?

[sluckey]

Major differences between the amps... B25 uses NFB and has a fixed bias cathodyne PI. V3 has no NFB and uses a more modern bootstrap biased cathodyne PI. Everything else is just minor.

[vampwizzard]

Make the NFB switchable. You get lowered headroom aka easier distortion when its bypassed. The grid stoppers are there to prevent blocking distortion and on a bass amp they can be higher. The removed bypass cap was for more distortion in the preamp Id try it with it in there first.

The phase inverters on ampegs are different than the standards we see on most Fenders or Marshalls. My bet is they tuned the phase cancellation to the power amp on the V3 with its topology or improved technology. I imagine there is less distortion in the B25 topology.

neat project!

[PRR]

Somebody red-pencilled a "3M9" on the V3. This is just wrong, won't work.

Agree that 97% of the difference is negative feedback yes/no. The similar with-distortion VT-40 was also no-NFB and was a BRUTAL BEAST beyond mere Watts.

[sluckey]

I don't see enough differences between those two amps to call one a bass amp and the other a guitar amp. I suspect the magic difference is the speaker cabs.

[fazeka]

Somebody red-pencilled a "3M9" on the V3. This is just wrong, won't work.

Hi PRR,

I penciled that in on the V3 as a way to try to show the difference between that and the B25, which HAS the 3M9 resistor there.

FWIW, someone on another forum responded to this similar post of mine there saying "see Paul C mod".

[vampwizzard]

The b25 is designed to prevent distortion. The V3 uses some tertiary design changes to influence the amp towards a guitar friendly amp. IMO the coupling caps and the preamp can be modified to have a bigger impact.. think the two channels on a silverface bassman 100.

These are heads, so no dedicated speaker arrangement. But I’d imagine the speakers would contribute a lot to the guitar vs bass voicing debate.

[PRR]

I penciled that in on the V3....

But the 1Meg/1.5Meg under it is connected different, also the 1.5k Rk...

The self-biased affair "may" overload a little more politely because it will de-bias itself.

Yes a bass amp wants BASS speakers and I can't remember what was sold as head and what was combo. (VT-40 was a 4-10 combo and HEAVY.)

The no-NFB is the big difference. As a geetar it lets the cones cry soulfully but as a bass amp (with most speakers) it would just be flubby. Depending: the VT-40 no-Dist was open-back stiff-cone and the deep bass was not offensive, just shy.

[fazeka]

But the 1Meg/1.5Meg under it is connected different, also the 1.5k Rk...

You're right PRR, my apologies. Admittedly not the best way to show the differences between the two...

The no-NFB is the big difference. As a geetar it lets the cones cry soulfully but as a bass amp (with most speakers) it would just be flubby. Depending: the VT-40 no-Dist was open-back stiff-cone and the deep bass was not offensive, just shy.

OK! The amp has a new 3-prong cord so the polarity switch is just sitting there. Thinking I can repurpose that to make the NFB switchable? In which case with no NFB I was going to run the head with a 2x12 open-back cab.