Ampeg V4: valve sections & their purposes

[jacobarber]

Referencing the schematic here.

I finished fully rebuilding my 1975 Ampeg V4 (non-distortion, non-MV). The short story is that when I bought this in 2008, I knew it'd been in a fire at some point in its life but a tech had kept it alive and working. I recently decided that it needed a new lease on life. When I cracked it open, the main and mids/reverb board were BLACK with soot, wavy as a rollercoaster, and home to more than a few unhappy lifting pads.

The work included:

• Replacing both main and mid/reverb boards with thicker boards that have fully plated thru-holes.
• Replacing the power supply with SNK Amps preamp & poweramp supply boards and discrete components.
• Replacing every socket except the compactron 6K11 socket with Belton micalex.
• Replacing every single capacitor, resistor, and diode to match the schematic.
• Cleaning every jack, pot, and switch as thoroughly as possible.

It works swimmingly. After months of staring at the schematic, I understand how just about everything works except for the 6K11. Here's the summary of what I think makes sense:

• V1 (Input 1) and V2 (Input 2) are both 12AX7s, and work as typical preamp tubes. Half is before the volume control, half after.
• V202 is the 6CG7 which drives reverb.
• V203 is just 1 section of a 12AX7 and behaves as a reverb recovery.
• V3 is a 12DW7, with the 1st section acting as an AC-coupled cathode follower (I think) and the 2nd section just boosting the line out signal for the PI or the external amp jacks.
• V4 is a typical AC-coupled long tailed pair PI.
• V5-8 are the 7027A / 6L6GC power tubes.

V201 is a 6K11, and I am out of my league in understanding it. I think the first section in the signal is a tonestack recovery. I can't tell what the middle section is doing except for boosting the gain for the 3rd section which may be what's driving the midrange signal coming from the inductor & pot? Very confusing. I think section 3 is an AC-coupled cathode follower, but would that be the case if the next section in the signal is also an AC-coupled cathode follower?

If you have any insight I'd love to hear. I really enjoyed learning every nook and cranny of this amp and would like to know why it actually works the way it does. Thing is a beast, BTW. Having what I can only describe as a brand new V4 is something I never thought I'd experience.

[sluckey]

Sounds like you have it mostly figured out.

V201-A is a bootstrapped biased gain stage and it's plate is AC coupled to the grid of V201-B which is a standard gain stage whose plate is DC coupled to the grid of V201-C, which is a cathode follower used to drive the mid-range tone circuit. There is also a NFB loop from the cathode of V201-C back to the cathode of V201-A.

Here's a redrawn schematic that may make it easier to understand...

[jacobarber]

Its all voodoo. I'm not sure if Ampeg was the first to come up with the idea of this circuit, but its wild to me that they used an entire 3-section tube to do it. The real confusion is why they didn't use a 12DW7 + the 2nd section of the reverb recovery 12AX7 to do it instead. I actually have a retrofit board for this, but the direct replacement one is more robust.

I had a lot of fun with this project. Need to figure out my next project, cause this nonsense is addicting.

[sluckey]

I wouldn't spend any energy trying to guess why the Ampeg engineers designed the way they did. The design was solid at the time that amp hit the market. Compactrons were a relatively new idea and they were everywhere in consumer electronics. Who could have guessed they would be such a short-lived fad. I was never a fan of compactrons. Thankfully silicon took over!