Power supply design questions for Ampeg V4-inspired project

[jacobarber]

I've been toying with designs for an Ampeg V4 inspired project. I posted about it a long while ago and only recently dove back in.

Let's assume that I've got the preamp & power amp sections mostly settled.

• The preamp section uses 7 triodes (mix and match of 12AX7s and 12AU7s, with 1 unused triode. Sad, I know).
• The power amp section (including Phase Inverter) is 1x12AX7 + 2x6V6S.

I grabbed a copy of Designing Power Supplies for Tube Amplifiers and am looking to understand what Ampeg was doing here. I built the V4 power supply in KiCAD and attached the PDF, along with the various output nodes and their target voltages as described in the original schematic. One thing to note is that in the PDF I have removed the standby switching altogether, but left the 180k pilot light resistor in because I forgot it. Also, for the project I'm working on I tentatively chose the Hammond 270FX, so please don't take that as what I feel the original Ampeg V4 power transformer might be.

In the book and elsewhere, typical power supplies are described as having a reservoir capacitor, followed by a network of smoothing & filtering stages. The V4 supply seems to do it a little different:

Instead of starting with a single reservoir capacitor right after the rectifier, it throws the voltage into a network of paralleled resistors and caps (R1, R2, C1-C4) before moving towards the filtering / smoothing stages.

• Does that parallel network act as a single reservoir? I know the voltages in this amp can be just absurd, so I'm wondering if this was their way of building up the power needed, as maybe a single capacitor wasn't reasonable?
• If, just hypothetically, one wanted to use a variation of this power supply but remove the entire reverb circuit from the amp, what is best done with R5 and C7? I can't imagine removing them entirely is the right approach, as it would impact nodes D and E. Thoughts?
• If anyone has deeper insight around the choices Ampeg made I would love to hear them.

[AlNewman]

Could you share the original schematic you used to make your drawing?

[jacobarber]

Could you share the original schematic you used to make your drawing?

https://el34world.com/charts/Schematics/files/Ampeg/Ampeg_v4_vt22_revg.pdf

I took some inspiration from a few versions, but this is by far the closest of the originals: Revision G of the V4 & VT-22. That would likely be the last before the Master Volume version and the Distortion version were released. Earlier versions had a few 70uf caps instead of the 100uf, and bias was set to -52v instead of -62v.

I updated the post with a revised copy of the schematic, as I realized that I had 70uf caps instead of the 100uf that I'd mentioned. I also disconnected the positive side of C3, as that's how Ampeg did it. I don't fully understand why, so if there's any insight I'll gladly listen.

[tubeswell]

That Ampeg V4 uses 4 x 7027A output tubes. JJ 6V6S are great tubes, but I highly doubt they'll last long at over 500V


The stacked filter caps in the power supply is a fairly standard arrangement for increasing the voltage handling of the filter cap section. And if you're going to run JJ6V6S, you probably won't need that. Just redesign the power supply for say 450V B+ and use 500V rated caps in a more standard configuration.


The real magic of Ampeg bass amps is the toroidal inductor in the tone control section. You will want to score one of those for your project methinks.

[jacobarber]

That Ampeg V4 uses 4 x 7027A output tubes. JJ 6V6S are great tubes, but I highly doubt they'll last long at over 500V


The stacked filter caps in the power supply is a fairly standard arrangement for increasing the voltage handling of the filter cap section. And if you're going to run JJ6V6S, you probably won't need that. Just redesign the power supply for say 450V B+ and use 500V rated caps in a more standard configuration.


The real magic of Ampeg bass amps is the toroidal inductor in the tone control section. You will want to score one of those for your project methinks.

I was looking for some thoughts on simplifying the power supply, and this gives me some good ideas on how to accomplish that.

Regarding the inductor, I have been toying with the idea of winding my own. Someone documented their process of doing it, and the parts are quite cheap. 1 ferrite toroid core of the correct size and a good length of magnet wire. There are some great inductor calculators out there to help determine # of windings to hit target inductance. It'd be a cheaper alternative than the $100+ choice from Mercury.

[AlNewman]

Keep in mind the front end of the amp uses nearly as much current as 2 6V6's would use, and also the 6V6's would need less signal voltage to overdrive.  You'll have to do a fair bit of math and figuring prior to building, or even picking a transformer.  Looks like a pretty daunting project, hope it works well for you.

[jacobarber]

Keep in mind the front end of the amp uses nearly as much current as 2 6V6's would use, and also the 6V6's would need less signal voltage to overdrive.  You'll have to do a fair bit of math and figuring prior to building, or even picking a transformer.  Looks like a pretty daunting project, hope it works well for you.

I'm diving in head first though, so I know that this is a very ambitious project.

My post a long while ago describes my hopeful changes. I'm reducing the total number of triodes from Ampeg's original 12 in the preamp down to 7. Adding that to the change from 4x7027s to 2x6V6S, I definitely need to fully redesign the power supply. That's the whole point of this exercise.

I've got Merlin's book on my desk and LTSpice open, so I'm in the thick of it now.