There are a few other components you need to make it work. You can't just throw some diodes on the pot and call it a day. There are a few chapters about clamping circuits in The Ultimate Tone books. But long story short:
1. Pre-tone stack master volume
2. 100K cathode resistor on CF stage
3. 10K from cathode, followed by decently sized DC blocking cap to MV (0.22uF - 0.68uF), to tone stack
4. Diodes or transistors of your choice from MV input to ground
I'd get rid of the PPIMV.
The lower the voltage rating of the zeners you use, the more clamping they exert on the signal. Diode clipping does not add gain. It clamps (clips) the top and bottom of the waveform off. This creates distortion, not gain. 18v zeners clamp at +18v and -18v. 6v zeners clamp the signal at +6v and -6v. That's why you get lower volume. You're literally cutting the amplitude of the wave off at whatever voltage threshold you choose. If you use asymmetrical zeners, then that clips the top or bottom off at different points for a different sound.
I think the sweet spot is 18v zeners. For more aggression and a more modern sound, you can try MPSA06 or 2N5401 transistors wired as zeners.
Use zeners, not regular diodes. And they need to be wired anode to anode or cathode to cathode.
Marshall currently charges $3,850 for this mod and many genius amp gurus have tried keeping it a secret for 40 years. You're welcome.
Thanks for all this info! I agree on the last point and since this is a clone, I’m fine experimenting with it, drilling holes, etc.
I tried the following, so let me know what you think/else you suggest. Just as a heads up I’m not good with a soldering gun, so my plan is to clip stuff in, decide what I like and then take it to a tech to solder it and perhaps add switches.
1. I have the diode circuit as two 15v zener diodes in series.
2. One end of the diodes goes to a 10k resistor and that to a 0.1uf cap (I also have a 0.68uf handy).
3. The cap is connected to the 100k CF on my turret board.
4. The end of the other diode goes to ground.
I think this is more or less what you suggested and it does sound pretty awesome. I took out the 10k resistor from the diode circuit and that sounds pretty cool too, though it makes the amp a bit more low mid heavy.
With a boost in front the amp sounds insane, so I tried a fewer more things to get the diodes to clip more at the volumes I’m testing:
1. Clipped a 10k in parallel to the cold clipper to get down to 5k.
2. Clipped a 100k in series to the 100k plate resistor.
Unfortunately, I can’t go below 5k on the cold clipper because my inputs aren’t shielded and the one 68k input resistor(?) is on the board instead of on the tube socket (I should probably get this all fixed).
For that same reason, I can’t bypass the cold clipper with a 0.68uf cap before getting a ton of noise. I read I can try putting the cap in pin 8 and then to ground, so I might try that (after discharging the whole amp).
maybe before that I’ll try your step #4.
I think step #1 is beyond my pay grade
