The PPIV Master/Rich mod is mentioned again and again on the board:



I know what it does, and how to do it, but there

They are 500k. You could use 1M as well. The only thing I could imagine using 250k for is if you dont have any fixed resistors and the bias feed is completely reliant on the pot tracks. Ive heard techies say thats playing with fire more often then not but also Ive heard others say your as likely to get hit by lightning while playing, thereby frying the tubes in your amp, as fry a tube because of a wiper making bad contact. If you leave the fixed 220k's for safety then you want a higher pot value. 220k and 250k paralleled make the value about 115k when its max'd. I dont know what the tonal ramifications are but its certainly far from stock value.




If you leave the bias feed resistors in there as fixed resistors for safety, which the Rich does, then you should change them to 400k. 470k is more widely available and will work but 400k will get you closer if it makes a difference. 500k+400k in parallel is like 222k. 500k and 470k is 240k thereabouts.



Ive never seen a picture but it certainly can be done.



If the amp doesnt need it at full volume it shouldnt need it with the master. Unless your refering to the 5 lengths of wire that go from the PI output to the pot and back? You probably should use some multi conductor shielded.



Yeah. IME they dont sound very good in any amp accept the really bright gainy 69-70s amps.

Make sure that the pot you decide on using is a dual ganged pot. I'm sure that you knew this I just want to clarify it in case others who don't fully understand what to pick up for this mod should get.

Installing the PPIV master in a pcb amp is somewhat a pain in the butt. When I installed mine, I had to take the two wires that go to the bias feed resistors and install ine on the top of the pcb and the other on the bottom. But this was on a pcb in a late era JCM800 where the pcb is attached to the pots which is what holds the pcb off the chassis.

Dan is right regarding how it sounds. Yes it does work to cut the volume, but I found that there was a very slight sizzle to anything I played. It actually sounded like how a snare will ring when in the same room as a guitar amp when not being played. Take that sound and introduce it to your signal just to the point where it is barely noticable and this is what it sounded like on MY amp. eventually it got on my nerves and I removed the Master.

Think about, that the tamed signal levels (from the PPIMV's wipers to the output tubes) become more and more sensitive, the more the pot is cranked down.

Therefore these wires more and more easily can catch other (stronger) electrical fields, what more and more can lead to a RF oscillation - and not seldom aready oscillation is there, although you still can't "hear" it.

But if YOU also are sensitive, then you'd feel it by playing. And sometimes, when the oscillation already is more than only "slightly", then you'd hear this very slight sizzle, riding on top of your notes.

To avoid this --> twist, twist and twist!!!

Twist all "push-pull-cables" accurately together. That means, twist the both cables from the coupling caps to the pot's inputs together. But still more important: Twist the both cables from the pot's wipers to the output tubes together...

... or use shielded cables there. You don't need to set the shield to ground, you also can use the negative bias feed voltage (which already is on the pot's output lugs) as the shielding potential. It's shielding as good as ground.

Larry

I wonder why its not common practice to use shielded cable in all amps from the PI output to the grids?

Thanks for the tip Larry. I will use this if I ever decide to reinstall this mod. Matter of fact I really need to go back in and rewire all of my heater wires as they look like shite from the factory. Im talking maybe three twists between tubes. Every one of the pictures I have seen of all of your amps the heater wires are for the most part tightly twisted.

There has to be a limit to how tight to twist them because at some opint youd only be adding more wire to get from point A to point B which means it would be more area to pick up signals. So it seems anyway.

In all of my Marshall rebuilds - YES!

In my DINO and British Purist amps - NO!

In my own amps I've decided for regulated DC to the heaters of all preamp tubes. They're dead quite, of course

Larry

Thanks all.

I edited my post, added a schematic and some pics.
I just needed some clarification, and I got it.

How does it work in Fenders ? Any changes needed ?

NO! It's all the same.

Larry

Would I be right in assuming that you'd need to change the splitters to 270k if you used a dual 1meg pot?

Yeah. Thats would be about 212k but thats close enough. Sounds like you have the idea. But theres also another thing that happens as you turn down other then less bias feed resistance. You also increase resistance between the output couplers and the grids. Thats kind of like adding to that 1k5 or 5k6 value by a lot. With 500k, 500k will be the max amount but with 1M it doubles it. I cant do anything other then speculate what that difference wound be.

The additional resistance between the output couplers and the grids is added as the PPIMV is turned down, right?

Am I wrong in thinking that the PPIMV works by basically summing the two sides of the PI? How would it affect the bias if the PPIMV was added in parallel instead of in series?

You're wrong!

The PPIMV usually is added in parallel to the stock bias feed resistors.

Larry