![Image](http://marshall.redpt.com/amp_pics/RichMod.gif)
I know what it does, and how to do it, but there
Moderator: VelvetGeorge
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.What value post are the best: 500k or 250k ? Log or Lin ?
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.Resistors: Do you need to change resistors so that it is completely out of the circuit when on full, and if so, to what value ?
Ive never seen a picture but it certainly can be done.I can find pics of it installed in PTP amps, but none in PCB amp. Anybody have one ?
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.Shielded cable ?
Yeah. IME they dont sound very good in any amp accept the really bright gainy 69-70s amps.Anything else.... ?
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.Necrovore wrote:... 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.
I wonder why its not common practice to use shielded cable in all amps from the PI output to the grids?novosibir wrote:Necrovore wrote:... 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
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.Necrovore wrote: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.
In all of my Marshall rebuilds - YES!Necrovore wrote:Thanks for the tip Larry. (...) Every one of the pictures I have seen of all of your amps the heater wires are for the most part tightly twisted.
NO! It's all the same.MacGaden wrote:How does it work in Fenders ? Any changes needed ?
Would I be right in assuming that you'd need to change the splitters to 270k if you used a dual 1meg pot?Billy Batz wrote: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.
You're wrong!Eoin wrote: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?