Shielded wire for Lar-Mar PPIMV?

[shakti]

I'm slow with this stuff...after all these years, only now the time has come to try one of these master volume contraptions. I tend to like amps in their stock form, but I have one particular amp which started off as a stock 69ish Super Lead, but has slowly been modded and tweaked. I've done the West Coast Hendrix mods (higher plate voltage, 6550s, 12AT7 phase inverter and a few preamp tweaks). Lately I've added a switchable cascade mod on V1 so I can actually get a very usable high gain sound, but that makes a master volume only more desirable. Will probably also add a zero-loss FX loop, but I am going to try out the master volume first to see if I like it.

Which brings me to my question; I see that shielded wire is recommended. I have a four conductor shielded wire, which is actually neat since it allows me to make this mod visually very appealing. I am going to have the master volume on the front of the amp, and I can use this wire to carry both the wires TO the pot from the PI caps, and away FROM the pot to pin 5 of the output tubes, as well as add a shield. But these wires would all be running alongside each other in opposing directions, with a common shield. Is there any reason not to do it this way? Typically people add this master volume pot at the rear of the amp, so you have one coax cable running from the PI caps to the pot, and then another coax wire away from the pot, but this way the wires are well away from each other.

I don't suppose it would matter, but since these are the AC signal wires to the output grids I want to make sure I am not introducing some kind of intermodulation or making them susceptible to noise or oscillations.

Also, as I understand it the 2M2 "safety" resistors are there also to make the 250k pot close in specs to the stock 220k resistors? So if I go a little lower on those resistor values, I would get an effective value lower than 220k, right?

Finally, I have thought of a way to make the amp appear stock and keep the 220k resistors and all the turrets and connections intact for easy reversal.

[shakti]

Well, I can answer my own question now:

YES, it works! Absolutely dead silent without any screeching, oscillation or artifacts. I will try to make photos to illustrate, because this is actually very useful. Someone else has probably done it before, but I have never seen photos:

- I now have a single preamp volume control where the volume 2 pot used to be. This controls both channels simultaneously. It also allows me to use a pull switch to enter cascaded mode for lots of gain.

- The PPIMV sits where volume 1 used to be, so I have both volume controls easily accesible on the front, both labeled volume.

- the cable routing can be done under the board, and makes this mod minimally intrusive. It also allows for very secure wiring of the shield/bias feed. By some visual tricks, it also allows me to keep all the onboard connections stock, so the amp appears like it is in stock form (I know, silly, but when you do all NOS part builds this actually kind of matters...)

I realize I should be posting this in the PPIMV thread, so maybe a moderator can move my posts into that thread?

Finally, one last question: I cut the bias supply that runs diagonally from the bias caps and 15k resistor to the junction between the two 220k splitters. In other words, that 220k junction has no voltage sent from the bias circuit. I take the bias feed directly from the bias caps and use the wire shield to send it to the pot. However, can I then keep the two 220k resistors wired up on the board? I thought initally it would work, but then I started wondering if having those two kind of bridging the two output caps would mess with the setup, even if I have removed the bias voltage from their path. For initial testing i lifted on leg so they wouldn't be connected to each other. Hope that explanation made sense.

Will order a PEC pot now and also a zero loss FX loop.