Cable for PPIMV hookup

[pjd3]

Hi, say what’s ya think.

I can get a few feet of Belden 24 AWG 2 conductor twisted pair with a foil/wire shield.

Rated for 600 volt peak.

Sound like a contender for PPIMV in a 6V6 plexi? My gut tells me it will work fine but, having not included shielded cable in an amp build yet I’d thought I’d ask. It’s an 8641 model cable from Belfon I believe.

Thanks!
Pjd3

[Stevem]

That should work fine!

I tipically us Beld 9451.

Just tie both sheild wires together and land them on the case of the pot, at the other end apply shrink tubing so any hanging sheilding does not short anything.

[pjd3]

Thanks Stevem.

Say, I watched Uncle Doug and he ties the shields to the negative bias supply claiming it created a common mode thing that is better than just grounding the shields.
Any idea how that plays out electrically? I couldn’t argue with that method but then again, I wasn’t able to fully wrap my head around it either.
I was going to do it that way since it isn’t difficult but, not totally understanding it.

Thanks,
Pjd3

[pompeiisneaks]

Thanks Stevem.

Say, I watched Uncle Doug and he ties the shields to the negative bias supply claiming it created a common mode thing that is better than just grounding the shields.
Any idea how that plays out electrically? I couldn’t argue with that method but then again, I wasn’t able to fully wrap my head around it either.
I was going to do it that way since it isn’t difficult but, not totally understanding it.

Thanks,
Pjd3

Just off the top of my head, the PPIMV connects into the same location as the bias supply, therefore using the bias ground means you're keeping it 'together' and it reduces the chance of injecting hum from ground loops. The case of the pot is also very physically close to the same grounding area 'usually' in an amp, as the presence is typically very close to the location of the bias supply to keep those wire runs as short as possible.

I don't like soldering to pots, myself as I've had bad luck with it, so I run a ground buss wire across the back of the pots area and use a jumper wire between the pots and the ground buss I connect at the input area to the chassis. I've done multiple presence pots on that same one w/o any issues myself.

~Phil

[sluckey]

Using the shield to carry the bias voltage to the pots makes sense. That's how I would do it. However, if you do this, the shield must be connected to the board and it must be connected to one leg of both pots. DO NOT CONNECT THE SHIELD TO THE POT CASE! There are plenty of diagrams on the net that show you exactly how to do this. Just google "PPIMV".

[pjd3]

Thanks guys, and also for the warning.

I do understand how the shield needs to be where it has to go to work properly and to not cause some terrible electrical damage and mayhem, which, I'm certain it would do if the negative bias voltage on the shield were to touch anything else but itself. Prudent warning it is.

I really wanted to spring for that PEC dual pot ($32-$37) but, its going to be hard enough winning the "good tubes" battle with the wife come time to buy good tubes. Will just have to live with an Alpha or something like that for now. Those high value resistors will be there in case of any trouble.

OK well gentlemen, you have successfully instilled enough confidence in me to move on to those steps. It will be interesting to see what comes out of having both a pre PI MV and a post PI MV, maybe some subtle variations in the electronic distortion texture which I always welcome.

Thanks so much !!
Best,
PJD3

[M Fowler]

Find dual 250KL pots that I like isn't easy.

Many use dual 250kA pots for the PPIMV.

I use dual lead shielded cable less cables flying around.

I resently found some great dual 250kL pots on ebay and bought a few. But they are large you need to plan your room but put on the 2m2 safety resistors is easy

Mark

56fa head 2.jpg

[pjd3]

Thanks for that M Fowler,

I did manage to come up with a dual conductor Belden shielded cable for this. It has a foil shield with a stranded wire that rides along with it. So, hopefully there is the good shielding properties of foil, along with the solderable wire thats attached to it.

I have failed to find any dual ganged pot that live between the apparent or imagined quality of a 4 dollar Alpha and a 32 dollar PEC. Of course, I want the PEC, but that will be more difficult to justify in terms of our house bills and over head! Hey, but I can try! My bands do bring in about 1/6th of our annual earnings so, its a good investment, right? ha. Right. I don't want to mess with my signal path. I've ruminated over every step as it is, even bought one of those snake oil Sozo 470pF super silver mica caps for 9 bucks! I love the fricken thing! I show pics of it to all the Engineers where I work. They laugh and bust my chops but, its all in good fun.

I really have to wonder if there are any tone benefits to the PEC pots. The folks over at Metro amp sure do. That makes me nervous.

Well, last shippment from Mojotone is in tomorrow with most of the last few components this 6V6 plexi will need. I'll figure it out. always do!

Thanks again,
PJD#

[pjd3]

Say, would you happen to recall who the source was of those pots you found on Ebay?

I did take a look on ebay but, seemed to only land on adds with no-name's and scant descriptions or brands.

Thanks
PJD3

[pjd3]

My other rumination was about the pot value. I see that 250K/220K is used across the board whether its 6L6's or 6V6's but was recently informed that 6V6's called out for 100K grid leak as the resistance on the data sheet or some design circuit examples.

I'm going to do see about that myself, see if we're barking up the right tree.

PJD#

[sluckey]

Keep it simple. The dual MV will be replacing existing grid resistors. If the amp has 100K grid resistors, replace with a 100K dual pot. If the amp has 220K grid resistors, replace with a 250K dual pot. If the amp has 470K grid resistors, replace with a 470K dual pot.

Read the data sheets on sleepless late nights.

[pjd3]

Yup, keep it simple I shall.

I'm feeling pretty cozy about 100K pots for the PPIMV.

Thanks!
PJD3

[sluckey]

I'm feeling pretty cozy about 100K pots for the PPIMV.

Does that mean you will be replacing 100K grid resistors?

[pjd3]

As I see, the pot takes the place of the two 100K grid leak/bias resistors. I'm not sure what the name of those resistors are now that they are also associated with bias but yes, it seems as though the dual ganged pot also becomes the two 100K "grid" resistors.

Say, I've seen it called out for both linear and audio in terms of taper. Often, when its a volume control it calls for a log or volume taper.
Do you recommend an audio taper for the dual pot?

Thanks Sluckey,
Phil

[pjd3]

By the way Slucky, just to remind, this is your schematic that I"m doing this build on so yup it is 100K on those gridbias resistors