PP/SE control for Liverpool

[bcmatt]

Hello Sirs,
I gotta admit, I got bored today, and started getting super curious about the SE/PP control. I know Dartanion makes use of some form of this on a regular basis... making think it's something I gotta taste for myself. I did see it on the Vajra 18 watt too, and used that as my guide. Of course, I have a few questions regarding implementation.

I haven't opened up my amp yet, but played around with measuring some pot/resister values. Managed to get a range from 0-224K using a switching 1M pot (measured 921K) and a couple resisters in series (measured total of 296K)

The reason I first tried with a switching pot leads me into my first question:

1) Is it a good idea to do some sort of bias boosting switch when going toward SE on a Liverpool? And if so what would be a good suggestion for that?

2) Is this pot sturdy enough for this application? I don't know the wattage rating, even though I found the same Alpha pot at AES.
http://www.tubesandmore.com/products/R-VPP-1MA
I don't yet know the voltage at this part of the amp... around 300V? I suppose the fact that it will be in parallel with a 1W resister indicates that a 1/2 watt pot should be fine... but still don't know what this is...

3)Is the practice to twist wires coming from both sides of the resister on the board to the front or back panel? Are we pretty safe from added noise problems at this point?

Any other thoughts or advice from anyone who has tried this? Is it worth it? How did you do yours?
I'm thinking if I really like it, I will replace my presence pot with a 25K resister since it is ALWAYS all the way down anyways, and then pop this thing in it's panel spot... rather ironic that I just wood-burned my faceplate last week to say "presence" 5 years after building my Liverpool, and now I want to replace that control with something else.

[bcmatt]

I found what I think would be the data sheet for this pot...
http://www.taiwanalpha.com.tw/english2014/p_e_046.htm
Since it looks like mine is an audio taper... it could be rated at just 0.1watts and 150V. Too small for this application?
I also have one of these rated at 1/3 watts it looks like with an SPST switch. Perhaps a bias switch is not necessary. But maybe if I replace my presence knob, a couple presence choices might be handy with the spst.

[dartanion]

There is no reason to use a switched pot for this application, and I would wager that it would pop loudly if you try it. Essentially the se/pp control acts like half a ppimv, you are sending signal to ground from one side of the PA. With your setup, when the pot is fully on pp, your resistance is 230k, thus very close to the 220k grid leak. The Vajra values work out to be equivalent in pp mode, so I recommend starting with those values. Grid voltage is small, so don't worry about the pot.

Hope you enjoy trying this out.

[bcmatt]

Cool! Thanks!
So I just finished installing and trying it out. It is kind of cool.
I think my range of total resistance in the end gave me 0-224k. I used the CTS pot with the SPST switch on it. It barely fit in front of the board; I had to turn it a bit so it wasn't contacting an eyelet!
Oh, I guess I should mention that I fully removed the presence pot and this went in it's place. So, the presence circuit is now wired up with a 22k resister instead of the 25k pot, with a 8k2 resister across the SPST switch.
So, now my normal setting is 30k2 and a very subtle presence boost to 22k when I pull the knob out. But both settings are very close (on either side of) my previous presence knob which was turned all the way down all the time.
It's subtle enough to be barely worth using the switching pot. But I'll play it like this for a while and revisit it at a later date if I have an inkling of wanting an option of more or less presence.
Here's how ugly it looks! A little crowded there next to the VVR knob!
I updated the schematic to better understand what I did.