LarMar PPIMV - different type of question

[DG45]

Hey folks.
I have a couple of Allen Bradley 250K dual pots. I'm considering using for a LarMar on a 45 type build. Problem is that the sections aren't balanced. I think the closest of the two measures 267K and 297K. Dumb question maybe but can I just use a different value resistor on each section to achieve the 220K parallel resistance? Or, do the pot sections need to be balanced?

Thanks

[Doug H]

I think this is an interesting question. On one hand, no nothing to really worry about, but...

The pot factors into the load on each side of the PI. Since the slightly mismatched PI is part of the Marshall sound, what kind of tolerances should be observed here to those that care to preserve that special mojo?

The pot you referenced has, what, a ten percent difference? Since that is in parallel with the much smaller plate resisters, it will impact the load even less than that, so we are talking about splitting hairs. The question would be, is it just enough of a hair to unfortunately balance the PI?

Maybe that would be a good thing?

[Haze13]

Actually the PI mismatched, but so tiny-bit. The differences in plate resistors there to compensate the difference in the gain of the 2 triodes. In other words the resistors should be mismatched to make the gain of the PI to be equal!

If you will solder the resistor in parallel than what you will have is the same value on the ends of the pot track but not in between.
Marshall sound is an asymmetrical clipping... If your signal will be a little more asymmetrical (not the same attenuation of the signal before the power tube), would you hear the difference? It's not that you care of THD like in Hi-Fi amp.

You can always put back what ever you've done, so why not to try?

[DG45]

Thanks for the replies. I think I understand the issue here. If we use a dual pot that is exactly 250k and balanced between sections, and we solder a 2.2meg resistor between the outside lug (ground) and the center wiper, then we have compensated for the removal of the 220k bias resistors from the board (very close, anyway). So I can still achieve that with the high reading and mismatched sections of my Allen Bradley pot by installing a different, appropriate value resistor on each section. But, the issue still will remain that the pot sections will still be mismatched throughout the pots range.

Bottom line is that It may or may not matter tonewise (it may further balance the pi or it may further unbalance the pi) but whatever the case, it won't be the same, electrically anyway, as using a pot with balanced sections. But it won't hurt anything to try it.

Do I have that about right?

For background ... There is an Alpha pot in there now, installed at time of build. I culled through a few alphas and found one that measured close ... 238k and 237k, and thats whats in the amp. Pretty darn good for an alpha, especially since I only had 3 or 4 to pick from. Its got 2.2meg resistors at the pot which is still pretty close to the 220k. Maybe I'd just be better off leaving it. I read some things about the better pots, especially pec, being an improvement but i was too skeptical to justify the 30 bucks. I picked the Allen Bradleys up at a surplus supply, for cheap. I expected closer to spec but maybe these were culled because they were a bit out. Which may explain why they were at the surplus supply.

Thanks.

[Haze13]

If there was only this pot and couple of more parts in your signal chain, than we could talk about quality or impact on a sound. Your real signal path is way longer and there is so much other components in it, so changing one of them even to a most expensive of a highest quality will not give you THAT difference. Live it as it is! PLAY!

[d95err]

There has been some discussion here lately regarding using high-quality (expensive) pots for PPIMV. High-quality pots are supposed to sound better than low-quality, especially at low volumes. As shown in this video:
https://www.youtube.com/watch?v=vypgBhGQwJA

(although without an A/B comparision with a cheap pot, it's impossible to tell if there is actually any difference...)

If the high quality pot does indeed sound better, I would assume it has to do with the two halves being better matched than with a cheap pot. The important thing should be that the reistance ratio (forming the voltage divider for the signal) for each side is matched, not the total resistance.

It would be very interesting to hear a proper A/B comparison using an expensive and cheap PPIMV pot in the same amp, ideally with measurements of the ratio difference between the sides.

[DG45]

There has been some discussion here lately regarding using high-quality (expensive) pots for PPIMV. High-quality pots are supposed to sound better than low-quality, especially at low volumes. As shown in this video:
https://www.youtube.com/watch?v=vypgBhGQwJA

(although without an A/B comparision with a cheap pot, it's impossible to tell if there is actually any difference...)

If the high quality pot does indeed sound better, I would assume it has to do with the two halves being better matched than with a cheap pot. The important thing should be that the reistance ratio (forming the voltage divider for the signal) for each side is matched, not the total resistance.

It would be very interesting to hear a proper A/B comparison using an expensive and cheap PPIMV pot in the same amp, ideally with measurements of the ratio difference between the sides.

Understood. And that Alpha does track pretty evenly between halves throughout it's range. The Allen Bradley does not. It pretty well follows the difference between the two halves, throughout it's range. Given the variation between the halves of the Allen Bradley pots that I have, I think I'll just leave the Alpha in there. For now, anyway.

[Haze13]