PPIMV

[dude]

Want to do the PPIMV mod to one of my Plexi's, LaMars. But every dual gain pot I have is not equal or close. I get 200k off on each pot. The 220k resistors need to be within 5%. I know a resistor in parallel across the outer legs will lower the values. Thinking what size to start to lower the highest pot of the dual gang. One side pot is .959 Meg the other is 1.159Meg. To lower the highest to .959 what resistors would l use across the outer legs, if this is possible?

[j_bruce]

5.56 megs

(algebra available on request)

Cheers!
Jerry

[dude]

Requesting the algebra.  You can teach a man to fish and feed him for life, might have screwed that up  .

[SILVERGUN]

Im no math teacher and I just use this:
https://www.allaboutcircuits.com/tools/parallel-resistance-calculator/

[PRR]

> The 220k resistors need to be within 5%.

Why?

[dude]

?, that's what l read on most schematics

[d95err]

The 220k tolerance is not critical. You should worry more about how well the PPIMV will work when turned down, if there is a big difference between the two sides.

I’d try to find a pot that is better matched.

[j_bruce]

Requesting the algebra.  You can teach a man to fish and feed him for life, might have screwed that up  .

We all know that 1/r1 + 1/r2 = 1/r  where r1 is one resistor, r2 is another resistor in parallel and r is the resistance of the combination.

so solving we find:
 r2(1/r1 + 1/r2 = 1/r)
        r2/r1 + 1 = r2/r

re-arrange to get:
     r2/r1 - r2/r = -1

Factoring:
  r2(1/r1 - 1/r) = -1

divide both sides by (1/r1 - 1/r):
                  r2 = -1/(1/r1 - 1/r)

setting r1 = 1.159 and r = .959 gives r2 = 5.56

or use that calculator link

Cheers!
Jerry

[shooter]

Factoring:
  r2(1/r1 - 1/r) = -1

I love when math ends in a negative apple 

[kagliostro]

Some time ago I had a similar necessity on a pot that was on a pedal

as to obtain a perfect value I used a small trimmer in parallel with the pot (instead of a fixed resistor)

this gived me the opportunity to achieve the perfect value I was looking for

Franco

[Curranm]

just to pipe in on the PPIMV. I have been very un happy with the LarMar PPIMV when i turned it below 80% on ANY amp I have installed it into. Do a google search for "the trainwreck pages" IF they are not already linked some where in this forum or on the EL34world pages! there is a PPIMV that I use from there (I think #3) which is a dual pot (250kA) and two capacitors. I love it and it really works very well in my plexi/800 amp I build! like SUPER GOOD!!!!

[kagliostro]

Do you mean something like this ?

https://el34world.com/Forum/index.php?topic=25903.msg281607#msg281607

Franco

[2deaf]

We all know that 1/r1 + 1/r2 = 1/r  where r1 is one resistor, r2 is another resistor in parallel and r is the resistance of the combination.

Product over difference.

r2 = (r1 x r) / (r1 - r)

[2deaf]

1/r1 + 1/r2 = 1/r

r2(1/r1 + 1/r2) = r2(1/r)

r2/r1 + 1 = r2/r

1 = r2/r - r2/r1

1 = r2(1/r - 1/r1)

r2 = 1/(1/r - 1/r1)

[dude]

I agree j_, 10th grade algebra, equations, I remember those days. Sometimes "use it or lose it" comes into play, the more we tax our brains the better it functions.As far the different PPIMV's, I've read the Trainwreck MV's, I,II,III pages, Curranm, is the schematic the one Franco posted ..? As most here usually suggest when "many ways to skin a cat" comes up, it's what sounds best to your hears.  The post phase inverter master volume I'm using in this Super Reverb clone is below, works well in this type of amp. I plan on installing the LaMar type in a 6V6 PLexi, there are several types. FWIW, I never turn the PPIMV below half as the highs are choked and I too find at that level it unpleasant sounding.I would be interested in why j_ thinks the  the LaMar sounds not so great at only 80% down...?Thank's for the refresher on ohm's law, much appreciated.

[JayCobie]

Having the pot slightly mismatched isn't the end of the world, but I like the feel of a well matched PPIMV better than a cheap non-matched.

I highly recommend blue Alps dual-gang 250k pots for this application. I bought a pair and the gangs on both were matched within 5% of eachother