Master Volume pot

[M Fowler]

Does the Lar-Mar or Type II PPIMV 250K dual pot have to be linear?

I've been using 250k Audio not realizing some PPIMV layouts have the pot listed as linear.

Mark

[Colossal]

Mark,

I've seen it done both ways. Larry uses 250kA || 2M2 but Rooster swears by 250kB in the Express and has documented his experience here.

[M Fowler]

Okay thanks Dave, I got a guy that bought a TW Rocket 1-12 combo and wants to have a MV installed and I have a drawer of 250kA but no linear dual 250k pots.

Mark

[Colossal]

Sorry yes, to clarify, you will need a dual gang pot. Larry's original was with audio/log, but either will work.

[rooster]

Linear taper makes this what it should be. I've tried both. Audio taper is pretty much a waste of time and does not deliver what the circuit can do.

Hm, Mark, did Larry actually use or suggest an Audio taper pot? I think when this circuit was originally posted of the Metro Amp Forum there was some small confusion by others but there was ultimately a drawing posted that confirmed the Linear taper pot - and this (Linear taper) is what Larry and Mark signed off on.

As a closer, on this forum, when I would see someone writing that they didnt like the LarMar MV, in every case it turned out they were using an Audio taper pot. In fact, when I would point out that the Linear taper is the correct taper, it seemed like I was shot down by all who used the Audio taper, saying that there could not possibly be any performance difference between the two tapers. Well, in a strictly resistive value way this is true. However, when a MV is used, it is MUCH MUCH easier to find an incremental volume result at the speaker when using the Linear taper pot. And that's why Larry designed the LarMar circuit with a Linear taper pot.

[Colossal]

Rooster,

On Metroamp Forum, on page 90 of the PPIMV thread Larry states:

250K log. double pot + 2.2M safety R's

Larry

In the Valvestorm store which supports many of the folks on Metro Forum, Robert sells 250kA and 500kA dual gang PEC pots for the Lar-Mar and Rich mods respectively.

Just sayin'

[M Fowler]

It's all over the place some for Lar-Mar say dual 250kL others state dual 250kA.

I have always used dual 250kA on my PPIMV.

[M Fowler]

The only dual 250kL pot I've found so far for sale was a 8mm shaft, but I want a 3/8" shaft to match my other Alpha or CTS pots.

[rooster]

Wow, that is interesting what you posted Colossol. Here's what I copied at the time the postings were ongoing. In the written response by Larry, you can see he failed to mention what the taper of the pot was. I'm not sure why this is the case but I will assume this part of the equation was somehow 'understood'. The drawing showed up on page 3 or 4 of the original post I think, a post Larry was still active on. My thinking here is that if the pic was incorrect, then Larry would have certainly corrected it. He did not. (Note: it is rockstah who asks the question directly to Larry for a detailed explanation and then, finally, it is rockstah who returns with the drawing.)

Mark, you are right about availability, I ended up buying the Alpha version from CEDist and grading the ones I use for balance. Reject rate is maybe 2 out of ten.

[M Fowler]

I ended up ordering 10 250kL dual pots from CED and some other stuff to justify shipping

Thanks guys.

[dorrisant]

I switched to linear taper and I like it better. I just installed a MV in a new build and used an audio taper pot... The taper does make a difference. The audio seems to not do much past 50% and below that it seems hard to dial in without passing the setting you want. With a linear taper pot the volume adjustment seems smooth all the way through the travel. I ripped out the dual 250KA and just subbed in resistors for now. Now where in the heck did I get dual 250KB pots before? And yes they were 3/8"...

How bad would it be to strap the audio pot with 2.2M resistors? I know it would change the taper, but I'm about ready to try it.

[Colossal]

Very cool Rooster. I've looked at that goofy picture a dozen times and never noticed that the pot was linear. Interesting.

I use linear for my guitar volume pot.

[Reeltarded]

How bad would it be to strap the pot with 2.2M resistors? I know it would change the taper, but I'm about ready to try it.

You are saying limiting both sides and using safety rsistors as well?

Go for it. Got anything bigger than 2M2 for the limiters? Won't matter much anyhow. You may find the sections balance much better through the travel as well, in fact, it might be a useful mod for this rig.

[dorrisant]

You are saying limiting both sides and using safety rsistors as well?

What I meant was to solder a 2.2M resistor from term 1 to 3 on each side of the 250KA dual pot. Then from 1 to 2 with the normal 2.2M on each side. The resistance across 1 & 3 changes the taper to a more linear taper, I think... Maybe not close enough. I'm just wondering if it would screw anything else up. I think I will try it next time I open it up.

Was that what YOU were thinking?

[Reeltarded]

It's a great idea, actually, for a couple reasons.

Yeah, that's what I thought you meant.