PPIMV on a jcm800/2204 yes or no ? .. I don't really like it

[copperheadroads]

I just installed a PPIMV on my marshall 2204 /jcm800 clone & i think i will remove it , as it sounds kind of flubby & looses some of the cutting tone .it does work as its supposed to . any one else feel the same way when doing this mod ?
here's the layout

[Chuck H]

You REALLY need an attenuator with that amp. Removing the power amp from the clipping reduces the voltage shift dynamic and that can make the amp sound/feel blatty and flat. PI clipping is an important part of the formula, but it's usually only adding harmonic distortion to clipping power tubes. Some amps make good preamp distortion and others need to clip the power tubes. I honestly can't think of one example where the PI clipping the final waveform was better or even as good. YMMV and sorry for your less than positive experience and efforts. Some guys like it.?. But for you, clearly, take it out and get a good attenuator. Look forward to using up a few more power tubes

[copperheadroads]

I built this amp in the spring & has been playing it & enjoying it with the gain maxed & the master on 3 or so .
the tone is certainly jcm & with

[g1]

When you are trying out the new PPIMV, are you running the regular master at full?
This would be the only fair comparison.
If done this way, you prefer the regular master to the PPIMV ?

[bob p]

i haven't listened to an 800/2204 with a PPIMV, though I have heard PPIMV mods in old plexis and metal face marshalls that i thought sounded pretty good at townhouse volumes on a half stack of greenbacks. those people were using the "Rich Mod" PPIMV before attenuators became widely popular. those PPIMV amps would have been relying primarily on PI clipping and maybe some mild speaker compression rather than power tube clipping and outright speaker IM distortion. so i guess my two cents is to say that a PPIMV doesn't have to sound bad, but it's only a part of the classic Marshall tone.

which PPIMV mod are you using? I haven't traced your circuit to figure out which one you're using, but there are several. I thought the "Rich Mod" sounded pretty good when I heard it, but that was a long time ago. You might want to consider the different flavors of PPIMV mods before scrapping the idea entirely.

my preference is like Chuck's -- I think the PPIMV mods serve a purpose, but I think attenuators give better results. IMO the clipping PI sound is an essential part of the Marshall tone, but it's only part of the formula. the output tubes really matter. You need them both.

[Zouto]

I installed crossline MVs on some 2205s for good results, sounding much better than the LarMar PPIMV to my ears. It's important to use a high quality pot (Bourns e.g.), 500K log gives a better range of control than 1M.

[Zozobra]

Another option is to add a pot in series with the LTPI bias resistor. Mojave amps do this on a few models. This has the effect of lowering the headroom of the LTPI so it clips earlier and it reduces the LTPI output swing too. I think the Mojave amps use a 47k pot.

Here is a sim I did to show the effect of increasing the bias resistor

[copperheadroads]

I used the same as in the diagram which is the same as in the triode kit 250k dual gang pot

[g1]

A schematic would be much better than the layout.

[big_teee]

It looks like the standard JCM800 2204 with the lar/mar PPIMV mod.
http://el34world.com/charts/Schemati...l_50w_2204.pdf
I've built it several times, and I thought it worked great.
With the PPIMV on 10, it should be like it is not there, and regulate with the pre MV.
Mine sounded best with the PPIMV above 5.
I did several other mods, and always changed some cap values to suit my tonal needs.
T

[bob p]

Another option is to add a pot in series with the LTPI bias resistor. Mojave amps do this on a few models. This has the effect of lowering the headroom of the LTPI so it clips earlier and it reduces the LTPI output swing too. I think the Mojave amps use a 47k pot.

Nice sim.

Merlin mentions an adjustable PI bias mod like that in his preamp book.

[copperheadroads]

I took it out ,It sounded best with it pretty much maxed & i guess then it was pretty much out of circuit then .

[bob p]

of course we know it'll sound better maxed and/or out of circuit... taking it out of circuit gives you both halves of the marshall tone, while using the PPIMV only gives you the PI half of the tone.

the real question is whether or not distorting the PI without frying your ears with the power tubes throbbing gets you something at lower volumes that you couldn't get otherwise. if the answer to that question is yes, then it adds benefit and is worth keeping.

perfect is the enemy of good.

[loudthud]

Has anyone ever tried half wave clipping diodes to ground at the top of the Lar-Mar Master Volume? These simulate output tube grid current as you turn down the Master Volume.

[g1]

I took it out ,It sounded best with it pretty much maxed & i guess then it was pretty much out of circuit then .

So I guess you prefer the sound of the regular master volume control, or do you normally run that maxed?