The LaMar (LarMar) PPIMV in an Express

[paulster]

Absolutely. It'll work just the same in a Liverpool.

[Markusv]

THX!

[mojotom]

Nice news !

I've installed this MV on a plexi with great results, way better than an aibrake.

For my Express I've gone to VVR, not installed yet and I have second thought.
If anyone here experienced both I would be glad to have an advise and what to choose.
For now I could use the dual 1M for the VVR for the master.

For practical reasons the MV will be easier to use, I could put it on the front panel too.

Do you guys think dual 1M will be ok ?

M'y main concern is to cut down some volume. As is it's too powerfull to be used on gigs, I'm not trying to make it work for bedroom volume either.

[PlinytheWelder]

No, a dual 1meg pot is not this circuit at all... Not even close.

[mojotom]

Well, it would have been too easy !

If you had to choose between vvr and MV ?

I could install both if it's worth it for sure but it could be too complicated
for my friend needs.

[rooster]

Just a note to add that I have updated the first page of this post (my last post @ the bottom of the first page) to include the results of the MV in a Rocket build. Well, OK, like everybody says, it does work equally well in the Rocket circuit. And much better than the Airbrake once again, IMO. HOWEVER, I just want to say that, where the adjustment is smooth, it does drop off the output volume sooner than it does in the Express circuit. So maybe at 12 noon on the control, the usable volume is pretty much gone in the Rocket.. while on the Express there is a little more to be had past this point.

[andrew]

hi all, using the LaMar diagram on this thread how would I wire it with a cathode biased amp? Thanks

[rooster]

Pull the 220K resistor pair that attach to ground at the post PI position - after the coupling caps, and before the screen resistors? With me? The dual pots will substitute here just like on the diagram then.

The only difference is the ground point of the 220K resistors. This is your cathode bias supply, (think 'zero' volts), so it will attach to the dual pots as if it was a negative voltage in the diagram. Make sense?

That's really the only difference. You will dig it.

[andrew]

thanks Rooster.

[rawnster]

Just a brief resurrection of this thread. I add the larmar mv to my express build last night. Played it for a while afterwords, and a bit this morning. I've got to say that I'm really liking it. The tone and vibe of the amp really does stay intact even when dialed way back on the MV. Very cool. I certainly wasted a lot of time building an airbrake last year. Thanks, Rooster, for starting this thread and helping me think outside the box a bit with these amps.

[PlinytheWelder]

Yeah, I keep on recommending this circuit and I think that some bad experiences with somewhat similar designs have turned people off on PPIMV's in general.

Oh well...

[Cliff Schecht]

TBH, this circuit is not something I see as a huge innovation that the "creators" came up with. It's an obvious and elegant solution for dialing back the amount of signal going to the power tubes. We're the "creators" really the first people to use a dual pot as a push-pull pair volume control?

[UR12]

Well, it would have been too easy !

If you had to choose between vvr and MV ?

I could install both if it's worth it for sure but it could be too complicated
for my friend needs.

The gain structure on both the Express and the Liverpool are set up to to give you wonderful sounding power tube distrotion first before the preamp tubes . If you install a MV (of any kind) you will be knocking down the signal going to the power tubes and loose the power tube distortion at lower volumes. The VVR drops the voltages to the tubes and keeps the gain structure intact so you still have the glorius power tube distortion at a much lower power level.

The gain structure on a Marshall is set up to overdrive the preamp tubes first and a MV works very well in those amps. Just my 2 cents

[Markusv]

True UR12

I find that you lose some of the "thickness" in the tone when the LarMar is dialed back substantially.
But if you want to just cut the overall volume a hair or two the LarMar is most useful between about 2 o'clock and up.
There, the power tubes still contribute substantially. They may not break up much, but they fatten the tone a lot and you have credible tone.

That said, the VVR is probably closer to the real deal- not sure as I have not tried it.

On a different note.
With my Rocket (KT66) the LarMar has been quite surprizing.
At about 2 o'clock with the gain pegged, I get something of a voxish ef86 sounding breakup out of it. Not the same-obviously- but not typical triode sound either

BTW I did not have a dual gang 220K pot available, so I used a dual gang 1meg pot with 250K resistors between the wiper and outer leg. Gives me a usable sweep and roughly 220K overall resistance. Works well!

I agree with a previous post that below 12 o'clock on the MV the Rocket loses its uhmph

Markus V

[rooster]

Ron - Yes, here I think that sometimes simple is better.

Cliff - No, not the creators of the dual pot PI power tube coupling section MV, per se. That might be Bell Labs or Western Electric, I don't know. But I do know that Ken Fischer included a version of this in his MV offerings, too, except with a 1M dual pot and no added resistors that serve as a failsafe should the pot fail. Eh, so I will still give it up to the two guys and call it the 'LarMar' MV. Why not?

Dana - OK, I get the design idea behind the VVR and I know that you are describing it correctly. And no, I am not trying to take anything away from the VVR. This said, please keep in mind that the Express has MORE gain in the preamp section than a Plexi Marshall. So when you concede that such a MV (LarMar type) might work in a Marshall, then it will certainly work even better in the Express. I also concede that preamp distortion alone is different than preamp distortion combined with output tube distortion, not a question. But in the vein of controlling an amp's output, an amp like the Express for one - at a gig or in a bedroom - I think the LarMar MV is a very good tool, and yes, easily a better tool than the airbrake.