The LaMar (LarMar) PPIMV in an Express

[rooster]

Over at MetroAmps forum many people are into this MV (Master Volume) circuit. It is a modified PPIMV Ken Fischer circuit basically but the dual pot value has been reduced to 250K and some smoothing resistors have been added and he cleverly uses the bias voltage to sheild the connecting wires. There is a builder who posts there and who is also a moderator, who makes "Larry Amps". He was using the smoothing resistors on a dual 500K and 1 meg pot, and then another forum member asked if Larry thought he could use a 250K pot so that when things were 'maxed out' the amp - in this case a Marshall Plexi style amp - would be dead stock. This poster's name is Mark, and I believe he works on amps, as well.

So Larry brings the circuit and Mark puts it in his Plexi and all is good. So they called it the 'LaMar MV'. I hope I have the facts straight. There is a long post there, 80 something pages, and a few links to the schematic that developed.

Anyway, I thought I would try it in my 35 watt Express since its so loud, in hopes that it would knock things back a bit. Since it takes the PI back to stock, I had nothing to lose.

I have only played it for 15 minutes at this point and will get into it tomorrow. I built it correctly, it is invisibly quiet, and it behaves like the typical PPIMV that Ken designed, however its adjustment curve is seemingly smoother. I suspect I will only use it to tame things just a bit in performance (turn the MV knob from '10' to '7'ish, but it might be just the ticket for this. Which is to say, there is bedroom volume available - easily - but for performance it starts to sound too 'grainy'. (FWIW, I prefer it over the Airbrake in just a quick run thru, but I will give it some time.)

OK, so can I compare notes with anybody else on this forum? Has anybody used the LaMar PPIMV in the Express?

[PlinytheWelder]

I have a LarMar PPIMV in a Laney GH-100L and it is the best MV that I have come across. I'm building an Express that should be done this weekend but I decided to go with a Hall VVR3 to tame it down.

I plan on gutting a Laney AOR that I have and putting a Wreck style front end in along with an effects loop and a LarMar and whole bunch of other heretical stuff. I should then be able to see how close it gets to the more standard design...

[paulster]

Larry's also a member here.

I haven't tried it in an Express but the only problem I foresee with it is that when you get the volume down a bit you won't have the power tubes working as hard so won't get the same degree of compression in the power supply.

This may mean that the sustain suffers a bit or that the clean to mean volume change is more pronounced.

It would be good to have some real world feedback though as it's a great master for a Marshall.

[UR12]

Larry's also a member here.

So is Mark. He goes by Rockstah on this forum IIRC

[Roe]

much of the express tone comes from driving the power tubes into saturation. a mv will mess with that. why bother? the express isn't loud compared to plexis. I use an alex attenuator at home and play unattenuated at gigs and rehearsals. my drummer likes the express since it is distinct, tight, gainy yet not very loud

[Darkbluemurder]

I have tried it in my Liverpool build but as mentioned here before it will not allow the power tubes to go into distortion when turned down. While I have not done it I have heard from various people having achieved better results with a VVR.

Cheers Stephan

[Cliff Schecht]

much of the express tone comes from driving the power tubes into saturation. a mv will mess with that. why bother? the express isn't loud compared to plexis. I use an alex attenuator at home and play unattenuated at gigs and rehearsals. my drummer likes the express since it is distinct, tight, gainy yet not very loud

Agreed. You just have to use either less speakers or less efficient speakers to get the volume down for stage use. The Express can hold it's own with any 50W amp with a 4x12.

[rooster]

Thanks for checking in all. OK, then I am alone here.

So I know a cranked amp will always trump a MV one in my world, but then there's the need for quiet. And you've got two, well maybe 3. There's the VVR thing, there's the attenuater thing, and then there's the MV thing.

I have some testing to do today, but I really am wondering if the guys using the attenuater wouldn't like this MV better. At first blush I think I do. I have not tried the VVR and read - I think - mixed reviews here in the Express.

OK, sounds clips are coming then, next week.

[PlinytheWelder]

Thanks for checking in all. OK, then I am alone here.

So I know a cranked amp will always trump a MV one in my world, but then there's the need for quiet. And you've got two, well maybe 3. There's the VVR thing, there's the attenuater thing, and then there's the MV thing.

I have some testing to do today, but I really am wondering if the guys using the attenuater wouldn't like this MV better. At first blush I think I do. I have not tried the VVR and read - I think - mixed reviews here in the Express.

OK, sounds clips are coming then, next week.

IMO, the LarMar MV made my Laney into a much better sounding amp. Like I said, it's the best MV I've ever used....

[JamesHealey]

The LarMar PPIMV doesn't sound as good as non-mv even when totally cranked so it's 220.. there is some kind of blurring to the tone that I can't describe probably due to the shielded cables and extra lead length.

I really hear this difference and do not like the LarMar because of it, It's deffinately the most transparent MV I've ever tried though.

Although Im a huge fan of the Matchless style PPIMV with a 250k pot and 180pF Ceramic Cap.. but having a defeat switch installed so you can disconnect the MV entirely without any intrusion into the tone when it's truely cranked.

Fair enough this MV doesn't sounds as transparent asthe LarMar at lower volumes.. but for me an MV has to be able to be disabled so I can use my amp as I intended to, yet whenI need to I can knock off a few dB using a rather reasonable sounding MV when a sound man kicks off at me during soundcheck to turn down.. Atleast for me as a working musician that is the cheapest and most viable option.

[rooster]

Alright, put some time on this MV by now. WOW! This is cool and completely blows the airbrake out of the water. I will do some recording on Monday and post some clips but I think this circuit is a great match for the Express. Using a modified TS9, turning the channel volume to 10 o'clock, playing guitar and then turning the MV down in increments has made me a believer. The pot I use is lineal, per instructions, and where I thought I would not be using it in performance at settings below '7' (down from '10' or bypass), I see that I was wrong about this. I can run it down to '4' and it sounds believable. No, the speakers are not jumping out of the cab, but the Express is still there. Folks - whatever your mindset is on something like this for an Express I think you might want to take another look. This MV actually works.

....And if you ever had a need to play this amp in your bedroom at 1AM, go no further.

As to it not being out of the circuit when the knob is on '10', well, anything's possible, but I don't hear a change myself, and electrically it is gone. Well, it does change the screen resistor value from 220K to 250K - and that's a change overall. So you could hear this maybe? But then with some luck you might be able to find some off tolerance dual 250K pot that came in at, say 240K or lower, and that's within 20% spec. The one I have is 248/249 and I bought 5, picked the best out of these. ?? Whatever change this has made I don't hear it. The sheilding idea, using the negative voltage, is actually genius, and I clearly think this is not adding anything at all to the circuit. My leads are short as I put the pot next to one of my speaker jacks - it sits the furtherest away from the AC connection and almost in the middle of the chassis back panel. Its a pretty sweet and quick install, very sano. My hat is off to Larry and Mark, you guys rock very very hard with this circuit.

[PlinytheWelder]

Actually don't the resistors in parallel with the pots knock the value back close to 220K ?

[img:683:599]http://verosurfcam.com/683px-Lar-Mar_PPI-MV.jpg[/img]

[rawnster]

Great report, Rooster. I definitely want to try this on my express. I hardly play it anymore due to it being so loud. I've read so many mixed reports about the LarMar mod (especially when mixed with a trainwreck circuit), that I had been skittish to try it.

anyway, nice job.

[rooster]

Pliny - Well, no not really. ***

Here, I cannot do the explanation justice, so this is what Larry, the originator of the PPIMV wrote regarding the operation of the circuit:

"Ok - step by step:

- The coupling caps are connected to the input lug's of the pot
- The bias supply voltage (which usually feeds the both 220K) is connected to the output lug's of the pot (of course to both)
- The pot's wipers are connected to the power tube's grids (or to their swamp resistors)
- Now simply solder each a 2.2M on each section of the pot from the wiper to the output lug - a small 1/4W resistor will do it

These are only for safety, because if the pot's wiper fails, then the output tube's grid is w/o bias voltage and will immediately jump over the Jordan.

But with the resistor applied and a failing wiper, although this side can't be regulated anymore, but nevertheless there's bias voltage on the output's grid - and the tube(s) will survive

Larry "

So, whereas the 2.2Ms are in fact smoothing the resultant resistance change in the pot's value as the pot is turned, Larry is saying that this (the 2.2M resistors) is a safety feature, should the negative voltage suddenly be disconnected from the tubes because of a failed wiper connection.

*** OK added note: Pliny, yeah, you are right about the 2.2M resister dropping the overall value of the 250K pot. Larry must have mentioned this somewhere in the lengthy post but I got caught up on the 'safety feature' he seemed to be focused on. When the pot is dialed out of the circuit or on '10'. If you do the math, I end up with 224K on a 250K pot using the 2.2M as a bridge. Meaning, for the sake of explanation, that when I say 'bridge' this would be like placing the 2.2M across the outside lugs of the 250K pot, which is what the pot would see when you turn the wiper to the furthest point away from the bias voltage, or '10'. In practice however, my 250K pot with the 2.2M resistor added actually measures 228K - which threw me. Eh, but the math is good, reality is slightly warped maybe. All of which would indicate that - math and reality considered, that 220K vs. even 228K is easily within a 5% tolerance range, so the amp should behave like stock when the pot is on '10'. Sorry to suggest otherwise, Pliny!!!

And thanks for posting that pic from the Metroamp site. I thought I would be polite and ask Larry if I could at least link this but he never got back to the post. If he does visit here, maybe he will check in. I thanked him and Mark for the circuit there, now I will thank them here - again. Thank you guys, this is a very nice addition to my Express clone!!

Oh, as to the 250KL dual pot, I used an Alpha brand. Also, FWIW, I purchased 5 from CEDist and found 2 usable ones. So if you are picky, be prepared to spend a few extra bucks. With the help of selloutrr, I purchased two NOS USA dual 250K pots - all the while thinking I had beat the devil or something - only to find that upon arrival the values are: 215/200 and 220/190. Man, what a disappointment. So maybe go with the Alphas here, that's my advice. ........AND, reading the long post over at Metroamps again, I now see that Larry is saying to use a 250K AUDIO TAPER pot! He states that if you use a LINEAL TAPER pot, like I did, you will experience a volume jump as you operate the pot. ?? Well, he's the man on the case so probably you should listen to him. I like the way the lineal pot is behaving, and I don't hear any volume jumping, but then I haven't heard the amp with an AUDIO TAPER pot. Whew, sorry for the edit but I wanted this info on the first page.

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(Updating this first page on 6/22/10: I have by now tried the 250K AUDIO taper pot, and this is THE pot to use! In fact, I would go further and say: Do not use the lineal taper pot!!! Sorry for the confusion but if you look at the pic above the lineal pot is listed - THIS IS INCORRECT! When you use the AUDIO taper the MV will just behave much better.)

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(Updating this first page on 6/27/10: I have now installed the 250K AUDIO TAPER pot in one of my ROCKET builds. It does work great here, too, as reported by others, and IMO it also blows the Airbrake away, yes, HOWEVER - while the adjustment range is very smooth here, the volume drops off sooner in the Rocket than in the Express as you turn the control. In fact, at 12 noon, the Rocket is pretty much done, where the Express has still got some volume dropping to go. But a total winner here, not to confuse anyone.)

[jerrydyer]

yep were all here
whats up !!!! ????????????

that MV is tried and true for most amps.

goes back pretty far though like Park days ken bran should get credit. Larry took it a step further to protect your amp should a pot fail .