The PPIMV (Post-PI-Master-Volume) thread.

rockgod212 · Post by **rockgod212** » Tue Mar 10, 2015 7:06 pm

damn, I end up getting the wrong pec pot, I got the 250k one, but after some calculations the 500k pot with 330k bias splitters on the board would get it to around 200k for the splitters, so that would get the amp closer to original spec. I might just take the plunge when im ready to do the ppimv on my latest build and get the right pot, I already have some 330k resistors laying around.

but damn georges amp sounds good like it is though.

CoffeeTones · Post by **CoffeeTones** » Tue Mar 10, 2015 7:28 pm

Post removed.

rockgod212 · Post by **rockgod212** » Tue Mar 10, 2015 8:08 pm

oh I know its fine I got some 2m2's for the pot. im just thinking out loud.

CoffeeTones wrote:See this post. Your 250k pot is fine http://forum.metroamp.com/viewtopic.php ... 47#p448247

Kenny Blue · Post by **Kenny Blue** » Tue Apr 21, 2015 1:48 am

Hello everyone... I am Kenny Blue and am a new member here.

I just recently bought a MetroAmp JTM45 built in the Metropoulos shop by someone with the initials J.M. .

I am wanting to get a Lar Mar PPIMV added to this amp. I really know next to nothing about working on circuits so I am going to find a good tech here in the Chicago area to do this mod for me.

The thing is that I would like to find a diagram and detailed instructions for doing the Lar Mar PPIMV on a Metro JTM45 so that I can provide them to the amp tech to do this mod correctly. I have tried to search specifically for this but the info I have seemed to find has been links to web pages from several years back and those links do not work any longer or those specific web pages no longer exist.

Can anyone help me find diagrams and detailed instructions on how to do a Lar Mar PPIMV on a MetroAmp JTM45 ?

Thanks ....

CoffeeTones · Post by **CoffeeTones** » Tue May 12, 2015 9:16 pm

http://www.vilo.net/ppimv/ppimv.jpg

http://home.comcast.net/~jbjdav26/1987PPIMVr17.jpg

http://ceriatone.com/images/mods/Plexi5 ... Ceriat.jpg

pjoliver182 · Post by **pjoliver182** » Thu Jun 04, 2015 3:09 pm

I've read a few times about using the larmar to replace stock 150k resistors by using a 250k pot and 470k safety resistors. But what about the other side of the pot? It'll still be 250k won't it?

CoffeeTones · Post by **CoffeeTones** » Thu Jun 04, 2015 3:24 pm

It's ideally 0 to 250k but it doesn't matter. It does not serve as the output grid resistors, AKA bias splitters, as the final maximum setting.

pjoliver182 · Post by **pjoliver182** » Thu Jun 04, 2015 3:59 pm

I guess I don't really understand how this thing works. I know from my #36 tinkering that changing between 82, 150 and 220k splitters changes the tone of the amp. One side of the dual pit is varying the resistance (that replaces the original 220k splitters) between 0 and 220k. That alone would affect the tone a lot. So the other side of the dual pot must be doing something to compensate?

CoffeeTones · Post by **CoffeeTones** » Thu Jun 04, 2015 4:29 pm

Draw the circuit on paper, with the dual pot as resistors and you will see that as one side of the pot comes down in resistance, the other side of goes up. Then look at how the wires are connected and what is happening to the signal because of those things. Remember, signal can go to ground through the bias circuit.

pjoliver182 · Post by **pjoliver182** » Fri Jun 05, 2015 2:48 am

Thanks Duane, I think I see it now.

So the resistance between bias feed and output grids will vary between 0 and 224k depending on pot position? And the signal will see between 224 and 250k resistance to the bias feed supply. And a variable resistance to the output grids?

bmwfreq · Post by **bmwfreq** » Mon Jun 22, 2015 11:22 am

I have a question regarding the PPIMV...

What would happen if...the grid shorts while you have the PPIMV set to zero?? Are we positive that the OT fuse would blow before serious damage occurred??

Also, I have been told to use 150k resistors with the 250k dual pot with my 6550s (for a total of 94k bias resistance), because this will increase the lifespan of the tubes.

Here's a quote copied from the internet regarding the 6550 tubes and their biasing resistance: "6550 based Marshalls should run at 50k (according to the RCA tube manual), but historically ran at 150k."

Can anyone offer any insight regarding my comments?

Thanks in advance to all that reply.

Sazabi · Post by **Sazabi** » Sun Jul 05, 2015 4:33 pm

Hey everyone!

Excuse me if this question sound stupid, I'm new to Tube amps.
Every diagram on LarMar ppimv says remove 220k resistors, but none says anything about wires from 0.1U caps lugs ( I have metroamps JTM45) to valve pins.
Should those be removed as well? And only connection to valves is going from dual pot lugs?
I've seen some photos where I can see (but hardly, this is why I'm asking) that only connection to valve pin 5 is from ppimv pot. But I need to be shure.
The other PPIMV mod (used 500K pots and stock resistors) has direct connectino from caps to valves.
Btw, as this is ppimv thread, I'd like to ask one more question.
I have installed first version of ppimv in my jtm45. But can't set bias voltage. Before ppimv mode it was just right (~42mV biased and ~36mV with bias pot fully clockwise).
After mod with volumes at 0 and master volume cranked all way up and bias pot all way clockwise, I got readings around 50. If I turn bias pot, it gets even higher.
What can be the problem?
Thanks in advence for any suggestions!

CoffeeTones · Post by **CoffeeTones** » Sun Jul 05, 2015 8:46 pm

http://ceriatone.com/images/mods/Plexi5 ... Ceriat.jpg
If your bias has problems, the PPIMV is very likely wired wrong. The layout above is about as clear as the correct wiring can be depicted.

Sazabi · Post by **Sazabi** » Mon Jul 06, 2015 2:31 am

It's for Lar/Mar PPIMV. Mine is Rich mod ppimv.
Like this:

CoffeeTones · Post by **CoffeeTones** » Mon Jul 06, 2015 3:08 am

You asked about the LarMar and referred to the Rich / Metro PPIMV, but the wiring is exactly the same for both the Rich / Metro and LarMar PPIMV. Only the pot and safety resistor values are different. The resistors can be on the pot or left on the board.

Sazabi wrote:Hey everyone!

Excuse me if this question sound stupid, I'm new to Tube amps.
Every diagram on LarMar ppimv says remove 220k resistors, but none says anything about wires from 0.1U caps lugs ( I have metroamps JTM45) to valve pins.
Should those be removed as well? And only connection to valves is going from dual pot lugs?
I've seen some photos where I can see (but hardly, this is why I'm asking) that only connection to valve pin 5 is from ppimv pot. But I need to be shure.
The other PPIMV mod (used 500K pots and stock resistors) has direct connectino from caps to valves.
Btw, as this is ppimv thread, I'd like to ask one more question.
I have installed first version of ppimv in my jtm45. But can't set bias voltage. Before ppimv mode it was just right (~42mV biased and ~36mV with bias pot fully clockwise).
After mod with volumes at 0 and master volume cranked all way up and bias pot all way clockwise, I got readings around 50. If I turn bias pot, it gets even higher.
What can be the problem?
Thanks in advence for any suggestions!

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The PPIMV (Post-PI-Master-Volume) thread.

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