Metropoulos Forum

shakti wrote: ↑

Tue Nov 06, 2018 2:42 pm

I wired up my Lar-Mar slightly differently. I removed the jumper wire with the bias voltage that runs from the 15k/bias cap junction to the 220ks. So the 220k node has no voltage on it. I take the bias voltage from the 15k/bias cap junction via the coax cable shield to the pot. With this setup, can I leave the 220ks on the board?

No, because the output tube grids are bridged through 440k

shakti wrote: ↑

Tue Nov 06, 2018 2:42 pm

I have asked before, but doesn't hurt to ask for more opinions; with the Rich Mod, a high quality/balanced pot like PEC is important and makes a difference. What about the Lar-Mar, has anyone experienced similar results in upgrading? The PEC pots are super expensive, but I will spring for them if they make an audible difference.

Buy 5 Alpha, measure all for balance, use the most balanced and throw the othe 4 pots into the garbage. Still cheaper than one PEC

Larry

Thanks Larry!

The one dual 250k I just bought actually turns out to be balanced extremely well - less than 2k difference throughout the entire range. So I'll try this one in a 1970 Super PA I am currently modding.

One more thing, and my apologies if it has been covered before, but not easy to find in this ultra-long thread;

- I mount the master volume on the front panel, very close to the PI circuit. So in fact, the output grid wires will be running more or less the same distance and direction as they currently do. My question; can I simply remove the green/Orange output grid wires from their respective turrets, and solder them to the pot wipers instead? Or do those wires *have* to be shielded with the Lar-Mar? I understand that they are sensitive, but seeing as they would only be moved an inch or two from their current location, is there any reason to expect them to be more prone to noise when running from the master volume control?

shakti wrote: ↑

Thu Nov 08, 2018 5:56 pm

I mount the master volume on the front panel, very close to the PI circuit. So in fact, the output grid wires will be running more or less the same distance and direction as they currently do. My question; can I simply remove the green/Orange output grid wires from their respective turrets, and solder them to the pot wipers instead? Or do those wires *have* to be shielded with the Lar-Mar? I understand that they are sensitive, but seeing as they would only be moved an inch or two from their current location, is there any reason to expect them to be more prone to noise when running from the master volume control?

The sensitivity of the grid wires with the PPIMV dimed is the same as w/o PPIMV
The more you turn the PPIMV ccw, the higher the sensitivity of the grid wires.
The grid wires at least should be twisted and running close to the chassis on the way to the output tubes.
Marshall sometimes has bundled the grid wires together with the OT's secondaries to the impedance selector and the NFB wire.
This is a major mistake and must be solved.
No OT's primary or secondary or the NFB wire parallel to the grid wires. One point crossing is ok.
To reduce stray fields by any means twist the OT's secondaries on their way to the selector switch.
AND twist the OT's primaries, but w/o the center wire included (only brown & red on 100W models, not the white included)
HTH

Thank you! I'll try my luck with the stock, twisted grid wires first, makes wiring quite a bit easier.

Hi to all,
finally i read all posts to know about this PPIMV i implemented successfully on a self built 2204.
Thanks to you all for any suggestion or solved doubt... and thanks to LarMar !!!!

I’m still not happy with the PPIMV I’ve got set up in a few of my builds. When wide open I have a slight hum (not too bad) but it’s there. I’ve talked to a few builders and they say their PPIMV is dead quiet when dimed.

A couple of things to consider. First, you should be using shielded wire in every case with this LarMar / Metro PPIMV, if you want to avoid problems. Yes, I have used this PPIMV with non shielded wire and had the same results you are having.

You didn't mention the use of safety resistors with your PPIMV. Are you using them and which value?

You should not have any ground wire to the PPIMV pot in a fixed bias amp and I believe that is what you described above, when you post "10uF, 15k and 47k components". The only ground wire in that bias supply should be leading from the two, 10uF bias filter caps. The wire from the bias circuit to the PPIMV is the bias voltage supply. A ground to the to the LarMar / Metro PPIMV pot would be involved in a cathode biased amp.

The best way I have found to help in getting away with using the LarMar / Metro PPIMV with unshielded wire, in a fixed bias amp, is to twist the bias supply wire leading to the PPIMV pot, with the wire pair leading to the 5.6k resistors / output tubes. This has worked every time, unless there was a separate issue. You can also twist a single point ground with the wires on the PI side of the PPIMV, but I have never had to do that.

The hard way to fix this in a cathode bias amp is to change the LarMar / Metro PPIMV ground wire location. You have to find a spot that will stop the noise by trial and error. I have found that will sometimes fix the problem, but it can take time to find that one spot that works. There are underlying reasons this can work but it is not the best solution. A better solution is to twist or wrap the PPIMV, output pair leading to the output tubes, with a single point ground wire or the cathode bias ground wire.

Sometimes the problem can be improved by relocating the PPIMV, unshielded wire routes but this has not been very successful.

Most of this leads back to shielded wire needing to be used in the beginning.

I have also found that the hum / noise can come from a preamp tube - the PPIMV reacts to it. Change that preamp tube and the issue is gone.

One thing i would like to understand in dept is why does NFB collapse with PPIMV, technically what happen ?

The ppimv reduces the drive voltage to the poweramp section which in turns means lower nfb voltage from the poweramp. You end up in a situation where the preamp is running hard but receiving a very small nfb voltage from the poweramp which affects the amp's sound and feel quite a bit.

Hi,
I'm trying to install a PPIMV on a VOX AC30 reissue. It works as intended but I'm getting serious hum.
I'm using a well balanced dual 250k Bourns pot, replacing the R50 and R53 resistors to ground and connecting to the 1.5k grid resistors.
I have the 2.2meg safety resistors installed and am aware of what their purpose is.
I've taken the 'Vib-Trem' pot completely out to use it's space by customers wish.

First run was just with twisted hookup wires. Those I've now replaced with shielded cables grounding them all at one ground point at the 250k PPIMV pot, not at the other end and sneaking them around the board away from the heater wires.
The hum is clearly dependent on the position of the 250k pot.

I'm a bit at a loss here. Maybe the removed 'Vib-Trem' pot needs to be replaced with something?

Any help appreciated.

Is there a way of recovering the lost NFB voltage when using a PPIMV? So, that the functionality of the Presence pot and the NFB resistor return to a noticeable usability? A 100% would be ideal.

I can't help but think that if we know why the NFB voltage is lost, that someone would know how it could be recovered without adversely effecting the functionality of the PPIMV.

On another note, my amp runs a little too dark for my taste when using the PPIMV at low volumes. So, I came up with a way to recover the lost highs at low volumes.

If you'd like a copy of the PPIMV schematic with the directions of how to mod your PPIMV pots, send me an email. Please include this in the email subject line: "PPIMV Bright Mod Schematic" wanted. I would have posted them here, but...as I'm sure everyone is well aware, the "Attachment Quota has been reached."

Email: bmwfreq64@gmail.com

PS: I've been using twisted pair wires rather than shielded cable with my PPIMV mods and have had absolutely no issues with it. Just my two cents for what it's worth.

You sure you came up with this mod? http://forum.metropoulos.net/viewtopic. ... 15#p469312 See #11. It's also been posted years ago in this thread.

bmwfreq wrote: ↑

Sat Nov 16, 2019 10:02 pm

On another note, my amp runs a little too dark for my taste when using the PPIMV at low volumes. So, I came up with a way to recover the lost highs at low volumes.

CoffeeTones wrote: ↑

Sun Nov 17, 2019 2:40 am

You sure you came up with this mod? http://forum.metropoulos.net/viewtopic. ... 15#p469312 See #11. It's also been posted years ago in this thread.

I don't know what bmwfreq did, but I've experimented with bright caps across the PPIMV pots already 15 or more years ago, but can't recall, whether I've ever mentioned it anywhere in this thread in the past.
Anyways - finally I've ended up with 330p or 390p ceramic cap (no SM) across input-wiper of a 250K pot, keeping the obligatory 2.2M resistor across wiper-output.

Larry, that is the mod. I have never seen it mentioned before, but no doubt you did this 15 years ago. You've been ahead of most of us here in the amp world. Thanks for your shared experiences. That's what happens when you keep circuit ideas circling around your head all of the time. As you may know, the LarMar master made it into some commercial amps without crediting the inventors. I don't like that sort of thing, but that is history.

CoffeeTones wrote: ↑

Sun Nov 17, 2019 2:00 pm

As you may know, the LarMar master made it into some commercial amps without crediting the inventors.

As well as my effects loop, yes I know about this.
It's like the indians, who stole the feathers of others to put it in their own hair
One of the reasons, why I became so quite on amp forums