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

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novosibir
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Re: The PPIMV (Post-PI-Master-Volume) thread.

Post by novosibir » Tue Nov 06, 2018 4:15 pm

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

Post by shakti » Thu Nov 08, 2018 5:56 pm

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?
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

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

Post by novosibir » Sat Nov 10, 2018 9:39 pm

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
The fault almost always is sitting in front of the amp :wink:

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

Post by shakti » Sun Nov 11, 2018 10:05 am

Thank you! I'll try my luck with the stock, twisted grid wires first, makes wiring quite a bit easier.
JTM45 RS OT, 1973 18W, JTM45/100, JTM50, JMP50 1986, JMP100 "West Coast", AC15, AC30, BF Super Reverb, Boogie Mk 1, Hiwatt CP103, DR103

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

Post by mancio791 » Wed Mar 27, 2019 10:20 am

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 !!!!
:thumbsup:

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