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

CoffeeTones · Post by **CoffeeTones** » Thu Jul 03, 2014 4:50 pm

http://forum.metroamp.com/viewtopic.php ... 40#p287143

sickman82 · Post by **sickman82** » Thu Jul 03, 2014 5:53 pm

Must have missed that diagram somehow. Thanks man.

That's exactly how I had it in my mind.

CoffeeTones · Post by **CoffeeTones** » Thu Jul 03, 2014 6:37 pm

Add a 2k2, 3k3 or 4k7 resistor in series with the presence cap to simulate the preferred pot setting. Then put that series combo in parallel to the 4k7 PI tail resistor.

sickman82 · Post by **sickman82** » Fri Jul 04, 2014 4:45 am

Thanks for the tip. Going to get this done this morning I reckon. It will certainly come in handy. I have 500pf and 5000pf bright caps on a switch, but rarely use them because well.. it's really bright! But the cut knob should now enable me to make use of them. Should be a pretty versatile amp. Thanks to all involved with this thread, you have all provided invaluable information..

sickman82 · Post by **sickman82** » Sun Jul 06, 2014 4:11 am

I've decided to redo the Lar/Mar mod as the previous owner hasn't done a very tidy job at all. In fact, the 'shielded' wire they used is just two covered wires and one stripped wire for the shield with some loose fitting heat shrink in purple! It doesn't look great. So, just a couple more quick questions..

1) The grid resistors - at the moment there are two 1k5 resistors between the output valves (see picture below), I've read that a 5k6 is recommended for each valve. Is this true for both 100W and 50W Plexis? The only reason I ask is because on a diagram earlier in the thread, it seems to say that it's dependent on 50W or 100W (I've outlined this in the red box). I would assume that 5k6 on each valve would be better, but I just want to be sure.

: 683px-Lar-Mar_PPI-MV.jpg (87.44 KiB) Viewed 3235 times

2) With regard to the cut control, I'm assuming that the wire needs to be fairly beefy as it will be seeing a decent amount of voltage on the PI Plates. Would the RG174U wire be a good option here or not? I'm asking because in the reissues I will have to either take the wires a long distance around the PCB, or take it over/under the PCB as the plate connections are on the opposite side of the PCB in comparison to the PTP boards.. the latter being my preferred option, but of course I would like to shield the wires.

Take a look at the amp below. These pictures were taken before I bought the amp, a hell of a lot has changed since then!

026.jpg: (274.23 KiB) Downloaded 1099 times

CoffeeTones · Post by **CoffeeTones** » Sun Jul 06, 2014 10:09 am

I'd use 5k6 to 10k. 50W or 100W amp doesn't matter as these are grid stoppers, The ones you likely read about are the output grid resistors (normally 220k in the PI, for both 50W and 100W EL34 amps or 82k for 100W amps / 150k for 50W amps using 6550 tubes.

I'd just buy a $10 Lar-Mar kit that has the right wire or use a shielded pair with ground and 20 to 22AWG wire.

Cut control - 20 to 22 AWG wire

SteadyEddie · Post by **SteadyEddie** » Fri Jul 18, 2014 2:14 pm

George says the PEC pots make a difference, and I trust his ears. So I ordered one.
But what about the resistors? Will precision metal film resistors on the pots make a difference?

sickman82 · Post by **sickman82** » Wed Jul 30, 2014 3:19 am

Ok.. so I have a problem. I was at a jam with my new band last night and past a certain point on the master volume I got a pretty nasty sound when I play. Up to around 2 o'clock on the master it sounds great, but past there when I play anything there's this awful crackling/fuzz sound. Any ideas?

CoffeeTones · Post by **CoffeeTones** » Wed Jul 30, 2014 8:19 am

Check for cold solder joints and make sure the PPIMV wires are not in proximity to other wires that they may pick up interference from. I assume you used shielded or twisted wire pairs to and from the PPIMV. Other than that, the pot may have a problem.

sickman82 · Post by **sickman82** » Wed Jul 30, 2014 8:39 am

I'm going to hazard a guess at parasitic oscillation. I still haven't wired up the MV with shielded cable as it's been on backorder for a while. I've found some elsewhere now, so that should be here by the end of the week.

Can somebody please put a photo up of their wiring, I just want to see an example of good wire placement for the MV, NF and Transformer wires. Whoever installed the Lar-Mar in this amp tied it together with a bunch of cables including the NF wire. This is not good!

Thanks guys, and a photo or two would be appreciated muchly.

mean_street · Post by **mean_street** » Thu Sep 04, 2014 4:20 pm

Hi, I originally wanted to do my first post here after I have installed the PPIMV in my 1959 HW sucessfully, but I have some problems with it... The PPIMV works correctly as far I can tell, but there is some high pitched buzzing coming out. On a low seting its only noticeable when I stop playing, when I turn the PPIMV higher, the noise becomes louder than the guitar signal. When I play with the volume pot rolled back on higher PPIMV settings, the notes get "swallowed" (sounds like a noisegate with wrong settings).

Here are some photos of the PPIMV.

https://imagizer.imageshack.us/v2/827x6 ... H9MRXS.jpg
https://imagizer.imageshack.us/v2/827x6 ... Wim7Qy.jpg
https://imagizer.imageshack.us/v2/827x6 ... WaVkux.jpg
https://imagizer.imageshack.us/v2/827x6 ... MXkdX1.jpg

I appreciate any help! Thanks!

CoffeeTones · Post by **CoffeeTones** » Sat Sep 06, 2014 11:00 pm

To begin, move the red and black wires leading from the PI output, away from the PI input components and the NFB connection. Then re-arrange the lead dress where you have the red and black wires routed above the rectifier and OT wires. Be safe, but move the wires around with a wooden stick, during operation and you'll probably find the noise changes or goes away. Your bias voltage is running through the uninsulated shield wire which shouldn't touch anything conductive. I've read of that method being used, but I'd run a new / separate wire for the bias supply and run the shield wire to ground. It also appears that you have the purple NFB wire running in close proximity and in parallel to the heater wires. I'd change that. If all else fails, swap the positions of the red and black PPIMV wires at the power tube sockets.

mean_street · Post by **mean_street** » Fri Sep 26, 2014 3:12 am

Thanks, everything is working now! I moved the shielded wire around and done some lead dressing.

Benjammin · Post by **Benjammin** » Thu Oct 02, 2014 9:49 pm

Hey everyone, i just got my lar/mar ppimv kit for my jtm45 and i was wondering if anyone could post some pics or give some detailed instructions on how to install it. Ive searched for images online nut theyre all for 1959slps and 2204s, thanks

CoffeeTones · Post by **CoffeeTones** » Thu Oct 02, 2014 10:05 pm

Not a JTM45 but the install is the same http://www.rig-talk.com/forum/viewtopic ... 00#p968290

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