Best High gain Mod/ Master Volume for a JMP 50 MK II

[john l]

Hello everyone, new to the forum. Just got a 74 jmp 50 MKII that has a pre PIMV already installed that Id like to rewire to post master such as the one in the OPs pic. What value dual pot did you use here? The lar/mar sugests 250k but the TW pages have a dual 1meg and dual 100k version.

Hi John,

Please see the enclosed drawing. I made the PDF pretty large so you can zoom in to see the details. If you are running EL34s, you'll want a dual 250kA pot and a pair of 2M2 1/2W (or more) resistors and some dual conductor shielded wire.

Wow thanks colossal! Im just getting my feet wet with working on amps can you connect the dots on the board for me so I see where everythings running as a reference? Heres what Im working with now.

Edit wrong pic lol
[IMG:767:431]http://img.photobucket.com/albums/v299/ ... 341461.jpg[/img]

This is how I bought it, Ive since recapped the cans. As it stands the amp gets me ALMOST there with a boost as far as crunch goes but if it just gave it up a little more Id be very happy and I was told that a post PIMV like the lar / mar might get me there since id be knocking it down after the PI. What do you think?

[Colossal]

Wow thanks colossal! Im just getting my feet wet with working on amps can you connect the dots on the board for me so I see where everythings running as a reference? Heres what Im working with now.

This is how I bought it, Ive since recapped the cans. As it stands the amp gets me ALMOST there with a boost as far as crunch goes but if it just gave it up a little more Id be very happy and I was told that a post PIMV like the lar / mar might get me there since id be knocking it down after the PI. What do you think?

Hi John,

I've attached another PDF which will show you the region where the PPIMV goes. You are replacing the two grid leaks (220k/220k) soldered to your main board. You will also need to re-route the bias voltage to your PPIMV pot. I am assuming you are running EL34s, right? (I can't really read the color code values on from your photo).

The PPIMV will lower the volume of your amp. As the pot value is reduced (turning it counter-clockwise), the grid leak values decrease. This loads the PI more causing a bit more preamp distortion which, as you have said, may be enough to get the crunch you are looking for. It will reduce the volume but it does sound good and might be the happy medium you are trying to get between good distortion and not bringing the cops every time you plug in. I found a 50W Marshall build I did for a guy (with a PPIMV) to sound very respectable with the PPIMV at about 12 o'clock. Really crunchy and mean sounding, not really pure Plexi anymore, but I loved the result. Big tight low end with a fast attack and a nice crunch. If the PPIMV doesn't do it for you, you might still want it for the volume control but there are lots of other strategies to get more distortion out of the amp.

If you can solder by numbers, you can probably do this, but if you're not sure or confident that you can pull it off, take it to a competent tech. It's not too tough to do, but it's something you definitely want to get right the first time.

Regards,
Dave

[Analog Assassin]

Regarding the "tightness" of the amp, that could be due to the negative feedback. I like 47k/8 ohm myself. Tight but lively.