Advice on new JTM45 mods, please

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Advice on new JTM45 mods, please

Post by Drew3419 » Fri Jun 01, 2018 8:14 pm

Hey, I'm about to embark on a JTM45 build and I wanted some advice on a few simple mods I'm considering.

1. Lower value cathode bypass cap on V1 - my understanding is this will reduce the bass, especially when cranked, to cut some of the farty bass.

2. Bypass cap across V2a, controllable by push-pull pot on the Mids knob - my understanding is that this will boost the mids and increase the gain slightly.

3. Changing NFB value from 27k to something higher - my understanding is this will liven up the amp a bit, but that the Presence knob would cease to function if I go too high.

4. PPIMV - I'll definitely be doing this one, but not sure which type. I have one that George installed on my Metro 2203 build and it works well. No idea what type that one is either.

Thoughts on these mods? Am I correct in my descriptions? Should I do them? One of them? All of them? Any of those I should avoid or replace with something else?

Here's what I'm looking to accomplish: I really like most JTM45 examples I've heard, but some do seem to have (too much) farty bass. Some seem super smooth and chewy and gainy when cranked (what I'm going for), but some seem harsh and choked off... Watching the Johan Segeborn vid "Marshall Plexi Comparison JTM50 Vs JTM45 Vs Super Tremolo Vs JMP50 Vs Super Lead Vs Super Bass Vs PA" (, my favorites are the JMP50, the JTM50 (not No. 2, which is the worst one), and the JTM45 and JTM45 Super Tremolo. They sound the beefiest with the most character, but are still clear and gainy without being harsh. Also, this one sure sounds good: As does this one:

Here's what I DON'T want to do: Thin it out too much or neuter it. Take away the magic of the JTM45... I know the sag is part of the magic. I WANT the sag, otherwise I'd just do a 1987 or 1986. Don't want it to be harsh. Don't want it to be too boomy or farty.

My application: I will be putting this into a 2x12 1973-style combo cab (28W x 20H x 10D) with 2 Greenback style speakers (either Greenbacks or Scumback M75-PVCs or maybe a M75-PVC and a J75). I'll be playing the amp cranked with with PPIMV engaged as needed. I'll probably also get a small attenuator such as the Tubular Mods Juice Box 50 for those times when I want the PPIMV to be higher, or maybe a combination of both. I have some clean boosts, ODs, and fuzzes to use, but I'd prefer to get my sound from the cranked amp, then use my guitar volume to back it off from there. Since it's a combo, I'd prefer to not bring my pedalboard.

I'd love to get some advice from everybody on where I should start out. I know I can make some tweaks post-build, but I want to get off to a good start. Also... while I don't want you to say it... say it if you must... should I get a JTM50 or 1987 or 1986? :what:

Marshall JCM 800 2203x w/ Metro JMP 2203 board w/ Sozo caps, Metro Dagnall Replica OT, and PPIMV.

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