JTM45 Build

Littlewyan · Post by **Littlewyan** » Wed Dec 31, 2014 4:19 pm

I'm looking to build myself a JTM45 next year and want to get the spec straight. I've already got a JTM45 OT that is 6.6K so thats set in stone. After doing a lot of reading I think a 400V B+ and a 3H Choke are a good combination with the 6.6K OT. For filter caps I'll be using 32ufx32uf for plates and screens, 16ufx16uf for the PI and Pre Amp. If I start to get ghosting issues then I can fit a 16uf Cap in parallel with either the Pre Amp 16uF or the PI 16uF Cap.

What do you guys think?

Edit: If anyone reads about the issue I was having with this amp I can save you time and say here that it was not an issue. You cannot play these amps with the Bass, Middle on 0 and Treble on 10 without the PI going haywire.

David Root · Post by **David Root** » Wed Dec 31, 2014 6:28 pm

Sounds about right to me. The only schem I have is attached, but there were differences in the early days. Main difference I see is power tubes B+ @440 or so.

However, lower voltages on old Marshalls do work well according to lots of folks on this forum. 375 to 400 or so.

Littlewyan · Post by **Littlewyan** » Fri Jan 02, 2015 12:01 am

Yeah the originals had a higher B+ but like you say the lower voltages seem to work well on old Marshalls. I'll stick to that spec then, thanks

.

David Root · Post by **David Root** » Fri Jan 02, 2015 5:23 am

As an option vs. the 3H choke, you might like to try a 10H (the standard one in the schem is 20H).

Dave Cohrs (Colossal here) is an experienced Marshall builder whose opinions I respect. He told me he likes a 10H in most builds, but the context was Plexi and 2204, not JTM45. Slightly more sag and more importantly some nice bloom. I'll be trying that myself on my current build.

Triad makes one that won't break the bank, the C-7X, and it is about 250 ohms DCR so it will take the screen voltage down slightly more than the C1999 3H, which is ~100 ohms, which won't hurt. The C7-X is what I'll try as I have two of them.

Hopefully Dave will see this thread and expound a bit.

Littlewyan · Post by **Littlewyan** » Fri Jan 02, 2015 1:03 pm

I know Metroamp use a 7H Choke for their JTM45 amps. 10H sounds good as I think 20H would be a bit stiff, 3H might give me ghosting issues with the bass set too high so 10H may be just right. I have seen Triad Transformers but never used them. They look quite good.

Btw it was this clip that sold me on the JTM45. I never thought it would have enough grind for my liking, but going by this clip it has plenty. I think the 1987 50W is a bit too aggressive for my liking.

http://youtu.be/E7Vksg3gQ2g

This guy has gutted a reissue and rebuilt it with a new board, filter capacitors but kept the reissue transformers. So he should have 400V B+, 6.6K OT and I THINK a 3H Choke.

And Dave yes please chime in

David Root · Post by **David Root** » Fri Jan 02, 2015 5:43 pm

That is an excellent clip!

I hope my Dual 50 build will sound as good. (Plexi/2204 footswitched). I will be using smaller filter caps, 40/20/40/20/20/20, than the Plexi or 2204 used.

I'm also putting a FSWed Dynacomp mid-'70s replica compressor board in the amp which fits the genre I think.

Littlewyan · Post by **Littlewyan** » Fri Jan 02, 2015 6:11 pm

Never heard of a footswitchable plexi/2204 amp before, sounds conplicated! Not to mention having a built in compressor! Have you started your build?

David Root · Post by **David Root** » Fri Jan 02, 2015 6:30 pm

Yes. It's a slightly modified version of Steve Luckey's Dual 50 amp. I'm replacing the manual Plexi/2204 switch with a relay and another relay switching the compressor board into the input jack.

Here's a pic of the Dynacomp board. The board is a real '77 MXR board. I populated it with vintage correct parts.

Littlewyan · Post by **Littlewyan** » Fri Jan 02, 2015 6:53 pm

Nice! I see what you mean with the filter caps, your values should give it a slightly softer feel and less of a boomy bass.

norburybrook · Post by **norburybrook** » Fri Jan 23, 2015 10:15 am

I built a JTM 45 last year and it's an absolutely great sounding amp. Every guitarist that plays it is always knocked out. A very good guitarist friend stopped by the other day after we'd been out together socially and sat down to try it, he was still playing it 1 1/2 hours later

The surprising thing is how good the clean sound is on these amps as everyone things about the cranked OD sound.

I'm building a. #102 at the moment and it's going to have to be very very good for it to replace the JTM 45 with my Celestion loaded Marshall 4x10.

Marcus

statorvane · Post by **statorvane** » Fri Jan 23, 2015 11:23 am

One of the best JTM45 demos I have found: https://www.youtube.com/watch?v=kFWQr8O6BJE

Post by **Colossal** » Fri Jan 23, 2015 12:03 pm

Hey, sorry I missed this thread earlier. 7H is the usual JTM45 standard choke value. Nice balance between too fast and too much!

Littlewyan · Post by **Littlewyan** » Fri Jan 23, 2015 5:54 pm

I am really looking forward to building one. That is a good demo in that video.

Thanks for chiming in with that info Colossal.

joeboo88 · Post by **joeboo88** » Sun Jan 25, 2015 3:49 am

Hi. Would 7h 40ma be enough for a jtm45? Or isi it too small. I do have beefier 4h one. I have the measurements of tht one but not with me at the moment. Thanks

joeboo88 · Post by **joeboo88** » Sun Jan 25, 2015 3:50 am

Hi. Would 7h 40ma be enough for a jtm45? Or isi it too small. I do have beefier 4h one. I have the measurements of tht one but not with me at the moment. Thanks

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