'72 Marshall Jmp 50 Organ

[Rasputin1176]

Yeah now comes the fight to return it after no one noticed and was sold as original

[Rasputin1176]

Put too much money into amp combined with price paid to keep it. And still not sounding or operating that well. Damn!!!

[Rasputin1176]

For those interested,
I again have (definite keeper now) the '72 jmp 50 back!
The seller agreed the OT was replaced as said & since still was a '83 Drake of right model I was willing to take amp at but at less than full price. Got over $1000 off price so I took it.
There's few online clips of 72 jmp 50 lead I love so for $ I'm happy!

I tried going more '68 style with NOS parts: 47k NFB to 8ohm, added v2a .68 mustard, lesser filtering on preamp to 32+32uf. All NOS Mullards. But it was harsh.

After hearing those clips I like put it back stock.
Replaced tube sockets with nos Amphenol brown w/ 1k 5w grid & 5.6k Carbon comp swamp. Also the filter caps were F&T and after some decent use kinda stiff so since had to change preamp value back to 50+50uf I used ARS for all 3 filter caps.

It's sounding really good now. I figure I could have paid more for original amp & still ended up having to replace or repair things for more $. I was very familiar with this one already too. So worked out

[Rasputin1176]

Also, before doing the work on it to get back stock ect the Mains fuse blew. It was not tubes. & I and tech both found no issues. With new fuse it worked 100%. ??

I know it's listed as 2amp fuse on back of amp However it's a UK model & that's rated for 220v use. I understand at 120 in US the fuse technically should be 3-4 amp which I see on many 50w el34 amps here. Normally u never want to put higher rated & prevent fuse from doing its job blowing to protect.... but in this case we opted for 3amp Mains.

So no idea what caused it. Least it did its job. & with recent work the amp is safer already. See how goes

[RockinRocket]

Never take your amp to a "tech" whos not familiar with the amps being worked on.

Unnecessary work (devaluing) by adding the bypass cap.

Glad you like it though!

[Rasputin1176]

Yeah I hear you! Luckily in this case with the OT replaced that devalues these quite a bit already. I have new guy now anyway who I like a lot. Once I decided this would be an amp worth just keeping I decided to replace the sockets. I have all the original parts & stash of NOS to replace anything really. But yeah I agree shouldn't have been changed...
It seems to be what I read though as far as trying to change spec on an amp. The 72 sounds good as a 72. & im really liking it that way.
Hope to start doing more work myself, I enjoy learning

[RockinRocket]

Interested in what you paid before you got a refund.

[Rasputin1176]

Interested in what you paid before you got a refund.

Got it for $1000 was originally $2000 when I was under assumption was all original

[Rasputin1176]

Any help here, or I can post in diff section....
Nothing wrong necessarily but since this is my only Marshall, how compressed are these? It's got 100k NFB & some nos Mullard xf2. So I know the power section is going to get gain & compress some.

I'm curious because full volume it's pretty nice sounding to me pretty accurate for what I've heard in 50watters.
I do have a Rivera Rock Crusher & it's tone stays basically in tact attenuated but it gets very compressed sounding.
(You ever pick hard directly over the bridge pickup on Strat it gets that thumpy compressed sound) it's kinda like that unless I play real light. I assume it's just attenuated you hear it more?? Chords are nice but single notes g string and up pretty damn compressed. Again loud I either don't notice as much or who knows?
Oh I should mention i am jumpering chs. 1 hi > out 1 lo > 2 hi. Mainly because a video demo of my exact spec & year amp sounds killer that way. Non attenuated I feel I nail the sound very close. But I feel that Normal ch is what's adding the thump & compression when attenuated? The more bring it up more it does it. I have backed it off.


other than that the amp is quiet operation now, it had the snubber cap leak & I believe the leak made contact because fuse blew. Took care of leak removed old snubbers & good to go. But it likely failed open, if not don't know if PT was stressed?

[neikeel]

First try just using hi channel 1, but dis unused volume up to 5 (it will add some thickness even not plugged into it)

For less compression, more open sound look at 47k on 8ohm tap NFB.

If you want more gain/upper mids go with 820R and 0.68uF on V2a and don't dial the volume too high (6-7 to retain the touch dynamics)

[Rasputin1176]

Thanks!
I swapped few diff PI tubes & that seemed to change it bit. I also had ch 2 vol little too far up. I still say Unattenuated it sounds like what it should. The Riverra Rock crusher doesn't alter tone much but I think at low settings u just hear it differently.

This clip of 72 jmp 50 lead is pretty spot on & I love the sounds

[Rasputin1176]

The only issue I know of, is slight red plating on v4 power tube closest to PI.
It's been biased few diff times few diff tubes & always does.
But to be clear, will not red plate unless amp is on vol 8 or higher & a pedal like fuzz slams the amp. Even then has to be really dark to see this happen, it's very slight & im sure it's red plating but very slight & stops when playing with pedal does. So it's under extreme conditions.

I do not want to kill good nos xf2 mullards though either. I want to start checking these things myself at least with a good bias probe. Hopefully no issues until I pick one up