Metropoulos Forum

I just bought an original 68 jmp 50 lead. It's in the UK so won't have for while yet.
I didn't buy this as a collector I plan to use this a lot. It's been serviced, even used in recent studio session.

Questions, When it arrives should I not even risk trying it out before adding Screen Resistors & Grid Stoppers as added protection in case something fails. And my tech doesn't have tester to check filter caps for leakage and operation under high voltage.... I'm thinking change them for safety, better new filters & original transformers being safer?? I would still have the caps and can get checked and potentially put back in if needed. I think at least the board mounted preamp filter cap for starters.
The amp does work though, I have a variac and a Brown Box so it won't get too hi voltage either.

I'd just like to plug in and play a little before it hits the tech. It's just not worth damaging it though if you think I should hold off as some I see do......

Thanks and I'll have pics by the weekend I believe.

Add the screen resistors and swamp resistors, then play all you want, but I would test it as in, use it before you work on it.

Thanks! I'll be sure to post more once I get it. Yeah I really should be able to give a quick run through it playing and then straight to getting those resistors added. Already have all the parts, and I have enough NOS caps and resistors to replace 1 of everything in there.
Can't wait to get this thing it's my dream Marshall! Like most I like all diff ones but I think for me if I'm going to for now only have 1 this gives me plenty of gain & I get my clean rolling guit vol. I was eyeing a 68 bass and I'm 100% sure it was all original as well but there was no bright ch mix capacitor? I know Marshall did some random things. I like bass models certainly but I passed and went for the lead. I'm happy

Congratulations.
Do you have pics?
Maybe we can tell you more about it...................

I know that some of the 50 watt models had a flaw in the bias supply wiring that caused many PT's to burn up over the years. It's a simple fix that requires moving the bias supply wire so if you can post a few pics we can let you know if it needs it or not.

I take it you were able to return the '72 JMP 50w with the replaced OT? If the '68 JMP 50w "BASS" you looked at didn't have a Bright channel mixer capacitor then it probably was a P.A. model (model 1985).

danman wrote:I know that some of the 50 watt models had a flaw in the bias supply wiring that caused many PT's to burn up over the years. It's a simple fix that requires moving the bias supply wire so if you can post a few pics we can let you know if it needs it or not.

A 68 should not have that issue.
The 1202-118 amps took feed from the HT on the PT.
It was in 1970 when the -164 and diode block came in that the problem started.

All the detailed photos are coming & then payment. It's fine because the buyer protection I'll have on it from being listed as all original transformers ect. would do just fine. & I'm more than willing to work and put money into a 68! I've done tons of research on this stuff over past few months and will be more involved doing work myself. Big part due to the forum obviously since so many past posts to check out. I feel the 72 was a good crash course for me with older marshalls. Defiteily excited and i'll post more!

Tazin wrote:I take it you were able to return the '72 JMP 50w with the replaced OT? If the '68 JMP 50w "BASS" you looked at didn't have a Bright channel mixer capacitor then it probably was a P.A. model (model 1985).

I agree, it felt like something was up and serial number wasnt' being shown which was kinda odd.

Need some help before amp arrives (and I'll have photos & details all when does).

Voltage? Has the voltage selector, I've heard some use 110v tap , some 120v here in US.
I have a Brown Box so I can bring line voltage down to 110v from wall too, so I'm thinking 110v tap?
Just for trying it out when I get it, then I'll certainly re-bias to voltage I choose to run at & add the screen & grid stopper resistors ect. But I do want to hear the amp least a little when I get it!!
(It was apparently used on the latest Stone Roses Ep in studio so fully working.)

Correct thing to do is to get your amp set up with your brown box and cab, select 120v and then measure your heaters.

Adjust to 6.3vac

Mark the settings on your brown box and check the bias there and then.

Don't be tempted by being impatient

Screen resistors do affect the tone (as can grid stoppers). If it does not oscillate now leave the grid stoppers off you don't need them, if it is as original as you say these things hurt value. The screens are a must with current production tubes but if you have boxes of old NOS Mullard xf2s then I would leave them off.

I recommend screen resistors even with original xf2s, unless you want to risk redplating and damaged tubes. At least that is my experience with a 445v plate voltage at idle

neikeel wrote:Correct thing to do is to get your amp set up with your brown box and cab, select 120v and then measure your heaters.

Adjust to 6.3vac

Mark the settings on your brown box and check the bias there and then.

Don't be tempted by being impatient

Screen resistors do affect the tone (as can grid stoppers). If it does not oscillate now leave the grid stoppers off you don't need them, if it is as original as you say these things hurt value. The screens are a must with current production tubes but if you have boxes of old NOS Mullard xf2s then I would leave them off.

Thanks! Definitely going to add the screens. Ok I will use the 120 tap. I had read something about the 110 tap bring preferred, that's only reason I asked. Yeah brown box must be used to set up bias on amp & check B+ ect.

I know it's safer to set it all up right first! I was just anxious to hear it little. Honestly not worth regret of a worst case scenario problem though.
I have just one NOS set of 1969 mini watt xf2 but my 72 wasn't right, and the left valve red plated slightly & I caught it immediately, only with amp on 10 & hitting with fuzz. (No further trouble shooting due to OT non-original)
So I may have to see about getting guy I know with tube testing equip that runs at full voltages to see if damaged. I tend to think it can handle it, maybe shortened life bit but was slight & very brief.

Any preferences on filter caps? I probably should replace them. Id rather have non original filter caps & be safer with original transformers.
Friday I should have all the info, detailed photos ect.
Thanks guys!!