1970 JMP 50 WATT PA

Jack Lickson

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Hi everyone, I'm a new member and first time poster. I joined because
I saw how helpful everyone was to the person who recently posted about
a '70 JMP Bass. I've got a JMP 50 Watt with an inspection tag date of
5-21-70 and a S/N of S/A6800A. I'll post pics, and was hoping you
could provide some input on the following...

I am a long-time player but I know nothing about schematics. I
understand the terminology and what (most of) the parts do, but I
can't look at a circuit and say this or that has been modded.

Known Issues:

- The mains is fraying and needs to be replaced
- The power and standby switches do not match - one is a replacement (standby?)
- The knobs do not appear to be original - they just don't look right to me

Questions/Unknown:

- Why does it have four caps beside the rectifier block? I've only
ever seen two, but I've also never seen gut shots of a PA
- I see one replaced pot. Do you see other replaced caps/parts
anywhere? The large blue caps on the top of the chassis don't appear to have writing on them
- Do you see any mods?
- Do I need to add screen resistors for modern EL34s?
- Does the head shell look "off" to you? It's a 26" shell, but it has
no separate holes for chassis bolts on the bottom. The chassis
attaches with screw-in rubber feet. The foot and screw are one piece - you can't see the screw heads through the bottom of the feet.
- There is no evidence that the back panel ever had that white square ID panel attached to it
- The shell had piping on the bottom at some point but it's long gone
- The corners of the shell seem a little too rounded off to me
- The handle and back panel look normal and the shell has significant
wear. It could pass for period-correct but I have no idea if it's
original or not.

Thank you all for whatever insight you can provide. I learned quickly
that there are some extremely knowledgeable people on this forum and
I'm eager to hear what you have to say.

I have no idea if I posted photos properly - I attached an Imgur link.
I've never posted photos to a forum before. Apologies for my noobness.

https://imgur.com/a/alziEGA
 

neikeel

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Here are the pics:
ZtjultO.jpg

KWDYFYd.jpg

Qn3Wz2w.jpg

GU3bKDV.jpg

uiHJImf.jpg

Mg1fmQB.jpg


Originally a 1970 Model 1985 (PA) amp (solid brown fibre board, small Iskras, WIMA ouptut couplers.
First cathode bypass cap replaced (correct uF)
Replaced treble pot
Yep non stock knobs.
Filter caps look stock and I can't see any bubbles or electrolyte leaking.
The cab looks ok to me although usually the feet are on dedicated screws.
Big snubbers in series presumably to get the voltage rating (1000v is usual)
Both output sockets replaced and both switches replaced.
Still has the dodgy bias feed switched off the rectifier (should be dedicated wire from one of the HT ac lugs).
Yes you should move the yellow screen feeds to pin 6 (not used) and add the 1k 3w Welwyn screen resistors between pin 6 and the actual screen pin which is pin 4.
Feet are unusual (maybe modded to fit someones particular cab?
I had a very similar Organ Model (1989) that found its way to one of our forum members here a couple of years ago, not sure if he still has it?
 

JParry335

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I’m no expert and have nothing to contribute other than, welcome to the forum, I know someone who actually knows something will be along shortly, looks like a great piece to me!
Thanks for sharing with us.
 

Matthews Guitars

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cFGPkB5.jpeg



Those caps are DONE. Replace them now before they destroy your power transformer. Bulged tops, something oozing out of them by the looks of it....and what's with the double layer of shrink wrap? Never saw that before!

Those are really just a moment away from blowing out. Get them changed out with new F&Ts BEFORE you even power the amp up again. Totally not kidding. I've never seen any that were bulged that bad that hadn't spewed goo already.
 

Jack Lickson

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Here are the pics:
ZtjultO.jpg

KWDYFYd.jpg

Qn3Wz2w.jpg

GU3bKDV.jpg

uiHJImf.jpg

Mg1fmQB.jpg


Originally a 1970 Model 1985 (PA) amp (solid brown fibre board, small Iskras, WIMA ouptut couplers.
First cathode bypass cap replaced (correct uF)
Replaced treble pot
Yep non stock knobs.
Filter caps look stock and I can't see any bubbles or electrolyte leaking.
The cab looks ok to me although usually the feet are on dedicated screws.
Big snubbers in series presumably to get the voltage rating (1000v is usual)
Both output sockets replaced and both switches replaced.
Still has the dodgy bias feed switched off the rectifier (should be dedicated wire from one of the HT ac lugs).
Yes you should move the yellow screen feeds to pin 6 (not used) and add the 1k 3w Welwyn screen resistors between pin 6 and the actual screen pin which is pin 4.
Feet are unusual (maybe modded to fit someones particular cab?
I had a very similar Organ Model (1989) that found its way to one of our forum members here a couple of years ago, not sure if he still has it?

Thanks so much. Not sure how you got the pics to show up - it told me they were too large. Thanks a ton for the info - really appreciated. JL
 

Jack Lickson

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cFGPkB5.jpeg



Those caps are DONE. Replace them now before they destroy your power transformer. Bulged tops, something oozing out of them by the looks of it....and what's with the double layer of shrink wrap? Never saw that before!

Those are really just a moment away from blowing out. Get them changed out with new F&Ts BEFORE you even power the amp up again. Totally not kidding. I've never seen any that were bulged that bad that hadn't spewed goo already.

Thank you very much for chiming in. I've seen a ton of your posts and I know the experience you have with Marshalls. Really appreciate you taking the time. I will not fire it up again until those caps get changed.

What are the specs on the F&T's that I should get?
 

Jack Lickson

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I’m no expert and have nothing to contribute other than, welcome to the forum, I know someone who actually knows something will be along shortly, looks like a great piece to me!
Thanks for sharing with us.

Thanks for the nice welcome
 

Jack Lickson

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Anyone have an opinion as to the originality of the shell? As mentioned, there are no holes on the bottom for feet. The rubber feet screw in through the chassis bolt holes to hold it all together. The corners of the shell look a little more rounded off to me than other shells from the same era. It does seem to have the period correct 6" white/gold pin logo though.
 

TAZIN

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cFGPkB5.jpeg



Those caps are DONE. Replace them now before they destroy your power transformer. Bulged tops, something oozing out of them by the looks of it....and what's with the double layer of shrink wrap? Never saw that before!

Those are really just a moment away from blowing out. Get them changed out with new F&Ts BEFORE you even power the amp up again. Totally not kidding. I've never seen any that were bulged that bad that hadn't spewed goo already.

Those filter caps look like the original Erie 50uF + 50uF/450v cans. The double shrink wrap is somewhat normal but usually they don't cover that much of the can; only the bottom section. As for what appears to be residue on the top of the can is probably the black or dark blue sticker peeling up which Marshall put on there for safety reasons since the can is negative/ground.
 

Jack Lickson

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Those filter caps look like the original Erie 50uF + 50uF/450v cans. The double shrink wrap is somewhat normal but usually they don't cover that much of the can; only the bottom section. As for what appears to be residue on the top of the can is probably the black or dark blue sticker peeling up which Marshall put on there for safety reasons since the can is negative/ground.

The stickers are in fact peeling. I can't find any evidence of leaks, but they are 51 years old and I'm pretty sure that's far beyond their expected lifespan, right?
 

TAZIN

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The stickers are in fact peeling. I can't find any evidence of leaks, but they are 51 years old and I'm pretty sure that's far beyond their expected lifespan, right?

Without knowing the history of the amp's frequency of usage and sound it would be tough to determine their condition based on sight. They need a thorough checking to determine their condition.
 

Matthews Guitars

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Honestly I've never before seen Erie caps serving as the main filter caps in the circular clamps sticking out the chassis. I've only ever seen Dalys in that application.

So I learned something new today. Cool.

I'd assume a 50+ year old cap to be suspect until evaluated and found to be still good.
 

TAZIN

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Honestly I've never before seen Erie caps serving as the main filter caps in the circular clamps sticking out the chassis. I've only ever seen Dalys in that application.

So I learned something new today. Cool.

I'd assume a 50+ year old cap to be suspect until evaluated and found to be still good.

Erie Capacitors (England division) acquired part of HUNTS Capacitors in late 1968. A lot of the 1967 & 1968 Marshall amps used HUNTS for their filter caps as did some of the early 1969 amps. Somewhere around March - April 1969 you start to see the Erie caps being used by Marshall. The Erie caps are identical to the HUNTS with the exception of the name change. During this capacitor acquisition/change Marshall supplemented any capacitor shortage with either TCC, RS, or Dubilier of England capacitors. Erie's were used until 1971 when the Daly brand were phased into use by Marshall.
 
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