Marshall 50w Jmp Ca 1970, Plexi Inside?

[Edwin Star]

hello!
i have this Marshall JMP since 20 years. When i bought it, the seller told me it’s the same inside as a plexi JMP. I recently had a look and now i‘m wondering how „original“ it is. i see a line out has been added and it has had a cap job. do you see anything else that is different from a regular JMP from that time? thanks a lot!

 

[stickyfinger]

Looks pretty good,
V1 cap is replaced witha .022 Philips 341 series that should be a .0022uf mustard.
Bright channel vol pot has a modern mica installed. Probably a 500p instead of 5000p.
Diode bridge has been replaced with modern diodes. Probably quieter now.

 

[neikeel]

as above although the white bias feed should come from a lug on the HT of the PT (i.e. unswitched side of the standby switch so the tubes get bias voltage on standby.

 

[Edwin Star]

thanks!
- shouldn‘t there be a cap together with one of the two 470k resistors, left of the newer yellow philips cap?
- do you mean the white wire connecting the diodes to the board?

 

[neikeel]

thanks!
- shouldn‘t there be a cap together with one of the two 470k resistors, left of the newer yellow philips cap?
- do you mean the white wire connecting the diodes to the board?

Yes you are quite right there should be, typically 470-500pF cap (either flat mica or tubular ceramic) you can see where the solder joint was disturbed when they removed it. Someone obviously wanted to tame the bright/lead channel with smaller bright cap, removal of channel mixer cap and the bigger coupler. Ideally I would put it back to stock but it is true some of these amps can be a bit spiky (unlike earlier plexi versions). I have experienced this myself and cannot fully explain why. As an example a few years ago I had a 1989 Organ model just like yours (mixer cap and ceramic Murata 100pF bright cap and WIMA 0.1uF output couplers. I wanted a lead amp so converted it with 100% correct NOS parts, it was harsh and spiky so I put it back to stock, much sweeter. I have had a few lead plexis and at the same spec they are much smoother - other than the OT and pots I cannot explain.

 

[stickyfinger]

I would recommend leaving the standby in the ON positions at all times since its wired to potentially blow the power transformer as mentiond by Neil.

 

[Edwin Star]

interesting, thank you! i found this layout. is it a period correct one?
if possible, i would like to fix the standby issue as well.

 

[neikeel]

Yes that is correct. You can wire it to one limb of the standby or how the plexis with lay down PTs do with the wire direct to one of the lugs on the PT.

 

[Edwin Star]

thanks for your help! i don't have a replacement for the 556p mixer cap. can i use a silver mica 560p 500v cap?

 

[neikeel]

The red tubular cap (Lemco) is actually ceramic and either 470pF or 500pF, you can use either value in silver mica or ceramic disc, there is not a lot of voltage there but any 500v cap will be the right size.

 

[Edwin Star]

Great, thank you!
Do you know if there is a source where i can buy original Lemco caps in that capacity?

 

[neikeel]

These will work:
https://www.ebay.co.uk/itm/6-pieces...552078?hash=item2391b71c8e:g:kXQAAOSwHHFY~M2c
I have some used ones spare but postage to the US is probably a bit silly?

 

[Edwin Star]

no gridstoppers, no screen resistors. should i do something about it?

 

[john l]

I absolutely would if I’d were you. There isn’t a new production tube around that could hang for any length of time with no screen grids. If your able to turn that thing up considerabley for instance if you use an attenuator I would highly recommend adding the 5.6k grid stoppers.

 

[ampmadscientist]

hello!
i have this Marshall JMP since 20 years. When i bought it, the seller told me it’s the same inside as a plexi JMP. I recently had a look and now i‘m wondering how „original“ it is. i see a line out has been added and it has had a cap job. do you see anything else that is different from a regular JMP from that time? thanks a lot!

Mr. Star, you need to install screen grid resistors.
If the resistors are not installed, the amp will blow fuses when it is cranked up loud...

This is due to a change in output tube design standards.
Let me put it this way: The tubes we have today draw more screen current, compared to older tubes.
The resistors must be added to limit the current.

 

[Edwin Star]

thanks! will install both resistors! great forum!