1969 JMP 50w NAD, but....

[Seanxk]

Ok, so I've had this a little while now but had meant to get on checking it out before posting some pic's ( it's super clean ) and hard wired for 240v.
Anyway, all seemed fine and everything checks out perfectly, it's done a few months of studio work @ 4ohms and medium volume and performed very well indeed, until last week where it blew the HT fuse, stupidly or perhaps not I replaced the fuse and carried on all was fine until the next day where 1 hour into playing it went dead, fuse replaced and no good still dead.
So I find one power valve ( a very healthy Mullard ), on my tester it has shorted g1 and g2, Pin 4 & 5.
Another valve in the amp and all's well, but it's sidelined now obviously.

I can't see any problems in the amp but for the brown hunts has got hot and melted leaving a mark on the black ground wire pictured, now I go through my pictures I first took and this is exactly the same, so it's got hot before. Do these things melt like that or was it from the day it was built?.

So, before I go ahead and fit the 1K screen grid resistors etc ( which I really don't want to do ), I want to know why it blew first, was it the valve and just bad luck that it had an internal problem.

Any ideas where I should be looking first?, anything look untoward in the picture?, many thanks in advance.

 

[Seanxk]

Ok, new pair of Mullards, 425Vdc on the plates and biasing up well at 1.70Vdc to centre tap, been sat on the bench for 1.5 hrs now and all is well, no excess heat, no hum, going to run it again tomorrow with a signal.

 

[StuC]

I know someone else is going to mention screen resistors... I haven't added them to mine either, but I'm fortunate to have enough Mullards to see me out.

I have the same thing on the Hunts cap in my JTM50. I suspect it's a reaction between the PVC wire insulation and whatever the capacitor is encapsulated with.

 

[Seanxk]

Now that's what I like to hear, thanks Stu.

 

[playloud]

It looks so clean (red dye on the V4+5 pins even!)

Any chance of more pics?

 

[Quinny]

My '73 had similar symptoms recently, detailed here: https://www.marshallforum.com/threads/have-a-guess-whats-wrong-with-my-73.131713/

One of the EL34 sockets had a trace which allowed arc'ing between pins. Also added screen resistors - sounds heavenly again now.

 

[Seanxk]

Interesting read, all good on the socket front, next test is higher volume and a dark room.

Edit, a tiny piece of what looks like plating flash from a valve pin in socket V4 tucked inside slot. I didn't think Mullard actually plated valve pins?, it's not solder anyway.

 

[Seanxk]

Interesting the label didn't get a date or a stamp, but it got the orange sticker.
Must go and do some work now, enjoy

 

[neikeel]

Looks very good.
Pin 4 and 5 would be an unusual path to arc so internal death of valve most likely cause (xf3 or 4?).
I would not rush to put screen resistors on if you have good Mullard supply. Only bit that would concern me is the bias caps and the screen part of the internal filter cap - but I’m pretty certain that you already checked those!

 

[Seanxk]

Thanks Neil, unfortunately it was an xf2 and a very healthy twin getter at that. I do now think this guy was at fault, you never know if it had a drop sometime in its life etc.

Yes all cap's checked and re-checked, they all have extremely low ESR and discharge quite quickly. I will check the internal cap again, the bias caps according to my notes last year were 11.10uf and 10.84uf with ESR around 0.35 and today they measure the same.

 

[gtomax]

I'd err on the side of replacing the bias caps if I was in your shoes. I've seen plenty of those blue caps go bad and it's not worth risking your OT. It won't change anything in your tone and just keep the originals.

 

[AndyD]

+1 on the bias caps. Keep the originals to one side and put some decent new bias caps in. Really not worth the risk. Lovely amp, btw.