JMP - missing resistors

[Joris]

I have a JMP combo here (50watt lead model) for a general maintenance check. The amp is not entirely stock, and I noticed (besides some minor preamp mods) that the powertube have no screen grid resistors (normally 1k/5watt) and screen grid stopper resistors (normally 5k6) on the tube sockets.

Question: is simply a mistake of some previous repair guy or might this be a modification of some sort? The B+ without load is fairly low: approx. 395VDC. I think I'll just wire the poweramp back to stock values, but maybe I'm missing something.

[pdf64]

The early marshalls with aluminium chassis and GZ34 didn't have screen grid resistors. Perhaps it was modded to be like those?
Maybe mullards of that time could cope, but modern EL34s can't, I'd think it would go through them pretty quick.
Best to fit screen grid and grid stopper resistors. Peter.

[dai h.]

do you have any pictures?

Some of the early ones were as you describe with no screen resistors or grid stoppers, relatively low B+ (i.e. came stock like that).

[oskar]

It's fairly common on 50w models (at least in europe) ,I see many JMP from early and late 70's and many 800 without the screens and grid stopper resistors , and this B+ range , .
My recomendation is to put them , the new current production tubes dont survive without them.

Regards
Oskar

[Joris]

Thanks for the replies: the photos can be found here: http://picasaweb.google.com/j.graaumans/MarshallJMP#

The poweramp is probably original: the solderjoints look fairly old and the amp still had Philips EL34 tubes. I'll add the resistors though, thanks for the help.