JTM30 bias and other tubes?

[fastfolkert]

I just acquired a JTM30 in, like it seems, the original condition. Unmodded and with original tubes. (5881)
It works and sounds good to my ears. I have not yet done any measuring inside.
However, I have two questions:

1 – I may stick with the 5881’s (perhaps throw in a new pair), but what I read here is that the bias (-39.1V) is fairly “hot”. I might add another 4 to 8 V zener to set it a little cooler. What is the approximate current through that zener-stack? (I have some 400 mW zeners at hand)

2 – I have a few NOS Philips EL34. Is it possible to put these in, and if yes, what do I need to do (set bias? Other?).

When I decided what tubes I use I will most likely keep it that way, so I think I do not really need a bias-pot.
TIA!

Schematics: http://www.drtube.nl/schematics/marshall/jtm3-61-02.pdf

[Enzo]

Before you go modifying teh thing, measure the tube current. That is the only way to determine if it is too "hot." Tube current and plate voltage.

[nickb]

Kinda old but I just came across this.

The current is about 1mA.

I have seen a few of these where the bias current sits over 60mA per side and with signs of heat damage. The plastic rear panel looking discolored and warped and the covering of the tube cage peeling off. I'd guess that Marshall used a particular spec of tube in production to keep the bias to a sensible level. I added a 100K pot in series with the bottom 9.1V zener and that allowed me to dial back the bias to 33mA per side. This loses the zener effect to some extent bad I don't think that is bad thing as you now track the mains better. At this level there was no evidence of crossover distortion. I mounted the pot on panel between the tubes and the power transformer and once set, locked in place with dab of (my favorite ) nail polish.

[fastfolkert]

Thx. I have (accidentally) started two identical threads.
The discussion and measurements on my JTM30 continues here: http://music-electronics-forum.com/t43465/