Any (electronic) valve freaks out there?

My Marshall JCM800 50W blue a fuse.
New2 fuse holds but one of the output EL34s is glowing red hot, but the other isn't. Amp works, but hums badly.
That's consistent with it drawing a lot mire current of course.
Question: the valves are biased by a single adjustable pot. Is it conceivable that anything else bar the valve itself has gone tits up?
As far as I can tell the grids are isolated from the previous stage DC by at least two capacitors, and so capacitor failure is unlikely.
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The Natural Philosopher wrote:

Swap the valves over and see if the fault moves. Check cathode resistors for imbalance The hum is propbably just excessive current causing greater than usual supply ripple
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Bob Minchin wrote:

Hmm just seen that it has no cathode resistors oops!
if the network of grid bias resistor is all symmetrical, it cant be much other than the valves. Unless one of the two 22n audio coupling caps from the phase splitter has gone leaky?
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On 07/05/16 14:41, Bob Minchin wrote:

Thank you for confirming what I suspected.
Now where to source an EL34...at less than a small fortune..
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On 07/05/2016 14:50, The Natural Philosopher wrote:

There are equivalents.
I'm assuming you have swapped the valves and the fault stays with the valve. Can you check the amplifier works with the faulty valve removed? A nagging thought is that the 'good' valve might be drawing more current than it should!
Otherwise it implies one of the grids have gone o/c and floating.
Suggest you inspect the base if you haven't already. The internal wires should be soldered to the plug pins. It might be worth considering removing the base?
I would say this sort of fault sounds rare and more indicative of a loose socket/pin connection.
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On 07/05/16 15:21, Fredxxx wrote:

Very weird.
I swapped the bottles and at first everything seemed OK... a bit hummy. Then there was a pop and the same valve started to light up like a Christmas tree.
Nope. Its new EL34 time. something is shorted or broken in one of them
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On 07/05/2016 16:04, The Natural Philosopher wrote:

Sounds like just one valve conducting.

At which point, did the speaker output have more hum, or go silent?

I don't feel you have ruled everything out quite yet.
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On 07/05/16 16:20, Fredxxx wrote:

more hum

I think I have
Its definitely in a given valve, not in the base or bias cqtry.
And its not worth trying to fix a valve - simpler to spend £12.50 on a new 'un
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On 07/05/2016 16:43, The Natural Philosopher wrote:

I didn't realise they were so cheap. Probably cheaper, in real terms, than in their heyday.
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On Sat, 7 May 2016 16:43:05 +0100, The Natural Philosopher

Is it a push-pull arrangement - do you need a matched pair? Is it worth replacing both valves anyway, as the 'good' one is probably fairly old?
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On 07/05/16 17:31, Chris Hogg wrote:

yes, and yes probably. I would never call a Marshal combo 'hi fi' but its probably worth getting a pair.
Old? yes. Cant remember when I acquired this amp, maybe in the 1980s second hand...worth far more now than then.
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On Sat, 7 May 2016 17:52:42 +0100, The Natural Philosopher

http://tinyurl.com/h4j4p9f
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On 5/7/2016 5:52 PM, The Natural Philosopher wrote:

I have a much less illustrious Vox. So, are you a Fender or a Gibson man?
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On 07/05/16 20:39, newshound wrote:

Built myself a 'Mighty Mite' Strat. All from 'spare parts'..;-)
cost about £350, sounds like about £1350...
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Check with local ham radio clubs, usually a few valve nuts around in those places. The amp on my puter at the moment has two valves in it. Its made by rogers in the 60s. brian
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On 07/05/16 14:34, Bob Minchin wrote:

I will.

What cathode resistors ;-)

Yup.
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On Sat, 07 May 2016 14:49:20 +0100, The Natural Philosopher wrote:

If there's no cathode resistors then there has to be a negative bias voltage for the grids. If that goes missing for a valve, allowing the grid to float, then it will pass a lot of current. Not for long though! Bias can go missing sometimes due to a faulty bias pot (depends on the circuit).
It's a good idea to introduce 10R cathode resistors on fixed bias systems anyway, just to check the current through the valve by measuring the voltage across them. 1v across 10R will be the equivalent of 100mA.
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On 07/05/16 19:39, mick wrote:

Yes. There is a rectifier fed off the heater circuits and a smoothing circuit and a pot. However swapping the valves still results in the same valve showing up with a problem, so it cant be associated with the circuitry, just with the actual valve.

If it was less of an expensive amp than it is, I probably would.
I bought it with resale value in mind, and its justified that. I don't want to fuck with it.
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If its push pull does reversing the valves move the problem the other way around?
Brian
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On 07/05/16 20:47, Brian-Gaff wrote:

Basically yes. The red hot anode goes with the valve, not with the position.
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