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Replacing output valve
Old 25th May 2003
Gear Nut
neilsby's Avatar

Replacing output valve

Hi there,

I think a valve in my 100watt Marshall JCM900 has decided to give up. The heater for it no longer heats up and there is a white precipitate at the top where the metal conductor used to be. I'm presuming the valve has leaked air into it, i haven't had experience with this type of thing before.

Anyway, i need a replacement matched pair of harma STR 6L6 valves but i've been told that the amp will need to be rebiased. Is this an easy process to undertake, i'm fairly adept at fixing things, i managed to replace a pot on the amplifier.

I also saw a pair of valves from groove tubes, they claim to be self biasing but they are very expensive, does anyone know of cheaper sources for these valves? I wouldn't say no to swapping to EL34s or 84s.

Any help on this subject would be very much appreciated before i blindl hand the amp over to an engineer to be fixed.

Thanks a lot, Neil.
Old 25th May 2003
One with big hooves
Jay Kahrs's Avatar
You can pull the tubes out and replace them with a new set of the same type of tubes. It's like changing a lightbulb in that respect. But, you really should have the amp rebiased by a tech. It's not that expensive to have done and the amp will thank you for it with better sound and longer power tube life. There was a thread on amp modding a few weeks ago and biasing came up. The general rule of thumb was don't attempt it unless you have a fluke meter an know what your doing. There's around 400-500 volts on those plates, enough to kill you if you aren't careful. Spend the extra $40 to have a tech do it. Also, since it's a 100 watt amp you really should have all 4 tubes replaced at once. The amp will run better and usually if one tube goes the others aren't too far behind.
Old 26th May 2003
Gear Nut
neilsby's Avatar

Thanks a lot for the advice, i'm going to have the amp serviced by torres, they come highly recommended.
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