Tube amp problem... I need help!

[Tony Allgood]

Hi all,

All this talk about tubes makes me feel that I may be able to pick your
brains on a little trouble I am having with a Marshall 100W head. It has
four EL34s in the output stage. It came to me with 'blowing fuses
continously'. I ran it up on the bench, via a variac, and found two of
the tubes, the top pair, were running red hot. You know that glow, light
up a small hall type of thing. Checked the bias voltage, all was OK on
all tubes. Marshall amps are set to be biased at -40V. According to JMP,
there is no need to set individual current. All other voltages seemed to
be OK. Took out the tubes, cleaned the sockets, and put them back,
deliberately in the wrong order. Again the same two tubes glowed red
hot, even though they were in a different place. Duff tubes, I thought.
So I replaced them with another pair of Marshall EL34. Rechecked the
bias, and all was well, even after a long run on the bench. No bias
balance on standard Marshall's, so I thought it should be OK, having
unmatched pairs in there.

But after a long run the older pair started to glow again. Replaced all
four with a matched set. All bias present on tubes and no tube getting
too warm even under load. So it seems OK now... but... The thing is,
surely all four tubes can't go at once, or is there another subtle
failure mode that I haven't understood??

Any help is much appreciated.

Regards,

Tony Allgood  Penrith, Cumbria, UK

MidiDAC Superior midi-cv, SuperLadder, TB303 clone and Filter Rack

http://www.techrepairs.freeserve.co.uk/projects.htm