Tube amp problem... I need help!

[Paul R. Higgins]

Responding to the message of <000c01bf4737$c3b73380$d01b883e at default>
from "Tony Allgood" <oakley at techrepairs.freeserve.co.uk>:

This is an interesting problem.  It sounds to me like ALL your original tubes 
might be flaky, if I'm reading this correctly.  Either that, or the bias is a 
little off (too much idle current).  If you swapped out the two EL34s that were 
glowing cherry red with new ones, and then the OTHER pair of original EL34s 
started to glow, I'd venture to say something was wrong with all four original 
EL34s.

I would recommend that you solder in four 10 ohm 1/2 W resistors, matched to 1% 
with a DMM, one from each EL34 cathode to ground.  This makes setting the bias 
point a snap.  According to Ohm's Law, the current-sense resistors measure the 
current as a voltage: 1 mA = 10 mV.  I do this mod for any tube amp that comes 
across my bench.  Then use the tube manufacturer's recommendation for idle 
current under Class AB conditions: my data are for a plate voltage of 500 V (a 
bit higher than some Marshalls), idle current for each tube should be 57 mA, 
corresponding to a bias voltage of about -44.5 V.  Maximum output power should 
result in a current of 112 mA for each tube.

My best guess is that the original EL34s were the "wanna-be" Chinese EL34s.  
There have been some problems with Chinese-made '34s that simply are not up to 
the demands of most guitar amps, especially Marshalls.  They sound great and 
work fine as long as they're not pushed to maximum power.  We're talking here 
about anything over about 25W/pair.  Maybe a little more downhill in a tailwind.
These Chinese-made EL34s are conspicuous due to their rather slim glass 
envelopes when compared to the more robust European-made EL34s.  Marshall would 
never put such wimpy tubes in their amps, but an uninformed player might have 
done so by mistake when replacing old, worn out tubes.  Few players probably 
know there's a difference.

The other thing that could be causing this problem would be excessive plate 
current due to inadequate bias.  This should be pretty obvious once you solder 
in the 10 ohm cathode resistors.  One or more of the cathode voltages should be 
way out of whack.  (As 1 mA = 10 mV, proper bias should be about 570 mV).  Also,
use a matched quad of EL34s in a Marshall at all times, unless you want to have 
to mod the amp for individual biasing of each EL34.

Oh, by the way--I've always felt that Marshalls would greatly benefit from a 
small fan to help with the thermal dissipation problems.  I've done this for 
every amp that I've owned (or modded for someone else) and I've never regretted 
it.
 
Hope this helps.

-PRH

 
> 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??


_____________________________________________
Paul Higgins
email: higg0008 at tc.umn.edu
University College, University of Minnesota
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