My M101 has had a hum for some time and although I replaced the large
electrolytics and the rectifier bottle (sorry tube) I still cant get rid of
it .. With a great deal of care <ouch> I have measured the output from the
cathode of the rectifier and I suspect I have a heater to cathode leakage.
Why do you say that? (Do you mean heater to cathode leakage in the
rectifier? That doesn't make sense --- the 5U4 does not have a
separate cathode: the header is the cathode.) (If you mean heater-to-cathode
leakage in some other tube, that could well be the cause of your hum
(see below.))
Are you sure the hum is coming from the preamp/amp? Does the swell pedal
affect the hum volume? Does the hum get better/worse as the preamp warms
up? If you ground the inputs to the preamp, what happens?
If you're sure the hum is coming from the (pre)amp I'd check the rest of
the tubes, particularly the finals. If you can get your hands on known
good spares, the best thing to do is swap them out one at a time (except
swap the finals as a pair).
See also
Not having a clue how to check that I wondered if it was worth (or even
dangerous) to use a bridge instead of the old bottle rectifier.
You can, but that's a whole nuther kettle of fish --- IMHO, it's
not worth the effort and worry. If you just replace the rectifier
with a pair of diodes, your B+ voltage will go up (probably
doing bad things.) Also the rest of the tubes see full B+ will before
their cathodes are warmed up: this is known to be bad for the life
of the tubes ("cathode stripping"). So you need to at least add a resistor
(to drop some voltage) in series with the diodes, and possible add some other
stuff to delay the "turn-on" of the B+ supply.
I have having difficuly getting hold of good quality valves
and the last replacement I tried in it started to spark over on power up.
They shouldn't be that hard to find (though you probably will need to resort
to mail order.) Where are you located? If you're near the US, see:
for several good sources of tubes.
Sparks are definitely bad :-) --- either that tube was already bad (or was
one of the real el-cheapos) --- or it might indicate further troubles
with your amp.
Jeff
PS Note to Guillermo who's thinking of buying a Trek preamp to cure hum in
his A-105:
I've heard nothing but good about the Trek preamps. Nevertheless, replacing
the preamp solely to fix hum seems rather extreme. Unless you're absolutely
sure the hum is coming from the preamp, it might not even fix the problem.
Why not just fix the preamp (or have it fixed)?
Geoffrey Dairiki