There are relatively
few causes of output tube bias problems. All of them
involve the grid not being held at a negative enough
voltage with respect to the cathode.
In
any amplifier:
If the output tubes have just been replaced with new
ones, "infant mortality"/early dying of the
new tubes. Leaky/shorted coupling capacitor from the
driver tube plate to the power tube grid. Note that
the chances are that only one of the multiple output
tubes will have this problem. Dirty, corroded or just
old tube socket not making good contact to the tube
grid pin Note that the chances are that only one of
the multiple output tubes will have this problem.
Eagle-eye the socket hole contacts with a magnifier
and try to see if they are corroded or dirty looking
or maybe the contacts have lost their springiness,
and don't make good contact. Try removing the tube,
squirting a little tuner cleaner into each socket
hole, then wriggling the tubes back in. You can also
very gently pry the contacts out so they have more
pressure on the tube's pins.
The socket could be broken. Rarely, a very poor or
inexpert rebiasing attempt by an unskilled
technician. Rarely, an output tube itself that is
shorted grid to cathode Note that the chances are
that only one of the multiple output tubes will have
this problem. Rarely, the resistance from the tube
grid to its bias supply will be open. This can lead
to "runaway" in power grid tubes. Measure
the resistance from grid socket pin to bias supply
point in fixed bias amps or to ground in cathode
biased amps and make sure that both (all) output
tubes have the same nominal resistance - usually 100K
to 220K, never more than 470K. If one grid has a lot
higher resistance to ground or to the bias supply,
there is a problem with the grid leak resistor on
that tube.
Even more rarely, a wiring flaw in the wires getting
the bias voltage to the grid. I have seen an
amplifier that came from the factory with all of the
wires inserted into the socket lugs and wrapped
around the contacts, but not soldered. It worked that
way for 20 years, and had intermittent troubles the
whole time.
In
fixed bias amplifiers:
If the tubes have just been replaced with new ones,
failure of the owner to get rebiasing done, or
"infant mortality"/early dying of the new
tubes. Failure of the bias section of the power
supply, especially the adjustment pot or the bias
supply filter cap
In
cathode biased amplifiers:
If the output tubes have just been replaced with new
ones, "infant mortality"/early dying of the
new tubes. Leaky/shorted cathode resistor bypass
capacitor. Rarely, a noninductively wound cathode
resistor with an internal short. Rarely, a broken or
incorrect wire.
