There's lots of info floating around on the net, some good, some bad. At Mojo, we are
not in the business of telling you to change out your tubes regardless.
You should know what the indicators are that a tube change might be in
order. Tubes can have one of the biggest effects on your amps
performance so changing them when a change is due can be very
gratifying in a cost versus effect comparison.
This one is tricky because, in older amps, it can also be due to the
output transformer. But, if only due to complexity, the tube change
makes sense to do first.
Tubes show signs of overheating, darkening of the glass
This can also be a sign of OT damage or decay or just overbiasing.
With any output tube change the bias needs to be checked but with this
one it is especially important.
Arcing over
Make sure and check the tube sockets and screen grid resistors if this
happens. Usually accompanied by B+ or mains fuse failure.
Bad tone
This one is highly subjective and if your output is not lowered it is
likely that your tone issues lie elsewhere - the preamp tubes may make
more sense in this case.
Looking for a different sound
Again, pre-amp tubes are probably a better choice for this but if you
have some experience and have heard a lot of tubes, the output section
can be a place to tweak your amp.
Cracked glass
No choice here. Pony up and get the best replacements you can.
Things that do not directly indicate time for an output tube change:
Blue light in the tubes that pulses with your playing When
tubes are flashed, the residual impurities in the vacuum are more or
less removed. The less perfect the vacuum was the more blue pulsing you
will see. By itself this indicates almost nothing.
Dirty tubes
Wait until they are cool and clean them. Your amp will run cooler and love you for it.
Humming, noise, hiss, etc None of these directly indicate power tube failure. If you don't know for
sure and your tubes are relatively new anyway (say 1 or 2 years of
moderate playing) then taking the amp to a tech may be less expensive
in the long run. Hum, for instance, could come from out of balance on
the output section. But that could be coming from a problem with
biasing or the OT or it might have nothing to do with the output
section and might be a crossed heater wire in the preamp or an
unbalanced hum rejection circuit for the heaters, failing filter caps
or any other number of things.
