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First check all your solder joints and make sure they are all solid & shiny.
Check the voltages. Then:
Start at the Output end and touch the input to them with an insulated screwdriver, see if you get a noise/click/pop/hum - anything. If you do, move forward one stage, to the PI, etc. and trpeat until you dont get a noise any longer. Look around that area for the problem.
This Could be:
Faulty Power Tube(s) - swap to check if possible.
Bad preamp cathode resistor - An unbypassed cathode resistor
Faulty phase inverter - If for some reason the phase inverter input side is good but the inverted side is bad, the power amp will still work, but power will be very low. This can be a bad 1/2 tube, a faulty socket contact, a broken or open plate resistor or coupling capacitor to the output tube, or a bad solder joint on any of these.
Open cathode bypass capacitors in preamp - if they go open, the stage they're in looses gain, but does not otherwise fail. If they short, it dramatically shifts the bias point, and may cause distortion as well as low volume.
If you think that a preamp tube is bad, swap in a new, known good spare until you find the problem. Just swap and see if the problem is corrected. If the problem does not change, then the tube you replaced was not the problem.
If you dont have a spare, use another preamp tube of the same kind from inside the amp. Swap a pair of them, noting which tube is which so you don't lose track of which one you suspect. It is rare for 2 preamp tubes to go bad at once. You'll usually be able to tell if the problem moves around when you move one particular tube. If it stays the same, you don't have a tube problem, you have trouble with the circuits around a tube.
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Stephen
Web: www.trinityamps.com. Facebook: facebook.com/trinityamps. Twitter: @trinityamps
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