1. Problem - No sound and tubes don't light up
Areas to check - amp unplugged, fuse blown, bad power switch, bad primary winding on power transformer
2. Problem - All the tubes light up but no sound
Area to check - Nearly always a bad rectifier tube - just because the filament lights doesn't mean the rectifier is good. Check the bias voltage - A no bias situation = bad rectifier tube
3. Problem - Fuse blows immediately upon turning the amp on
Area to check - Nearly always a bad power transformer winding or one of the windings is somehow connected to chassis ground
4. Problem - Fuse blows about 10 to 30 seconds after turn on and/or sparking inside rectifier tube
Areas to check - bad rectifier tube, bad DC high voltage power storage (usually a bad quad cap), bad (shorted inside) output tube
5. Problem - Amp has a hum that doesn't change in level as you advance the volume control of your preamp
Areas to check - bad quad cap, bad ground inside the amp, bad tube filament that is leaking AC into the audio
6. Problem - Hiss, noise, crackling sounds on idle
Areas to check - Noisy driver tubes, noisy resistor on driver board (especially older carbon composition resistors), bad solder joint or bad solder trace on the driver board. Also bad contact between the tube pins and the socket pins. This is more likely on a driver tube than an output tube because the driver tubes have less contact surface area on the pins. Remove the driver tubes and clean the pins with contact cleaner and then reinsert the driver tubes into the sockets again.
7. Problem - Popping or snapping sound
Areas to check - Usually means that high voltage is jumping to chassis ground somewhere. Could be anywhere even might be inside a tube
8. Problem - "Motorboating" (audible osscillating sound)
Areas to check - bad quad cap or other electrolytic or coupling capacitors
9. Problem - All tubes have a low bias that can't be turned up high enough
Areas to check - Weak rectifier tube, weak quad cap, bad bias circuit capacitors or the high voltage AC winding on power transformer is bad
10. Problem - One output tube or output tube pair will not bias
Areas to check - One or both of those output tubes are bad or that (those) tube socket(s) has a problem (usually pins 5 or 6 on an octal socket). Also check on VTA amps (with the amp OFF) that the 10 ohm resistor that goes from pins 1/8 to chassis ground does measure about 10 ohms.
11. Problem - Amp makes a "crackling" noise at high volume levels or on musical peaks (especially bass peaks)
Area to check - Not enough DC power storage - replace the quad cap
12. Problem - Unequal volume on the two channels of a Dynaco stereo tube amp
Areas to check - weak tube(s), bad cathode resistor, bad resistor in the negative feedback line, bad output transformer. On VTA amps, try replacing the LM334 current regulator on the driver board for that channel.
13. Problem - One channel cuts out completely or much lower volume than the other channel
Areas to check - Bad solder connection, bad solder trace on driver board, bad coupling capacitor, bad LM334 current regulator on VTA amps
14. Problem - Low volume on BOTH channels - try a new rectifier tube
More detailed tube amp debugging at the link below ..
Tube amp debugging link
If you like a flow chart for general troubleshooting of all amps and preamps see the flow chart below ..