Before we get to the checklist, remember these basic rules:
Your power amp should always be the last thing turned on and the first thing turned off. Never turn your preamp on or off while your power amp is on.
Never connect or disconnect your speakers while your amp is on.
One channel is out on your system and not sure what's causing it?
Start with your speakers by swapping the speaker cables left to right at the amplifier. If dead channel moves to the other side, it is not your speakers. If the same channel remains dead, you have a problem with your speaker. Put the speaker cables back in their proper positions.
Now go to your preamp. If you have an integrated amplifier, move on to the next step. Swap the interconnects going between your preamp and power amp left to right at the preamp only. If the dead channel moves to the other channel, it's not your power amp. If the same channel remains dead, there is a problem with your power amp. Put the interconnects back in their proper positions.
The next step is testing your sources. If the same channel is dead no matter what source you're using, most likely your preamp is the cause. Let's say you have two sources: a CD player and a tuner. If both channels work with your tuner, but one is dead on your CD player, try moving the CD player to another source input on your preamp. If the same channel is still out, you have a problem with your CD player.
This checklist is also helpful for isolating noise that may be coming from a noisy tube in one of your components. The steps below will help you to isolate a problem tube:
Once you have isolated the component with the dead or noisy channel, try swapping all the tubes from left to right. If the offending channel moves, then we know at least one of those tubes is bad. Isolate the bad tube by swapping tubes left to right one pair at a time, until the offending channel moves again. Then we have the bad tube isolated and can get a replacement