I doubt a tube sounds more like a capacitor discharging. Does this happen every time or only the one time. How old is the amp?
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Exactly what kind of noise? Pop cracking fading ocean wave? Usually tubes make their aches and pains in the rear known when full power to the grid is present not de-energized.
Im gonna say for an experiment switch the speaker leads to the opposite channels and see if the noise follows to the the corresponding speaker. Lto R R to L This will eliminate the speaker for starters.
Well, do the speaker lead swap to eliminate the speaker. If the speakers are eliminated then put the leads back to the correct L & R connections and swap the amplifiers input IC's from the preamp now if the sound is still in the same speaker we have determined it to definitely be an amplifier issue and im gonna say a cap is acting up.
For now I would let the symptoms develop and become more pronounced because I may not rule out a tube completely as a failing tube has a way of playing havoc on other components such as caps,
Go through your tubes with a tester for starters but thats usually a long shot
What kind of power output tubes are we talking about?
You should swap the tubes between the left and right channel; if the sound moves, then it is in the tubes and not the capacitors. If it doesn't move, I would suspect an electrolytic capacitor. If it moves, then swap the small signal tubes to determine if it is those tubes or the power tubes. Keep doing the swapping until you figure out which specific tube is at fault.
I've had several small signal tube (6sn7) that made noise (popping and whistling noises) on power down, but were otherwise quiet.