Question about popping Noise

[Derrick]

I recently acquired a used Audio Valve Assistent 20 Integrated Tube amp with 2 set of brand new RCA tubes. The amp seem to work properly. But every time I power off the amp, it made a popping noise. It's not real loud, but it does pop.

Base on my limited experience with tube amp, I thought this probably caused by a bad tube so I swapped all the tubes out with the second set of tubes. Now, the popping noise went away and reinstate with a little thump noise via the speakers (sort of a small puff of air pressure that push the speaker drivers outward.

Any thoughts or suggestions on what would cause this?

Thanks in advance,
Derrick

[Thatch_Ear]

I am going to make a wild guess and suggest you measure the voltage coming out of your wall.