Reason for whooshing sound when turning an amp off.

[Mark]

I was recently asked why some amps make a quietish whooshing sound on power down.

I could say what I thought it could be, but I don't know for sure.

Anyone have experience with this sort of thing?

[Stevem]

I will offer up a guess, and someone please correct me if I am wrong.
My guess is that killing the power supply brings up the noise floor and what your hearing is called shot noise, the actual noise of the electrons that have left the cathode of the tube(s) and slamming into the plate.

[martin manning]

It seems to be associated with failing tubes, and of course plate current damping up or down, but I don’t know what the mechanism is.

[Mark]

Thanks for your replies.