[morzh]

Basically the question is - is the crackling present when you have NO TUBES in OTHER THAN 80?

Install 80, remove the rest. Use a known good one.

See if the noise starts even before the 80 is able to warm up, that is the second you turn it on.

Also see if this by chance your power switch doing the popping? Or the transformer? The 80 presents the filament load; without it there is no load when other tubes are pulled out.

So you could also look at the 80's heater (5V) winding and all the connections.

Also look at the panel of 80.

Basically the big difference is, whether the noise starts immediately on turning the power knob when the 80 is in, or it waits a second or two before 80 starts warming up.