[maxhifi]

I had this problem on an old DeVry movie projector amplifier. It turned out to be a defective plate resistor thermal cycling, it was fine when cool but when fully warmed up would go way high in value, which had the combined effect of making noise and reducing gain, then it would cool down and the whole cycle would repeat - annoying! Eventually it failed to the point it was crackling all the time and was easy to troubleshoot.

I would try to operate the amplifier so that the bottom is accessible and measure the plate voltage on all the small tubes, then try and catch it when it starts misbehaving and take the same measurements. My guess is the plate voltage will change drastically on one of the tubes or tube sections. I would begin with the first voltage gain stage, since if the problem was in the phase inverter it would more likely be an increase in distortion rather than a drop in gain, as one half of the push pull amplifier came and went. My guess is one of the resistors is packing it in after it warms up - would be much nicer to sort this out by changing just the defective part rather than a shotgun approach, RCA Camden is mostly torn down and the likes of this amplifier will never be seen again as a commercial product.

If the problem doesn't occur with the amplifier on its side (bottom of chassis well ventilated) try hitting the underside with a blast of hot air from a hair dryer, specifically near the input tube sockets to try and provoke it.