Fender 59' Bassman RI Issues

[Yurka]

The amplifier has really loud cracking and popping on idle, regardless of any kind of guitar signal. I have tried replacing all the tubes, problem still persists. Eventually I was able to isolate the popping and crackling to the DC Cathode follower driving the tone stack. The voltage reading on TP4(On the schematic, link provided below) would jump plus or minus 20-40 volts every few seconds, which was a quick giveaway. I have tested all the B+ supply voltages, seems to be okay. Ill be combing through the circuitry looking for failed components in the DC Cathode follower, also checking the pins of the tube sockets. Any other suggestions?

http://support.fender.com/schematics..._schematic.pdf

[MarkusBass]

Last time I had such problems with similar Fender (loud crackling), they were caused by noncontacting potentiometers.

Mark

[acorkos]

i have a 5f6a homebuilt amp that had similar popping/crackling....it went away only after replacing the 250pf treble cap