What you heard was microphonics. try tube dampers rated for output tubes.
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Since you were tapping on the chassis, how do you know it is a power tube problem? Usually, microphonics develop in the gain/driver tubes (most often the smaller 9-pin twin triodes like 12 (AX,AT,AU) 7's, etc.
Try tapping the top of each tube individually with a WOODEN instrument like an ice cream stick or a pencil (not the eraser end ;-) to find out which tube(s) it really is.
Personally, I don't like dampers, but . . . . My position is either a tube is microphonic or it isn't. If it is, I replace it.
I am with Nsgarch for the most part. Microphonics used to be less of an issue in the day, as they were selected out and often rebranded. If the tubes are truly loose in their sockets (common) you should carefully tighten the pin connections of the tubes. Tube dampers usually shorten the life of the tube by not allowing the heat to escape...
Sounds like you've had this amp for a while. If you only hear the noise (microphonics) because you intentionally rapped the side of the amplifier, I would do nothing. If you hear it while the amplifier is sitting quietly, absent of any external forces, than I would consider following Nsgarch's advice. Personally, I would not chance placing anything within the amp that could potentially melt. I have even read somewhere that you should not use tube dampers on output tubes because of the heat factor. In my experience, I have been fortunate in that I have never had microphonic problems with output tubes in my amp. Wish I can say the same for the smaller gain tubes in my preamp.
thankyou for your replies, I have done my own research and have found that the base of a kt88 does not get anywhere near 500 deg so I am in no danger of the silicon tape melting, also since the tape is only on the base of the tube and not the glass, it should not effect tube life, however I am really suprised at how it changed the sound, more testing will be needed to see if it makes the sound better or worse, thanks, chris
When you did your research did you find any mention of the possibility that the tape might inhibit the ability of the metal base to dissapate heat? Maybe that limited dissapation, as small as it might be, could change the internal temp of the tube's components and change its sound. Just a thought, FWIW.
hmmmmm, interesting theory, but I would think that the room temp would have a bigger change ie summer vrs winter ect.I think that dampening the tube base would change the resonant frequency of the tube, and that could change the sound. from my understanding any resonance in a tube is a bad thing, it gets amplified along with the signal and causes distortion. in a couple of days I will take the tape off and see what the difference is, If anybody else has any information on tube dampening I would greatly appreciate it ! thanks, chris