IF it is only one tube doing it, then you'll need to retube at least that channel. Tubes are *microphonic* and the degree to which they do that varies quite a lot, as does their internal self noise - so that's one set of parameters which are used to select tubes out of a batch. Keep the 300Bs... they're nice... Check the drivers, as suggested, and swap channels for certain identification... Putting anything on the outside of the tube won't do very much to eliminate the internal microphonics - of course the less you *excite* the resonance, the less of it you will hear. |
We have found the 300B tube to be among the most problematic of all the power tubes in regards to microphonics. You can find excellent, inexpensive damping instruments for the 300B tube at Herbie's Audio Lab. Herbie's Audio Lab Warm regards, Steve Herbelin Herbie's Audio Lab |
Though anything may happen, the 300B are rarely the culprit of the symptom you describe. Same with the driver tube, not sensitive to microphony. It is usually the first stage input tube. Never tap on tubes without knowing why. You stand a chance of causing a problem. The simple way to determine the problem is to switch the input tubes left to right, since they are most likely the culprit. See if it swaps channels. Next do the driver, them power tubes. If you plug this link into your browser you will see an explaination of microphony quoted from the tube "bible". The RCA Radiotron Designers Handbook 4th Edition. http://www.upscaleaudio.com/rare/noisetest.htm |
