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A client of mine brought a 1974 B-3 I'd worked on a few months ago back to me for an intermittent scratchy/hissing sound. I fairly quickly identified that it was coming from the organ preamp's 6AU6 input stage, but that's when the diagnosis got difficult. I'd previously replaced the screen and plate resistors (standard procedure), the cathode resistor, and the cathode bypass capacitor. (Note that none of these components connect directly to the tube socket.) I couldn't find anything wrong. I cleaned the tube pins and cleaned the socket contacts with Deoxit Gold, and I swapped tubes to rule out a noisy tube. Each time I tested it, it would start out sounding fine, but after a while, the noise would reappear.
Finally, I noticed some gunk on one of the 6AU6 pins when I removed it, which looked like solder flux. I've noticed on several Hammond preamps that they really slopped flux on the tube socket terminals in original construction. So, I put a cotton ball underneath the socket and used a dental brush I sometimes use on miniature tube sockets with a generous amount of alcohol to clean the tube socket more aggressively. The brush showed yellow gunk on it, so I flushed the contacts until it came out clean. I was shocked when I looked underneath and saw the amount of flux that had washed out onto the cotton ball. Someone, in original construction, managed to wick a LOT of solder flux up into the socket.
After more cleaning, I dried the socket and fired the preamp up again--no noise. I'm theorizing that, as the tube heated up the socket, it would soften the flux, which then crept up between the tube pin and socket contact.
Finally, I noticed some gunk on one of the 6AU6 pins when I removed it, which looked like solder flux. I've noticed on several Hammond preamps that they really slopped flux on the tube socket terminals in original construction. So, I put a cotton ball underneath the socket and used a dental brush I sometimes use on miniature tube sockets with a generous amount of alcohol to clean the tube socket more aggressively. The brush showed yellow gunk on it, so I flushed the contacts until it came out clean. I was shocked when I looked underneath and saw the amount of flux that had washed out onto the cotton ball. Someone, in original construction, managed to wick a LOT of solder flux up into the socket.
After more cleaning, I dried the socket and fired the preamp up again--no noise. I'm theorizing that, as the tube heated up the socket, it would soften the flux, which then crept up between the tube pin and socket contact.