I was going to suggest the microphonics possibility as well. I think you should switch tubes between the amps to determine which tube might be the culprit. After 12 years, though, it might be time to try new tubes to see if you've lost some frequency extension and dynamics. You may be pleasantly surprised.
After 12 years,it’s time to replace them !
Microphonics and Noise
Problems with preamp tubes are often due to microphonics and noise issues. A microphonic tube will ring and amplify any outside noises such as bumping the amp, tapping on the bottle, or even footsteps as you walk across the floor. All tubes will amplify tapping to some degree but an unusable tube will be very loud and often feedback or squeal. In a guitar amp with many preamp tubes it can be difficult to determine which one is microphonic. This is because tapping any tube near the bad tube appears to be bad as well. Rest assured it is very unlikely to find multiple bad tubes all at once. We suggest you tap each tube gently with a pencil or chopstick (something wooden or plastic, not conductive) and often the culprit will be louder or noisier than the others. Replace that tube and likely all will be quiet. In a home stereo amp or preamp these same steps can be followed. However one more trick that is useful is to swap a suspect tube to the same position in the other channel. If the noise follows to the other channel you know you’ve found the bad tube. If it doesn’t, you know the noise is caused by another tube and you can repeat this procedure one tube at a time until you find it. In a stereo amp you would use this same procedure to find tubes with any noise issues.
Tube noise such as sputtering, hissing, popping can be tougher to find in a guitar amplifier. If you have a spare tube of the same type it’s good to substitute it in place of one in your amp and then listen for the noise. If it’s gone you know you removed the bad tube. If not, re-install the original tube and move your spare to the next position. Repeat until the noise is gone and at that point you’ll know you’ve removed the noisy tube. Some guitar amps with more features can give you clues to where the issue is. For example an amp where the reverb is malfunctioning indicates the tube in that part of the circuit needs to be replaced.
Output tubes can also be microphonic. If you hear a rattle or ghost notes on certain lower notes you likely have a microphonic power tube. One way to confirm this is wear a glove to protect your skin from the heat and then gently hold the tube while you play the note that causing the rattle. Usually gentle pressure on the bottle is enough to stop the vibration and rattling, and will give you a clear answer as to which power tube is to blame. This problem will arise in guitar combo amps but is much less common in heads or hifi stereo amplifiers as vibrations are greatly reduced in these applications.
After swapping tubes around, the problem seems to have disappeared. I also tried the tap testing, with no obvious differences. Perhaps I should try cleaning all the pins. A friend of mine suggested that I bring him all the tubes and he'll test them on his Hickcock.
I have to say that 12 years doesn't sound all that long to me for amps that only get used 2 to 3 times a week. Maybe 15-20 hours? I have radios with 50 year old tubes that sound great, but they don't get used much.
I still think it prudent to get some new output tubes. That family of tubes run hard, and even with limited use, it's time.
he tap test, no matter how widely used , even by some dealers, isn't a good test. Even the gentle focused tap test isn't what I would use. A microphinic tube with make bad sounds, when moved and more importantly, when played- they produce a feedback sound and ring and they also transmit any physical perturbation.
As for figuring out which of multiple potential tubes go I would be inclined to remove the suspect and roll in another tube. That does require a set of "good" back up tubes.
I was going to say remove and replace but I didn't want to get political.
I tend to "roll" signal tubes and preamp tubes more than the power tubes. This has made larger differences in my mind. But once I've got it sounding good, across different sources, I don’t want to mess with it.
In any case, just shifting the tubes around and cleaning connectors seems to have solved the problem. But I'm going to find those damn Philips tubes in the basement. It's bugging the hell out of me.
They are Philips brand tubes. The seller told me they were made by Mullard, but he was Chinese and I may have misunderstood. The amps were recapped in the 80s and may need it again, but for now, they seem to be working well. There's a little hissing when they power up, but it goes away pretty quick.
Yes, tomorrow I will dig around again in the basement. I'll be mad if I don't have them anymore. I found all sorts of cool stuff down there including reel and Elcaset tapes, so I think I'm getting close.
Ok, I found those NOS Philips and swapped them in. Maybe a slightly better treble, as the cymbals are louder and seem to fade slower.
But the RCAs are back in. They have much better bass and mids. More balanced across the whole spectrum. I don't hear a difference in width or height.
My girlfriend doesn't hear any difference, even though she's got better ears. She thinks I'm crazy.
I haven't decided if the better cymbals with the Philips is real, or just a consequence of a suppressed and less liquid midrange. But it's true, the Philips only have about 12 hours on them. The Chinese guy raved about them, but then he was selling them, so...
At any rate, I'm looking for more RCAs on the net. Then I find that there's at least 4 different kinds. It's never simple, is it?
Thanks for the advice. I'll check it out.
I've been looking at these, including Telefunkens at $190 per pair. Seems a bit high, no?
I have no relationship with this seller, and this link is not an endorsement. I posted it for information purposes only. (not sure what the rules are here).