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Discussion Starter · #1 ·
My Fender DR, probably 10-15 year-old model, has recently developed a peculiar, pronounced hum. It happens only occasionally, following a cold startup, gradually (1-2 minutes), increasing loudness, only up to a certain level. Its steady pitch is about the frequency of an open D string. It is not acoustic feedback, it is not affected by the position or proximity of the guitar. It appears to be an oscillation that coincides with the time the amp is warming up. Switching the amp off, then on again fixes the problem (yes, just like a computer). Could this be some power tube issue?
 

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Preamp tubes tend to squeal when they get microphonic, but power tubes do more of a lower frequency rumble or hum. That sounds like it could be the case here. When it does it try holding the power tubes one at a time with a rag (or put on an oven mitt). If it stops, you have located the defective tube.
Or when it's not doing it, you can tap on them and see if you can make it start.
 

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+1
Power tube likely
I have an old Stromberg Carlson head with very good sounding mullards in it. But the one tube hums like a village idiot for about a minute and then it goes away.
The hum goes away - not the idiot- thats me
 

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J,
Definitely sounds like a microphonic tube. Often a quick hot cold hot cycle can stop it as you've found out, but it will eventually rear its ugly head permanently. Time to think about some new tubes. Try what JB suggested first, or tap them lightly with the rubber end of a pencil. If it starts upo you have found the culprit.

But the one tube hums like a village idiot for about a minute and then it goes away.
That's actually fairly common, especially for older tubes. They heat up and conduct at different rates, and the mismatch causes any hum in the power supply to show up. The output transformer normally cancels this hum, but conduction must be the same for each tube for that to happen. Eventually the hum will gradually get worse, and then not go away at all. Then it's time for new tubes. Nothing to worry about though.
 

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Discussion Starter · #5 ·
Yep, turned out to be one of the power tubes, which are a pair of vintage RCA's. I'm not the original owner, and fairly new to this amp. I'm going to tolerate the hum, as long as it's confined only to the startup behavior. When the time comes that I have to replace them, I would want to do so with another NOS pair.
 
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