Yup, sounds like a microphonic preamp tube. They’re as easy to change as a light bulb, this is not a big deal. Tubes go bad when they wear out. And sitting in an amplifier, they get vibrated a lot, which makes them go microphonic.
Tubes are actually pretty simple devices, and we’re talking largely 100 year old technology. In side the glass there’s a vacuum of course, but there are also little tiny parts. When those parts get vibrated and move out of position, they tend to go bad sometimes.
New tubes are particularly susceptible to this - “they ain’t makin’ ‘em like they used to.” That’s because (a) the folks who made really great tubes back in the day are either pretty old now, or pretty dead. The USA and Western Europe stopped making tubes around 1980-ish. The old machines that made the tube parts were pretty worn even then, 40 years ago, and those machines got sold to Eastern European, Asian, and Third World countries for tube manufacture. But the folks who designed the machinery didn’t go with the machines. Many of them simply aren’t in great shape, and they turn out less-than-great tubes.
Plus costs were trimmed by cutting back on high quality tube parts. If you were to disassemble a modern tube, you’d see lots of mechanical differences between the new ones and old ones.
I tend to buy NOS tubes made before 1980, but you don’t have to do that, there are still some pretty decent Czech and Russian tubes on the market. Just plan on knowing how to change them out when they wear out or go bad.
99.9% chance you have a simple microphonic tube that needs to be replaced. Which is why they sell tubes at just about every music store. This is definitely not rocket science, and not a problem with the amp, as I said, 99.9% of the time.
Most tube amp owners retube their amps themselves, and touring pros retube them several times a year.
