I don't know how to check them - going to google it. Ordered replacement, but would be nice to know how to check the wrong tube.
The trrrr sound you describe might be what’s called ‘motorboating’ and if so, it’s a preamp tube. But it might be something else, of course, there are no guarantees with this stuff.
First, I always turn the amp off when changing a tube. Safety first!
OK, unplug your guitar/pedalboard/dc loop. You want just the amp. Turn it on, let it warm up, and then switch it out of standby. Then turn up the volume a bit, not all the way up, just so you can hear the problem.
The first thing I try is to tap very lightly on each preamp tube with a pencil or chopstick. The first tube near the input jack is the V1. Note that even if the V1 tube is good, you’ll hear the tapping in the speaker.
But if it’s really loud, or really sensitive, it could be a bad tube. If it’s not the V1, move on to the next tube. In any case, tap each tube lightly, and see if they make any strange noises you can hear through the speaker. If you can hear ringing, or loud tapping, or anything unexpected, simply replace the tube.
If tapping the tubes doesn’t tell you anything, then the next step is to swap each preamp tube - one at a time - with a known, good tube. Turn off the amp, replace the tube, then turn the amp back on. This is a process of elimination. Be patient, and replace each preamp tube with the known good one. If the problem isn’t solved, move on to the next tube.
Also remember that more than one tube could go bad. Modern tubes aren’t as well-built as the old ones, and they tend to be more fragile. This is especially the case in combo amps, where the vibrations from a nearby speaker in the same enclosure vibrate the hell out of tubes. So you might find more than one bad tube. It happens.
As an aside, I use NOS (“new old-stock”, or “unused”) preamp tubes from the 60s, 70s and 80s that tube dealers dig up, because they tend to sound better with less brittleness and ringing, and last longer. But even NOS can occasionally go bad, even though it happens less frequently. Then again, I’m a freak and maniac about tubes, so, take my advice with a grain of salt!
