Hi all: Long time player, first time tube amp owner here. I've been following the Champion 600 mods here the past few years, and finally picked one up a few months ago (used). Mine appeared to be stock, so I changed the speaker, grill cloth, and tubes. All was running just fine 'til yesterday. I had changed the functional power tube from an RCA to a Sylvania I had just to hear the difference (both old tubes rated around 95%). The Sylvania worked fine on the low input. Then when I switched to the high input it gave me some weird harmonics, made kind of a "crinkly" sound, and then the amp cut out. I am wondering if the tube was just dying/old and couldn't handle being put back into service (got it as part of an eBay lot). No burnt smell from either preamp or power tube, though. I put the RCA and another JJ preamp tube back in the amp, powered it back up and get no guitar signal. The amp hums still, but it's a very low hum that is fixed i.e. does not increase as the amp warms up (like before) and the volume knob doesn't affect it. The amp lamp is behaving normally. I haven't opened up the chassis yet to check for burnt stuff but will do that. Am just wondering what issue this might sound like to more experienced folks here. Did I possibly fry one of the transformers? Btw I bought this little amp to mod and learn more about amp circuits (kinda like the guitars I've built from various Squier guts). I guess be careful what you wish for!