I've got a PRS (Sewell designed) "30" 1x12 combo. This is not the "SE" - it is the handwired, US made amp. All tube, 30 watts, 4xEL84 power section. Lately, it has developed a hum - or more appropriately a buzz - that is audible when the amp is first turned on, but grows progressively louder and louder the longer it's on. The odd thing is, it only does it when I have a guitar plugged into it. If I just turn it on and let it sit, with no cable in the input jack - no buzz. I can turn it up, mess with the tone controls - nothing. Dead quiet. As soon as I plug in a guitar though, instant buzz. Doesn't matter which of my five guitars I use, or whether it's single coils or humbucking p'ups - same buzz. I've tried using different types of cables (normally use George L's), bypassing my pedalboard and plugging in directly - same buzz. The amp almost never leaves my guitar room, and has not been abused. Just started this out of the blue. The only thing I've done recently, is swap out the original JJ power tubes, for a matched set of EI Golds that I had laying around (which I know were all good tubes - had them briefly in another amp at one time). It's not a hum like a microphonic tube, it's a buzz - like a short or something not grounded correctly - except that it progressively gets louder, the longer the amp is on. Anyone have experince with a problem like this, or have any suggestions? This one has me stumped.