This 2A3 ran for a month or so in my previous amp without issue. I put it in my new amp (which has only slightly high filament voltage - below the 5% tolerance) and could not adjust the hum to settle - measurements were jumping all over the place. Then I noticed the tube was not as warm as the one in the other channel. Thinking the problem was with the amp, I opened it up and measured: 496VDC plate-gnd 170VDC grid-gnd 4.43VDC cathode-gnd This is quite different to the baseline measurements (obviously!) I had for this amp. For comparison, putting the tube from the other channel in the amp measures: 455VDC plate-gnd (perfect!) 154VDC grid-gnd (close to perfect!) 204.6VDC cathode-gnd (close to perfect!) A quick inspection of the problem tube shows that the getter flashing near the base has blackened with the appearance of what might be heat rings: So, it seems like the voltage from the cathode has been added to the plate - is this even possible? How would this happen? Any thoughts most appreciated. Cheers. P.S Yes, those are EML mesh plates - not an inexpensive tube to go south.