I was operating my amplifier a couple of days ago, when all of the sudden it stopped transmitting. I had loaded it up fully (using CW carrier) and it was looking good, power out was about 900W and grid current was about 400 milliamps, and plate current was looking normal. SWR was a very good, maybe 1.5, 1.6 to 1. Everything looked ok. Then all of the sudden while transmitting SSB during a QSO, I looked down and there was no power output. And my signal had dropped off the air. Grid current was showing about maybe 125 milliamps constant, with no input from the exciter with the transmit light off (Amp not keyed). I then looked at the 3-500Z power tube and it was glowing cherry red! I then switched off the amplifier and turned it off for the day. The following day I turned the amplifier back on and loaded it into a dummy load. It seemed to load up and work just fine. I transmitted into the dummy load for a while and nothing seemed to be a problem. What happened?