I bought a Heathkit SB-200 linear from a local University radio club a couple of years ago. It was my first amplifier and I finally got around to getting it on the air last month. It is in decent physical shape and tuned right up (tuned for peak power). It seems to put out the full 600 watts on all bands according to my external antenna tuner power meter. When setting the SB-200 up initially, I accidentally plugged in my Kenwood TS-480 exciter output into the ANT RELAY port of the SB-200 instead of the RF INPUT port and transmitted for a bit. I saw some smoke and checked the internals of the linear. I found that the R16 resistor was physically fried but still Ohm-ing out at proper value. I swapped it anyway and began using it. This time with the correct wiring There were a few non-standard mods done to the rig like a replacement antenna relay and a quasi-softkey that lowered current but not voltage of that circuit. I was still using an external relay box to interface with my Kenwood TS-480 just to be safe. I also added diodes across the internal meter to try to protect it (for whatever that is worth). After using it for a couple of months I have noticed that CW keying for more than 10-12 seconds (for tuning) and during prolonged SSB transmissions, I am getting red plating on one of the tubes in the unit. Here’s what I have tried to diagnose and solve the issue running into a dummy load and testing primarily on 40m using mostly lower power output from the exciter (20w): 1. I have swapped the position of the 572B tubes but the particular tube that glows red will do so in the left or right socket. 2. Thinking that maybe I have a bad tube, I have tried carefully running just the one "good" tube solo at about 20 watts exciter power but then that one started red plating. So it appears that when they are run together, the plate of the more powerful tube of the pair turns red. However, either tube will red plate when run solo. 3. Thinking maybe these old tubes were just tired, I have tried swapping in new 572B Taylor tubes from RF Parts. Even with two brand new tubes, one tube glows as before. 4. I thought perhaps some mods made by the radio club were causing a bias circuit issue so I removed the home-brew softkey and replacement relay mods restoring both circuits to original configuration per the manual. I found a used original Heathkit antenna relay and swapped it in since the coil resistance actually matters and is part of the bias circuit on this rig. 5. I changed out the parasitic suppressors to new ones. 6. I changed out the factory fan to a higher flow fan from Harbach Electronics which moves much more air over the tubes for cooling. Even after all of this, I am still getting a cherry red glow from the plate of one of the tubes. Looking for any help.