The Eimac datasheet shows the max base seal temp rating to be 200C. Proper cooling across the pins is an issue in some amplifiers, such as the SB220, with this limit being exceeded in rare cases, especially when the amp is over-driven, operating AM, or CW by long-winded ops. This isn't normally an issue for me, as I mostly listen, and am partial to SSB operation on HF. But if a tube gets stuck "on" in a welded-contact relay failure, (or a cat napping on the TX key), the seal temperature theoretically would soon be breached, not to mention, the stresses the entire amplifier's components are subjected to during such an 'event'. So, I've been kicking-around the idea of securing a N/C thermal switch, such as the one in the pic above (below?), to one of the grid pins of each tube, with a tripping temp rating of 180C . It would connected in-series with the amp's keying relay circuit. My theory is that, in the event the pin seals ever reach 180C, this simple addition would force the amp into STBY until the tubes cooled, and the thermal switch resets (which is around 110C for the ones I have). I'm wondering if I should have concerns over stray RF interfering with this addition, or with the way the plate interacts with the grid, and the switch being that close to the two fields. Also, maybe a lower temp-trip rating on the switch could be better, or worse? Do you think any of these concerns are valid, or do you think I should just try it? I have no immediate plans to attempt this, but if/when I have one of my 3-500Z amps on the bench for repair, I've been planning to. What are your thoughts / concerns / suggestions?