gpsparky,
I remember, a couple of years ago, in a freezing works where I was working, they had temperature controllers on the gut cooking vats that used a set of valves(tubes).
My memory escapes me as to what type they actually were, but the electrical technicians there had been replacing the solid state parts every 2-3 months, due to the heat.
They tried this valve circuit and that solved the problem.
Good to see that our old school solutions are still working in modern industry.
