Does amplifier stability have anything to do with the temperature of the output tubes? Can tubes go into thermal runaway?
Yes. Emission in tubes increases with temperature, but not a whole lot, as the tube gets hotter. The predominant effect is that as the tube gets hotter, you cause outgassing from the metal, glass, and other materials in the tube. The gasses are attracted to the grid as the most negative point in the tube and stick to the negatively charged grid, causing a decrease in grid bias. If the tube is too gassy (which it can get to by being too hot) you can get into a condition where the grid leak current changes the bias in the direction of more current, which makes the tube hotter, which causes more current. The solution here is to lower the value of the grid leak resistors. This increases the available current to the grids and keeps the tube out of runaway. Back to the GEO Home Page Amplifier Modifications A. OK/Recommended amp modifications Read the SAFETY WARNING first, before you put your hands – or other personal parts – into a tube amp. • Put a fuse in the B+ line after the rectifier(s)
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