Vacuum tubes are much more idiotproof than transistors. Grid current does flow during a significant part of the drive cycle in some classes of amplifier operation. Many high power tubes have really low grid current ratings. Why are the ratings so low? Are other things like electromigration limiting factors before thermal issues? What actually happens when you exceed the manufacturers grid current rating?
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1"Vacuum tubes are much more idiotproof than transistors." well, if you don't include the idiot burning down their house or electrocuting themselves or their children, yes. – TimWescott Mar 09 '21 at 02:16
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What actually happens when you exceed the manufacturers grid current rating?
Generally, the grid melts. Finer grid wires are an advantage, because they have lower capacitance and because they intercept fewer electrons on their way to the plate. So if a tube is designed to operate at low or zero grid currents, the grid will be made of finer wire, and no particular effort will be made to pull heat out of the wire.
TimWescott
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1Nor will the wire be an expensive high melting point conductor like the cathode. – Mar 09 '21 at 12:33
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The main limitation on grid current is grid thermal dissipation.
The grid cannot get rid of heat easily - it has the red hot cathode on one side and in the case of a high-power device an anode that may also be at red-heat on the other side. The grid being a fine mesh of wires does not have much thermal capacity or dissipation ability.
Kevin White
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