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Something I don't Understand...

Posted by Lee of Omaha (D) on November 11, 2014 at 15:13:18

...OK, there's a lot of that. But here's one I've struggled with and can't figure out by thinking about it.

There's always at least a little heater-to-cathode leakage because the insulating material conducts "some" at the high temperatures at which the heater operates.

To combat hum (when operating heaters on AC) it's a common practice to raise the heater voltage above that of the cathode.

That's what I don't understand. The cathode is good at emitting electrons, so I would think that operating the heater positive with respect to the cathode would make the problem worse. It seems to me that operating the heater negative with respect to the cathode would be a better idea, as the heater is not a particularly good electron emitter (tungsten at red heat is a poor electron emitter).

Clearly my reasoning is flawed, or heater negative is the way it would be done. So where have I gone wrong in my reasoning? What's the real story?

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