"John Larkin" wrote in message
...
What's the physics of the left part of the curve? Thermonic emission
from the warm grid to the plate or the cathode?

Should be something like that. Although I wonder, since negative current
[magnitude] should rise somewhat with decreasing grid voltage as it becomes
a better cathode. Of course, it may also be that, as a cathode, it is
already saturated (no substantial space charge), where current wouldn't
vary much with voltage.

I wonder what conditions these measurements were taken in -- probably
static, in which case I wonder if the plate temperature, and therefore
everything inside the plate, was at thermal equilibrium when these
measurements were taken. If so, then towards cutoff, the triode will be
running a goodly bit cooler and the grid won't emit much. It follows that
high plate dissipation and grid voltage just under the virtual cathode
potential (about -1V) should produce the maximum leakage.

Are there secondary emission effects at the grid?

I don't think so. I suspect the electric field doesn't increase much until
the electrons are past the grid (i.e. not enough leaks through the grid
spaces), so in the grid region, there aren't any electrons with more energy
than the grid's work function. A hot, contaminated grid might have some
secondary emission though.

I haven't thought much about tubes in a long time.

People keep threatening to make a device with a semiconductor
electron-emitting cathode, a vacuum gap, and a metallic anode. That
would be very slick, but none seem to be practical so far.

I would love to see a 300B in a TO-247 package. Well, no, I wouldn't love
to *see* one, but it would be quite fun to use.

Tim

--
Deep Fryer: A very philosophical monk.
Website @ http://webpages.charter.net/dawill/tmoranwms