R.Measures wrote:
The high grid-I during a vhf parasite creates a lateral EMF on the hot
filament helix. This occasionally bends the filament far enough
sideways to short against the grid -- which is grounded through the
choke. Since the filament xfmr is connected to the +110vdc power supply
on Rx, it shorts the supply to ground, causing a large current flow--
which smokes the choke.
3-500Zs (perhaps certain vintages and manufactures -- I'm not sure) sometimes
fail with grid-to-cathode shorts. The short often only appears when the tube is
hot, so removing the tube and checking with an ohmmeter does not show it. I
have seen this myself a couple of times, and have heard of several other cases.
Without speculating as to the cause of the short, I will say that such a tube is
likely to repeat the performance the next time you turn it on. So if the
circuitry is such that a g-c short will damage components, it's a very good idea
to replace the tube after you replace the components.
--
73,
Vic, K2VCO
Fresno CA
http://www.qsl.net/k2vco
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