[Amps] SB-220

[Vic Rosenthal]

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