Fw: SB-200 plate current

[Bob, WB2UJD]

In a message dated 6/13/2003 7:25:52 PM Eastern Daylight Time,
stewgoat at INDY.NET writes:


> I just did a visual and a 33 ohm resister off the grid of one tube R21 or
> R22 is cracked in two.  The tube on that side has a discoloration on one side
> (the gitter appears to be at the bottom) so it may be toast.  Now to figure
> cause and effect I guess before I replace the resistor.
>
> Jeff
>

Jeff,

You may not find anything conclusive as a cause, the cause was likely high
plate dissipation for whatever reason, high VSWR, parasitic oscillation, key
down for too long at high power, etc...  The discoloration on the side of the
tube is likely vaporized plate metal, I have seem 3-500Z's with this.  The plate
on the one I saw was so thin that you could see through it (even though
physically it was still there) when the filament was glowing.  The grid resistor
likely let go because of excessive grid current when the tube was under the same
stress which caused the plate to partially vaporize.  As is usually the case
with these sort of things, a conclusive answer is not always evident or
available.  Sometimes the signs only add up to a likely scenario based on the damage
which resulted.  In your case I would replace the tube (if the plate is
damaged), replace the resistor, make sure the other side tube and components are
good and begin testing at low drive and output.  If everything is good at low
power, ease your way up to rated while watching the meters carefully.

73,

Bob, WA2T

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