>Good morning to the group.
>I experienced a grid-filament short in my TL922 while testing
>a pair of unused 3-500Z tubes which I had kept in storage for
>several years. The reason for trying the unused tubes was that
>the amp was giving low output after service in the CQWW SSB.
>The grid short took out several small components.
...snip...
Grid shorts seems to result from an intermittent parasitic-oscillation at
the TL-922's anode-resonance of roughly 130MHz. To measure the anode
resonance freq. in your 922, couple a dipmeter to the 12 gauge bus-wire
between the DC blocking capacitor and the HV-RFC. IMO, the nature of
this dip suggests that the anode resonant circuit has a high Q.
- To determine whether a parasite actually occurred, measure the
resistance of the parasitic-suppressor resistors. If the resistance is
within tolerance, a parasite is not indicated. If the resistance is
substantially above the marked value, a parasite is indicated. However,
if the parasite which bent the filament in the shorted 3-500Z took place
in another amplifier, the suppressor resistors in your TL-922 may not be
damaged.
Note: If a factory-stock 922 is operated on Standby with a grid/fil.
shorted tube, the unfused filament transformer will melt down. To avoid
this problem, remove the +110v source from the cathode bias relay
contacts that connect to the fil. CT.
- An article on circuit improvements for the TL-922 is available at:
http://www.vcnet.com/measures
good luck, Martin
Rich---
R. L. Measures, 805-386-3734, AG6K
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