I am restoring a Swan 500 transceiver. The PA seems to be essentially
inoperative although all voltages on the tubes are normal and there
seems to be adequate RF drive. Everything points to the conclusion that
the tubes are dead. However, they look like new visually, the filaments
light, and there are no shorts between elements per ohmeter.
The plate voltage at the caps on the tubes is 800v. The SG voltage is
250 at the socket. The bias is pot settable between -60 and - 120v and
that is measured at the socket. The plate current on the panel meter is
near zero and increases less than 10ma under drive.
RF drive at the grid is 250 v pp per my scope. RF on a 50 ohm load at
the output is 40v pp. The grid and plate tuned circuits tune normally
and peak within their tuning range.
The obvious conclusion is that the PA tubes are kaput. Can anyone
suggest any explanation other than this?
The tubes are 6HF5's and are listed on several tube dealers lists as
available for $14 each. I am about ready to order a pair but thought I
would look for any other ideas. It bugs me that both tubes are
apparently bad, look like new, and show no visual signs of abuse.
Any suggestions on a good dealer to buy the tubes from?
73, Dan, N5AR
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