There's been a fair bit of discussion on this, and Rich always suggests making
sure that there's some extra cooling around the tank coil and the bandswitch.
I've seen amplifiers where the 'cooling' was such that tinned copper wire
discoloured, so this overheating does happen. However, tank circuit heating
didn't seem to be a problem in the 'good ole days' of AM with the 20 minute
monologues. Were 1kW input AM transmitters more conservatively rated, or what?
OK, so they were about 650 watts of carrier output, 2600 watts PEP as opposed to
1500 watts PEP of today, and they weren't run under single tone modulation for a
long period. Even so, I'd expect similar problems.
Doing some sums suggests that the losses are not negligible. If the tank coil Q
is say 200 (coil Q, NOT working Q) then in a 160m pi with a 20 microhenry tank
coil, I calculate tank coil losses of around 70 watts at 1500 watts out. Of
course, that's single tone.
Anybody remember if tank coil heating was much of a problem with AM?
73
Peter G3RZP
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