I only got a couple of responses to my query about reducing the high-speed
fan noise on an LK550-ZC. Both indicated that the cooling in Amp Supply amps
may be inadequate for the tube seals.
Tom, W8JI, said " Every amp supply amplifier I've ever tested has operated
the tube seals far over the factory rating, even with the fans on high
speed. I've found that to be true with no RF drive, and the tubes just
idling with normal quiescent current."
I called Bill Edwards, K4BWC, of Omega Electronics and asked him about the
issue. Bill was formerly the service manager at Amp Supply. He said that the
cooling was indeed inadequate on earlier models because there were not
enough holes to permit proper air circulation under the tube deck. They
later modified the deck to increase the airflow. My amp happens to be an Amp
International unit, which means it was one of the last to be manufactured.
Bill said my fans should provide 105 CFM at high speed.
Bill said there's a 300 ohm 25W resistor in the AC line to the fans that is
bypassed by the thermal switch when it kicks into high speed mode. He
suggested using 150-200 ohms in high speed mode, most easily done by
inserting another 300 ohm resistor in the bypass path so it will be
paralleled with the stock resistor. I'm sure it would be easy to add a panel
switch to defeat this modification by bypassing the second resistor.
Bill felt that it would be safe to operate the amp this way, especially
given that the LK550 is the S&P amp in my SO2R contest station, and
consequently has a very low duty cycle. Still, I plan to do some temperature
measurements just to be safe.
73, Dick WC1M
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