NK7Z wrote > > Initial testing here indicates no power level is safe... At 150 watts > the KPA500 reaches 70C pretty fast, in under two minutes, and 70C is my > upper limit for testing... At 500 watts it reaches 70C in about the > same time... > > Can I assume the amp is not the most efficient thing in the world at 200 > watts?
Correct! Up to a certain limit, the output power increases with PA current quadratically. So the maximum dissipation may be below the max power, and decreasing power does not reduce dissipation much. Assume that at 600W KPA500 operates at 60% efficiency. 1000W total, 400W dissipation. Now reduced the power to 1/4 or 150W. The input current is reduced 50% to 500W. The dissipation is 350W, not much below 400W at full power. With KAP500, the dissipation at low power can actually get worse than predicted since the power supply is unregulated and has higher voltage at lower current. Reduction of dissipation at low power can be done at least 2 ways. One is reduction of voltage as done in SPE amps. The other one is changing the the transformation ratio of output amp, e.g., in KX3; hard to do at high power. One can have the action of a different transformation ratio with a manual tuner. Once my 20A power supply died but I had a 5A supply available. But the max power was only 5W before the power supply switched off. 1/4 curent = 1/16 power. By adjusting a manual tuner for max output I got 20W. But KPA may not let this trick go because of SWR. protection. Ignacy, NO9E -- Sent from: http://elecraft.365791.n2.nabble.com/ ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[email protected] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html Message delivered to [email protected]
