>"The KPA does have diodes, but I don't know why the KPA diodes seem to make it >and others don't. Maybe not stressed as bad switching into an open load, which >might be the real culprit that folks don't want to admit to."
The KPA500 has substantially less field operating time compared with the Alpha 86/89/87A series. After checking this evening, apparently Elecraft doesn't post schematics of their amplifier. It would be an interesting exercise comparing notes. If anyone has a set, please send to me by PM. Late-generation Alpha 87A amps have ~ 150VDC higher Rx bias voltage to better withstand high VSWR conditions. In later QSK-5 devices, Ameritron doubled-up on the number of Rx diodes and placed 1M equalizing resistors across them. That's a reasonable way to restrict PIN diode voltage without spending a lot of money on higher-priced parts -- although Rx loss may be slightly higher. Possibly Elecraft mitigates lightning-induced voltages with a spark gap or other similar device, but without a schematic, it's only speculation. Given a choice between vacuum relays, or dealing with potential PIN diode issues, I'll still take a PIN structure every time. OTOH, I respect folks who prefer not opening their amps, and so a T/R relay is best for them. Guy made several excellent points concerning T/R relay mounting and the need to completely shock-mount the relay, mounting material, and isolate all relay wiring. Thin silicone HV wire works well; even better if short loops are used to add further isolation. >"Anyway, back to the original post, I will recommend the Sony MDR-7506. " Bose QC25 and Sony MDR-NC7 used here. Both are a NC type. The Sony headphones work almost as well and at a fraction of the Bose price. Paul, W9AC _________________ Topband Reflector Archives - http://www.contesting.com/_topband