>"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
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