Tom
While increased voltage on the relay coil does not improve the current
capacity of the contacts, it does improve the pull-in time. In the 77DX and
perhaps other amps, Alpha momentarily put 50 volts on the 26.6 volt coil of
the HC-1 vacuum relay to improve the pull in time and hence the QSK
performance, which by the way was excellent. The HC-1 coil is rated at
26.5volts, the pickup voltage is 16V and the dropout voltage 1-10
volts.
Reduced hold-in voltage might reduce the shock rating and explain why some
HP engineers didn't like the form C relays, but the HC-1 is rated for 50Gs
with 26.5 volts on its coil so it would likely be fine with 12 volts on it
for band switching applications unless that application was in a mobile.
Thomas, AC7A, who started the thread, asked about a circuit to operate the
26.5 volt relays from 12 volts for use in band switching hardware. That type
of hardware may not require the fast switching times needed for QSK
circuits. The K6XX circuit which Paul, W@9AC provided the link to, should
work well to provide adequate voltage to pull in the relays (at least 16
volts for HC-1) yet deliver adequate hold-in voltage (at least 10 volts for
HC-1) from a 12 volt supply.
http://www.k6xx.com/radio/fastrely.pdf
73,
Paul, VE3TA
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