Keying with tubes or relays

Don Ehrlich K7FJ 72254.1270 at COMPUSERVE.COM
Thu Apr 10 16:44:47 EDT 1997

Re: Bry's comment about using a mercury wetted relay and no-success with reed

I agree.  However, you can use a reed relay if you protect it from the high
voltages and currents
inherent in cathode keying.  I didn't have a mercury wetted relay AND am using a
laptop computer
to generate CW.  I needed to provide absolute voltage protection for the
computer interface.

I used a simple single transistor - reed relay interface which offers absolute
isolation from the
high voltages of the transmitter ... but I cheated a little by converting my
homebrew transmitter
to grid block keying.  Grid block keying still results in very high voltages
(mine goes up to
about 300 volts!) but at extremely low current.  Reed relays can handle the
voltage if the current
is essentially nil.

When Bry's relay started to stick closed it was because the contacts welded
together.  That will happen if the relay, when closing, has to discharge a
capacitor, probably the
case in most keying circuits due to the anti-keyclick shaping circuits
universally used.  The peak
discharge current may very well be several amps for at least a millisecond or so
... plenty of energy
to weld together those tiny contacts..

All I'm saying is that you can use a reed relay ... but only if you add the
triode and VR tube
needed for grid block keying.  All of the handbooks show workable circuits.  I
used a spare
1/2  12AT7 and an OA2 together with a -300 volt supply stolen from the existing
HV supply with
a pair of diodes and filter capacitor.

Don   K7FJ   Seattle

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