|[AMRadio] Keying a Gates BC-1?|
k4kyv at hotmail.com
Mon Apr 18 19:27:02 EDT 2005
My BC1T is keyed with the main T/R system on AM, and also has cw capability.
For the T/R keying, I used a large 2PST relay to interrupt the 220v a.c.
line to the primaries of the LV and HV power supplies. I turn on the
filament start, then when I am ready to transmit, hit the plate start. The
transmitter is ready to go. Normally the filament start automatically turns
on the oscillator after the time delay relays kick in, then the plate start
turns on the HV. But with the T/R relay in line, the transmitter is in
standby until the relay contacts close to turn on the LV and HV. If you
intend to use AM only, no need for protective bias. The overload relay
kicks the plate relay off once the plate current exceeds a set amount. If
you lose grid drive to the final, the o/l relay simply kicks the transmitter
off the air. That's the way it was designed of broadcast operation.
For CW I ended up cathode keying both the pair of 807 drivers and the 2nd
12BY7 stage. Just keying the 807's left enough backwave to be visible on
the scope, so I knew it would be audible on the air. The combined
807/807/12BY7 cathode current exceeds 200 ma, so I use a TV sweep
transistor, rated at something like 1500 volts at several amps to key the
cathode circuit, and my bug only has to key a few milliamps.
For protective bias, I use a 6AS7G in a DC amplifier circuit taken from a
1947 or 48 west coast Radio handbook (the original circuit used a 2A3 or
6B4) but I had more spare 6AS7's handy, plus they will handle more current.
This circuit allows use of the existing bias circuit, but the cathode
metering is not affected, so that the meters still read accurately, and less
than 1 ma is drawn from the bias supply. I use just enough protective bias
to cut off the 833A's at full strap key-up condx, and the rest is developed
with grid leak bias.
I use a coil and capacitor between the sweep transistor and cathodes to
shape the keyed envelope waveform.
I get excellent reports on 160 both on AM and CW using that transmitter.
73, Don K4KYV
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