Rob, what I used was a PAIR of 30 amp rated, DPST relays, Like P+B types,
in an external 8x8x4 standard metal electrical box...with loads of
knockouts (home depot).
1st relay is used to apply the 240 vac...(both poles used). One pole of
2nd relay was used to shunt the 20 ohm resistor. 2nd pole of 2nd relay
was wired to a pair of RCA jacks. The keyline is fed in / out of those
RCA jacks.
Done that way so there is NO way to key the amp, and apply
drive...UNTIL the 20 ohm step start resistor is shunted.....via relay #2.
Small control cable runs between the 8x8x4 box on floor....and a small mini
box on the desk, with a pair of mini toggle switches. Toggle #1 for relay
#1. Toggle #2 for relay #2.
Dead simple. Note, I also put the resistor back into the circuit, when
switching from CW (1900 Vdc) to the higher SSB ( 2650 vdc) position.
I also put the resistor back in the circuit, prior to shutting things down
for the night. IE: shut off toggle #2....then toggle #1.
By using the 2nd pole of the DPST relay for the key line, it's then a dead
simple mech interlock.
Note, IF the step start resistor on any step start circuit was still in the
circuit, and amp keyed, and drive applied, the V drop across the step start
resistor would be Immense. You would know asap, since the B+ would sag to
an extremely low value.
On one of my HB 6700 vdc B+ supplies, I used an 8 pin octal style plug in
timer module (P+B / Agastat etc) plugged into an 8 pin flush mount socket.
Is adjustable from 0-15 seconds. One and only toggle switch used to apply
120 vac to the coil of a big 2 pole contactor..... whose contacts applies
the 240 vac. Same toggle also applies 120 vac to the input of the timer
module. Timer module has DPDT 10 amp contacts. One pole of timer module
used to apply 120 vac to the coil of contactor #2..... which shunts the big
step start resistor. 2nd pole of timer module is wired to a pair of RCA
contacts...with key line run in / out of them.
Same deal. Amp can't be keyed until the timer module activates after a few
seconds.....and resistor is shunted and key line closed.
Jim VE7RF
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