[ARC5] SG Keying of ARC-5 TX

[Dennis Monticelli]

Whether using a transistor to switch the screen, plate or cathode I highly
recommend taking advantage of the relatively high Z of the driving
electrode of the transistor switch (gate or base) to do some wave shaping.
Otherwise it is easy to generate pronounced clicks.  A little chirp can be
charming but clicks are always bad news.

FYI, I happen to use a HV NPN bipolar transistor to key the cathode and
wave shape with an RC in the base circuitry.  There is a backwave from the
continuously running osc, but it doesn't bother the receiver because key
down will pull the osc down by about 2KHz on 40M in my T-22.  This places
the signal 2Khz away from the receiver's passband which is sufficient to
avoid interference.  If your transmitter pulls less than mine then the
screen cutoff keying method would probably be a better choice.

Dennis AE6C

On Fri, Nov 18, 2016 at 6:32 PM, Tom Bridgers <tarheel6 at msn.com> wrote:

> Recently found out that you could key an ARC-5 by using an appropriate
> sized SPDT 28 VDC relay that toggles between a cutoff voltage and normal SG
> voltage.
>
> In transmit mode, after applying B+ and VFO voltage, on key up a large
> negative voltage (about -200 volts)  is applied to the screen grids.  On
> key down the relay applies the normal +275 volts to the SG.
>
> This circuit eliminates back wave, and was recommended in GE Ham Tips in
> the early 1950's. Though not specifically to the ARC-5, but for use
> generally to transmitters.  This method was implemented successfully by Ron
> Baggs , which is where I found out about it.
>
> Instead of using a mechanical relay, it would be really cool to figure out
> how a MOSFET could be used instead to implement this circuit.
> Tom KE4RHH
>
>
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