[ARC5] SG Keying of ARC-5 TX

[J Mcvey]

For me, running a stack system has been the goal. I was always curious what a completely original setup was like.There is a certain charm to the whirling dynamotors and clickety clack of the relays.They do work very well with the original modulator/power supply and ancillary equipment.The "hot keyed" B+ works a whole lot better than I thought it would-even though that method has been long abandoned by RF engineers.
However, The hacked units were fair game for doing various experiments such as Tesla coils where I pulsed a mosfet in the cathode circuit.There are a host of mosfets which were designed for HV transformer drivers used in CRT TV applications, etc. The Vds is around 1200V or so.Doing a shunt to ground would be easy to implement, but a series pass switch presents quite a few interface challenges.
Now the question is if grounding the screen grid going to damage the tube?  
 

    On Saturday, November 19, 2016 12:42 AM, WA5CAB--- via ARC5 <arc5 at mailman.qth.net> wrote:
 

 The Command Set transmitters worked just fine box stock.  If you can build a MOSFET DPDT relay that will handle 300 VDC, you can just as easily build one rated for 600 VDC and run unbutchered transmitters.

Robert Downs - Houston
wa5cab dot com (Web Store)
MVPA 9480

In a message dated 11/18/2016 20:32:23 PM Central Standard Time, tarheel6 at msn.com writes: 

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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