K1JJ Maul PTT Scheme

[PA4WM]

I suspect that letting the screen float at many times its spec is asking for a screen to grid arc. Using the series screen resistor as half of a voltage divider with another to ground at the socket is one way to limit the soaring as well as eliminating negative screen current during tuning. This can happen especially with the small ceramic tetrodes.

Excellent suggestion, Carl!

There ya go, Martin. Add another power resistor to ground from the screen and the voltage will not soar.  The downside is it will add some power waste even when the rig is unkeyed, unless you add a HV screen relay in there.

Carl:
I think you are right. 1500v (or even higher when there's no load on the HV supply) is asking for troubles...
Dropping the screen voltage with a voltage divider is a good option, but my plan was to modulate the screen of the 813 partly by using two dropping resistors. One from the modulated B+ and the other from the unmodelated side of the mod transformer.. (partly selfmodulating at a ratio of 60/40) I probably could use voltage dividers on both side of the xfrmr... But that doesn't make it easier.

Tom:
I still like the idea of switching the 50k bias resistor in and out the cathode lead to PTT the transmitter.
Letting the screen voltage soar is maybe not such a good idea then , so I have to look at a different way to supply the screen. I might have to search for a 20h or so inductor.  

Thanks everyone, for all the reactions so far.

Martin

[K1JJ]

Martin,

Thinking more about it, if you use a 1500V HV supply tied to the screen thru the resistor, there could be spikes of over 3KV on the screen when the rig is unkeyed - with a spark  produced in the modulation plate circuit. The modulation transformer adds to the 1500V at the plate as normal and it is common in the early stages to see keying/unkeying transcients when the rig is first shaken down.  The floating screen could get arced over, though the current would be limited by the resistor.

However, I know many guys use the screen resistor off the plate as a modulation technique. Maybe someone can tell me why there are few reports of screen damage? What if the HV keying or added grid bias was sequenced poorly and produced damaging voltages for an instant?  Maybe the risk is not as bad as we think.  I mean, even if grid bias is applied, the screen voltage still soars if the HV is left on. Safety may depend upon opening the HV circuit quickly with a relay..


That said, small 10-20 H chokes are quite common in the states at the flea mkts. I know there are few parts in Europe from what they tell me. Too bad the shipping cost is ridiculous to Europe.

If you find a choke, I would go with the small screen supply and a Variac. Being able to control screen current on the fly when antennas changes are made, power levels are changed, etc., is an advantage - as well as eliminating the potential HV risk to the screen as we discussed.

T

[W2PFY]

I never did but I have a couple of those FG17 tubes.