SV: [AMPS]Plate choke at the antenna end instead?

[Peter Chadwick]

Hans said:

>>Let us start  a discussion on placing the plate choke instead at the
antenna
>> end of the pi/L filter to eventually get rid of these series resonance
>>problems? It works fine according to what I have seen in, I believe, PAT
>>HAWKERs RADIOCOMM column?

This appeared in Pappenfus' book back in 1964, and I started that discussion
in a letter to Pat many years ago now. The problem is that the tuning caps
now have to be rated for at least twice the plate voltage - both of them.
The 'Princess' transmitter that appeared in the 1970 RSGB handbook tried to
minimise this by putting HV  fixed capacitors in series with the tuning
caps, but without bleed resistors, this doesn't help too much. The other
problem is that the blocking capacitor now needs to be fairly big, and
capable of handing at least 5 amps - 12 for safety.

By putting the choke to the middle of the pi - L, you can use a smaller
blocking C and a bigger second inductor, but the current rating still needs
to be about 12 amps for a 1500 watt amp (safety factor of 2).

But you still end up swapping one problem for another. If you use vacuum
variables, that gets around the problem. But Pappenfus says:

' This transmitter uses a unique plate feed circuit to overcome the
plate-feed-choke problem of sufficient inductance without self resonances in
or near the 2-30Mc range. High voltage vacuum capacitors are used which can
withstand the d-c as well as the r-f plate voltages. The tubes ( 4 off,
4CX5000A - RZP) are series fed from the 50 ohm end of the output network
where the r-f voltage is much lower than on the tube plates. The plate feed
choke, the blocking capacitor and the antenna static-drain choke are chosen
to form a high pass filter of 50 ohms characteristic impedance. In this
manner, relatively low inductance chokes and a low capacitance blocking
capacitor can be used. An arc in vacuum capacitors, due to a lightning
transient, (my italics!) for example, would be damaging because it would
provide a dc path. For this reason, a "crowbar circuit" is employed to short
circuit the high voltage power supply in the event an arc starts.'


I guess the bottom line is that there ain't no free lunch!

73 

Peter G3RZP



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