Jim said:
Aha... wondered why you hadn't mentioned it before -
unfortunately, the guard circuit is not shown in the simplified
diagram in the LF handbook.
What it is - the tank coil has a link winding on it. This feeds the 'AC'
side of a bridge rectifier made up of 4 fast diodes, via a series
leakage inductance tuning capacitor. the 'DC' side is connected to
some big plastic film reservoir capacitors, and then across the PA
supply rail. In fig 3.20 in the handbook, it is assembled on the 3
heatsinks largely concealed by the tank capacitor boards.
At the time I put the Low Frequency Handbook together I had not done
any work regarding the modification of the Decca transmitter. I felt
that the circuit should be included in a simplified form because it
showed a design not in use by amateurs at the time. Furthermore, no
one else had any experience of this type of PA circuit and I didn't
know how the guard circuit worked; and it is only possible to
simplify a circuit whose function is understood.
Andy sent me the data sheet on the bridge configuration PA driver and
that was included in the Appendix.
I can confirm that the guard circuit works if the output is short
circuited. I had shorted the output of each amplifier at the output
of the tank circuits so that I could measure the tank resonances. I
forgot to remove the shorts when I fired the transmitter up the first
time. The only indication that anything was wrong was the output
current was zero and each of the amplifier guard circuits indicated
current flow. When the shorts were removed the guard current fell to
zero and the dummy load got very hot.
I have never noticed any guard current during normal operation - even
when tuning. I think they only seem to come into play in the case of
a severe mismatch
--
Regards, Peter, G3LDO
<g3ldo@zetnet.co.uk>
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