Message-ID: <2000121823082968199@zetnet.co.uk>
Date: Mon, 18 Dec 2000 23:08:29 GMT
To: rsgb_lf_group@blacksheep.org
From: "Peter Dodd" <g3ldo@zetnet.co.uk>
Subject: Re: LF: Guard Circuit
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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>