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From: "MAL HAMILTON" <MAL@g3kev.freeserve.co.uk>
To: rsgb_lf_group@blacksheep.org
Subject: LF: Re: Guard circuit - impressive performance
Date: Tue, 26 Dec 2000 16:54:31 -0000
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-----Original Message-----
From: Andrew Talbot <G4JNT@thersgb.net>
To: rsgb_lf_group@blacksheep.org <rsgb_lf_group@blacksheep.org>
Date: 25 December 2000 17:03
Subject: LF: Guard circuit - impressive performance


>Well .............
>Just installed a version of the Decca guard circuit on the 600W PA, and its
>performance is seriously impressive !   Still using the cheap and nasty
>IRF840 FETs with just a minimum of turns adjustment for setting up, the PA
>can be set to deliver 600 W, and no more.   It is completely short circuit
>proof and when the load resistance  is progressively lowered, the output
>current stays constant - power actually falling where before it would have
>just kept going up until the devices popped.   The Make-B4-Break switch for
>the output transformer tap selection is how quite happy being hot switched.
>
>Only one spare IRF840 left now, out of an original total of 24 so can't
blow
>up any more by routine testing.   I reckon this PA is now finished - looks
>like there is no need for current monitoring circuits so its time to think
>about the BIG one - four FETs in bridge.

Go for it Andy.
As far as I can remember the commercial Loudenboomer RF amp had no mains
transformer, just rectified/smoothed mains, and 4 x tv line output tubes. It
produced around 500w out and was very small and light. Not sure of the tube
configuration.
The man I know is getting mains sag with his 4 x fets in a bridge, but I
think he is pushing it too hard. Are you using a series resonant cct then 50
ohm matching at the o/put similar to the decca ? The decca guard/protection
cct is good, surprised no one thought of it before.

>
>Getting this rather strange circuit concept  going was far more satisfying
>than this Xmas thing, congratulations (pass please on Walter) to the person
>who first thought up the idea.
>
>Now to forget RF design and watch Titanic !
>
>73
>Andy  G4JNT
>
>
>
>