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.
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
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