-----Original Message-----
From: Andrew Talbot <[email protected]>
To: [email protected] <[email protected]>
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
| 