On Thu, 16 Sep 2004 07:22:44 -0500, lncarman wrote:
>I've heard over the years of some amp builders floating the filament
>transformer. I was
>told that you isolate the filament transformer from ground and add two chokes
>in the primary side with bypassing. Since the primary side is 115V or 220V the
>amperes demand would be much less on the primary side. Looks like any RF would
>be suppressed greatly by the filament transformer and you wouldn't need a lot
>of
>uH in the primary chokes to finish the job. Has anyone tried this???
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I haven't tried it myself but a couple of thoughts occurred:
1. I doubt if you'd need fewer uH for a primary choke. The
transformer has lots of capacity between the windings and would couple
the RF through itself quite well. You'd still need the same choking
action.
2. Now you've moved the choking point several inches away, so the
wiring between socket and transformer introduces some capacitance to
ground plus some series inductance, not to mention a possible source
of RF radiation and/or pickup.
3. How well can you really "float" the transformer for RF? There
surely will be some capacitance from the transformer to ground. Will
this introduce unwanted resonances with the now-longer wires to the
socket? Surely there will be a series-resonance present. What
frequency will it be?
The end result: I wouldn't do it. It's best to choke off the RF
right at the source rather than farther away. The only advantage I
see is you could use smaller gauge wire on the primary side, but the
uHy's would need to be the same. Not worth the possible problems,
IMO.
It is an interesting idea, though. As I said, I haven't tried it so
this is all theoretical. Contrary opinions welcome.
--
Bill W6WRT
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