Hi Andy I think I have some lengths from a Decca coil left that would be
long enough for you (If I can remember where I put them :-)) ) I stripped
down a movable coil a number of year ago and spent a long time unwinding it.
There were some nice long lengths but there were also a number of shorter
ones....I will dig around.
Alan
G3NYK
----- Original Message -----
From: "Andy Talbot" <[email protected]>
To: <[email protected]>
Sent: Saturday, January 22, 2011 5:14 PM
Subject: LF: Thick Litz wire needed, and more on class E PA development
> Is there still any of the really thick Litz wire from the old Decca
stations
> around? I'm after about 3 to 3.5 metres of whatever is available that is
> thicker than 3mm, and preferably nearer 3.5mm diameter. Enough to make
a
> 9.3uH coil
>
> The Tank coil in the latest version of the class E PA under test is now
the
> power-determining item. At 450 Watts output the heatsink with fan is
COLD
> - but the coil annoyingly smells of hot varnish.
> It is wound with the cotton covered Litz, about 2.5mm overall diameter
which
> itself is a big improvement over the plastic covered stuff I was using for
> the breadboard. But, I have the surreal situation that as teh coil gets
> hot, teh wire expands, inductance falls and due to the nature of the
> combined Tank + L-Match circuitry causes a resulting *increase* in powser
> output - leadign to higher dissipation and you can guess what will
> eventually happen if left unchecked.
>
> One soluition will be to duct air from the fan through the coil, but I'd
> feel happier if loses were lower to start with.
>
> At reduced Vdd it reaches stability running at 400 Watts without getting
too
> hot and increasing itself spontaneously.
>
> Efficiency, by the way, variously measures at 86 - 94%. So I'll say
around
> 88%. Hard to believe, but I have been over everything and double
checks,
> and can't get teh value below 84% however much I try to push errors in the
> worst direction.
>
> The FETs are now a pair of IRFP360, and the driver chip had to be
replaced
> - the poor ICL7667 just went bang and split itself open driving them. The
> copper of the PCB tracks running under it became discoloured so it must
have
> bene running very hot before dying Now using a pair of SMT TC4420
devices -
> one per FET - with additional copper heat dissipating fins on the PCB.
> They've survived so far.
>
> Its been delivering 320 Watts for an hour now off a 37V rail, so at that
> power level is certainly reliable; brief forays up to a 55V rail gave
> slightly over 700 Watts - and I could see current rising as I watched due
to
> the coil expanding, so didn't hold it there for long, but does mean the PA
> as it stands could be used with AM/envelope modulation.
>
> The shack is getting quite warm now !
> A second chapter to the write-up will appear in due course
>
> Andy
> www.g4jnt.com
>
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