Hi Gary, I have exchanged a several e-mails with Fritz Rabb, who is
cordinating the ARRLs 500kHz experiment. He has done extensive work on high
efficiency PAs, and has co-authoured papers with Nat Sokal on Class-E unit
design. It is his opinion that problems occur almost entirely due to aerial
"effects". Unfavourable reactance / impedances can lead to very high
voltages appearing across the FETs. This does not happen with 50 dummy
loads. Half wave filters can help, but even then there are conditions that
can be transformed to dangerous voltages. I think this is more likely that
core problems, unless your cores are getting very hot. If that is the case
then stack a couple together instead of just using one. That should give
more bulk to absorb the heat and keep it well below the Curie temperature,
and it also reduced the possibility of saturation.
You mentioned that you have had some "arcing" or corona effects, this is the
kind of step change in the aerial parameters that could cause very high
transients across the FETs. At the level of power you are running on a
realively low capacitance aerial the voltages on bends or end insulators
will be very high. Double insulators are probably necessary and even 3in
insulators may not be enough. You may need to double up, particularly if you
operate during any rain.
Fets can "blow" in one cycle of RF so SWR protection may not be able to act
fast enough to protect the devices, but it may help in some circumtances
where conditions change more slowly. I am not sure how effective the use of
MoVs across the FETs is, but I believe it has been used by the two Dave's
Cheers de Alan G3NYK
----- Original Message -----
From: "Gary - G4WGT" <[email protected]>
To: "Steve McDonald" <[email protected]>
Cc: "LF (Rsgb LF Group)" <[email protected]>
Sent: 04 July 2005 15:28
Subject: LF: RE: G0MRF 300w Tx PA mods
Hi Steve & LF,
On 28th January 2005 you included the following statement in your e-mail.
""I think Scott has found that anything over 43V at 15A is getting
very scary as the cores will saturate all of a sudden...""
It seems that t has just happened to my PA, although my readings were
showing 36 volts at 14 Amps (504 watts), I was testing again & tweaked the
volts up a bit from 34 volts at 12 Amps (408 watts), it only stayed on
about
30 seconds & failed.
During testing after the mods were carried out I bench tested into 50 ohm
dummy load & achieved 645 watts so I felt that I could procede a little
higher into the antenna, the antenna readings looked fine using Jim's
(M0BMU) LF Tuning Meter. So I wonder, was it core saturation??
The core I am using is the original which came with the kit from David
(G0MRF).
Any thoughts or ideas ??
Regards,
Gary - G4WGT - IO83qp
Web : http://myweb.tiscali.co.uk/wgtaylor
-----Original Message-----
From: Steve McDonald [mailto:[email protected]]
Sent: 28 January 2005 22:50
To: [email protected]
Subject: Re: G0MRF 300w Tx PA mods
Gary - it sounds like you are getting similar results as we are with our
'west coast half-killowatt' amps. I am wondering what toroid you are using
in the output xfrmr? Both Scott (VE7TIL) and I are using salvaged TV cores
which seem to be OK up to a limit...they all appear to be slightly
different. I think Scott has found that anything over 43V at 15A is
getting
very scary as the cores will saturate all of a sudden...be careful with
yours and get as much cooling on it as you can. I am thinking of rewinding
another output xfrmr with the same turns ratio but with two cores stacked
together. Good luck with yours.
Steve / VE7SL
Web: "THE VE7SL RADIO NOTEBOOK" at http://www.imagenisp.ca/jsm
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