QDC ?

[N3ADL at aol.com]

The practice at V26B is NO QRZ just 59908 V26B..the less said the better the
rate and you don't have to worry about some dummy hollering " WHATS UR
CALL??"
Must have done something right...Almost 24meg in CQ Phone from zone 8! This
was contest strategy and we worked hard to make sure we did this for every Q.

                                              73 de Doug // N3ADL // V26DX

>From Kim Östman <kim at emil.multi.fi>  Thu Apr 11 08:53:27 1996
From: Kim Östman <kim at emil.multi.fi> (Kim Östman)
Date: Thu, 11 Apr 1996 10:53:27 +0300 (EET DST)
Subject: WPX log by e-mail ?
Message-ID: <Pine.LNX.3.91.960411105159.9395B-100000 at emil.multi.fi>

Hello contesters !

What is the email address for sending the WPX log to Steve, N8BJQ ??

CU, 73 de Kim, OH6KZP (kim at emil.multi.fi)

>From Marijan Miletic <s56a at s55tcp.ampr.org>  Thu Apr 11 16:34:52 1996
From: Marijan Miletic <s56a at s55tcp.ampr.org> (Marijan Miletic)
Date: Thu, 11 Apr 96 11:34:52 EDT
Subject: FET vs Tube QRO
Message-ID: <14559 at s55tcp.ampr.org>

I hope some contesters are interested in the power amplifier topics?

Tom, W8JIT disputes some of my statements with a very wrong FET example.

I'll have to expand on the subject a litlle but still no math involved :-)

Graphic environment would be preferable for showing simple diagrams as
people like cartoons...

>>Theoretical peak voltage on sine driven amplifier is TWICE the power supply
>>DC voltage (including less than 180 deg. conducting class C configurations).
>That is true only for a properly loaded tank, or class A amplifier Mario.

While tube plate tank PI circuit maintains Q>5 and "tune" capacitor presents
low impedance for harmonics, instantenous output voltage only swings from DC
down to few hundreds of Volts while tube conducts and then returns above DC
for the same amount and time thanks to the free-wheeling resonance effect
converting DC into RF energy.

Equivalent RF circuit is represented by current generator (almost ideal in
the case of tetrodes) with output resistance of few kiloOhms driving tank.

>With 12 volts on the drain of a MRF 150, biased at the class A side of class
>B (185 degrees of conduction when properly loaded), I can measure 100 volts
>of drain voltage with an unloaded tank. I think the "two times rule" comes
>from oversimplification that the tube is somehow an energy source instead of
>a time varying resistance in series with a diode, and that failure to
>understand the conduction angle shortens as the load is removed.

Tube IS current (and energy as voltage is always present) source over the
linear power amplifying range while FET behaves in similar fashion ONLY for
a small input signals.  It becomes saturated switch very quickly as might be
observed with the scope.  MRF150 in above configuration will happily
generate harmonics all the way to 200 MHz producing nice square waves at the
drain.  FET output impedanse in ONLY FEW Ohms dumping tank to Q<<1 and we
end up with switched inductor which exibits high voltage peaks in OFF state.

>This is a whole chapter in a book to explain, but the test example is easy
>to build. Make no mistake about it, the *peak* tank voltage can easily be
>more than double the anode supply voltage in a mistuned amplifier. I have no
>doubt at all about that.

Apart from my old colege books, I checked Collins: SSB systems & circuits and
Motorola RF transistors: Principles and practical applications and couldn't
find a support for your claim.

Going back to proposed MRF150, it is 50V DC FET SSB transistor with breakdown
voltage of 125V.  They would be massively destroyed during a routine industry
tests with VSWR 30:1 under 50% overdrive if * several times * DC voltages
would occur.   Instead they happily generate even 900W output in the quad
Ameritron configuration!  If we can only make cheaper harmonic filters, tubes
would be history for ham power levels (with few notable QRO exceptions :-).

>The QC acceptance voltage breakdown of the cap in the SB-220 was 3500
>volts peak, allowing a tank voltage swing of 7000 volts peak to peak. If the
>two times rule was true, the capacitor would never be able to arc!

7000V is a good safety margin against 5500V peaks but cap plates get dirty
and wet and arc might eventually develop.  Beter over the capacitor then
inside the tubes!

>Try building the FET test example. It's simple and non-destructive.

My last PA was 80W on 50 MHz using 28V VMOS surviving 32V DC into high SWR
antenna.  I hope I'll not exceed Vds breakdown while gate is dumped with
47 Ohms against 10W transverter overdrive.  I've taken a good care of source
grounding, DC coupling, RF feedback and drain matching according to the books.

73 de Mario, S56A, N1YU.

P.S.  KL7HF remark on VHF oscilations should be given due consideration.