3CX1200-A7 and 3CX1200-Z7.
Little preface, my profession and my amateur radio activity brought me, in
the last 20 years to built a number of HF linear amplifiers.
Because of this fact I had the opportunity to check a number of commercial
amplifiers as well to try transmitting tubes.
Among my tries, 3CX1200-A7 was surely an "unforgettable" experience.
When it come out, the typical advertising was presenting this tube as a
"plug out - plug in" of a 3-500Z pair, with increased output power and plate
dissipation, having (apart shape and materials...) the only difference in the
filament requirements.
It was just an obvious consequence thinking about some "convertion" of
popular amplifiers (L4B, SB220 etc.), or to project and try some construction
with the new tube.
Finally it seemed possible to overtake the hi costs and precautions of the
8877 (3CX1500) in the 1500w out class amplifiers.
Well, the reality wasn't so nice and easy like advertising.
In spite of what aspected the tube was NOT enough stable in a grounded grid
configuration.
A good RF amplifier requires stability, a project that's not respecting such
condition shouldn't be used for production, amateur markets included.
The problem was found in the internal of the tube (electrodes'shape), and
because of this, no external fixture (proper component disposition and
wiring are maximally important in RF power amps) could solve it.
Unless some tricks (plate suppressor) or neutralizing circuit the tube was
always self oscillating not so much above 30 MHz.
If neutralizing would have been a (the only) real solution it was not a
practical one unless disregarding considerable complications and costs.
The plate suppressor, that coil with the anti-inductive resistor inside,
should be used to prevent VHF parasitic oscillations, but when the tube
self-oscillating frequency is not far enough from the operating one (at
least one octave), it becomes an handicap for the higher bands of the
amplifier.
Then, to have a stable condition with the 3CX1200 A7 the suppressor is quite
detrimental for the 28 and 24 MHz amp output power, its bulky because it's
also dissipating a part of the amplified signal..
It came out later (a phone call to ... .... confirmed it) that 3CX1200-A7
wasn't infact specifically designed as a grounded grid RF amplifier.
I think the 3CX1200 A7 is not any more producted, while there is existing a
very fine one (for RF amps purposes) named 3CX1200-Z7.
In spite of the name there is not so much simylarity with the 3CX1200-A7.
Shape and socket as well as internal electrode shape and parameters are
different from the 3CX1200-A7 makink not possible any easy replacement
between the tubes.
I don't know if all manufacturers are now using the 3CX1200-Z7 instead the
3CX1200-A7 like it should be preferable.
This is all what I wanted to point out.
73, Mauri I4JMY, one of IR4T.
E-mail I4JMY@UUGATE.AIM.UTAH.EDU
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