>Subject: [AMPS] 3CX1200-A7 & 3CX1200-Z7
>Sent: 3/25/1997 9:22 AM
>Received: 3/25/1997 11:30 AM
>From: i4jmy@uugate.wa7slg.ampr.org
>To: amps@contesting.com
>
>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.
...snip...
>.... the tube was NOT enough stable in a grounded grid
>configuration.
The reason for this is that the 3CX1200A7 (like its predecessor, the
3-1000Z) has over 5 times as much feedback C as a 3-500Z or an 8877.
More feedback equals more chance of oscillation.
>A good RF amplifier requires stability, a project that's not respecting such
>condition shouldn't be used for production, .....
Amen to that, Mauri.
>Unless some tricks (plate suppressor) or neutralizing circuit the tube was
>always self oscillating not so much above 30 MHz.
However, there is no way to neutralize a single-ended grounded-grid
amplifier--and even if there was, that would apply only at the operating
frequency. At the anode circuit's VHF self-resonace, all bets are
off---except perhaps in Conyers, Georgia.
>If neutralizing would have been a (the only) real solution it was not a
>practical one unless disregarding considerable complications and costs.
A friend of mine owned two AL-1200s. The newer one was 'neutralized'.
It exhibited more signs of VHF instability than the older one. The
technician who tested the original/un-'neutralized' AL-1200 in the ARRL
Laboratory, told me that he observed what appeared to intermittent VHF
arcing in the tank circuitry during testing.
>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.
True. However, the problem can be somewhat ameliorated by using a
conductor material that's a bit more lossy than silver or copper for
constructing the suppressor inductor. A comparison between a copper-wire
suppressor and a resistance-wire suppressor is available on my Web page
at: http://www.vcnet.com/measures
(measurements copyrighted by Wes, N7WS)
>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.
Yes, fine indeed, Mauri.......and the feedback C of the' Z' is less than
9% that of the 'A'.
>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.
The improved, more-stable, 3CX1200Z7 and special socket costs more. As
far as I know, Mauri, it is not being used in a manufactured HF hamband
amplifier.
R. L. Measures, [805]386-3734
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