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[AMPS] 3CX1200-A7 & 3CX1200-Z7

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Subject: [AMPS] 3CX1200-A7 & 3CX1200-Z7
From: (Tom Rauch (W8JI))
Date: Tue, 25 Mar 1997 16:06:18 -0500 wrote:
> 3CX1200-A7 and 3CX1200-Z7.

> 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.

The 3CX1200A7 is, as you found out, much less stable than the 3-500Z or
the 3CX1200Z7.
It is about as unstable or slightly worse than the 3-1000Z and 4-1000A.

> 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.

Yes it can easily be solved. The cure is not difficult:

1.) Be sure to ground the grid pins with a very direct connection. If
you use the Eimac socket, ground to the top of the pins with copper foil
through the slot direct to the chassis. Otherwise use a socket with a
groundplane attached to each grid pin.

2.) Make sure the anode to chassis path is short and very wide, to
minimize the impedance. This means the entire path form tha anode to the
plate tuning capacitor.

3.) Parallel tune the parasitic suppressor at about 80 MHz, or whatever
frequency you measure the oscillation at. Put the resistor across the
parallel tuned circuit.

Another approach is to put a series resonant circuit (at about 80 MHz,
or whatever frequency the PA oscillates at)in series with the suppressor
resistor, and place that series LCR circuit across the conventional
suppressor coil of 50 to 100 nH.    

> Unless some tricks (plate suppressor) or neutralizing circuit the tube was
> always self oscillating not so much above 30 MHz.

If the oscillation was not much above 30, maybe the grid leads in your
layout were too long. You need to get right at the tube pins with a wide

> If neutralizing would have been a (the only) real solution it was not a
> practical one unless disregarding considerable complications and costs.

Neutralizing will not work, because the reactance (and feedback phase)
shifts too rapid above the self-neutralizing frequency of the tube.

> It came out later (a phone call to ... .... confirmed it) that 3CX1200-A7
> wasn't infact specifically designed as a grounded grid RF amplifier.

It was designed as a grounded grid tube, it used the 3-100oZ grid and
filament in a ceramic envelope. I worked with Buzz Miklos and Jim Aurand
when the tube was being developed. 

> 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.

The 3CX1200Z7 was a product of my complaints about grid impedance. It
uses a grid flange instead of grid pins, that extends operation of the
tube to VHF and makes it more stable at HF.

The 1200 D7 is about the same as the A7, but the Z7 is a big
improvement. The only chnge necessary is mounting and filament voltage.

73 Tom

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