On 06/06/2011 21:24, Dr. David Kirkby wrote:
> Are there any published designed for 2m linears using the 4CX1000A tube? I
> tried
> to build an amp with a pair of them in push-pull grounded screen, but it never
> worked well. Now thinking about something more modest - a single tube, using a
> conventional socket with cathode grounded.
>
> I know there was a design in the ARRL handbook for a single-tube grounded
> screen
> amp, which gave me the idea of a push-pull amp, but I never got it working
> very
> well (it was drawing positive grid current with an output of only 300 W. Had I
> pushed the input power, it would have probably damaged the tubes).
>
> I've seen comments that the grounded screen amp in the ARRL handbook was not
> such a good idea, which is making me think it might be better to go along the
> route suggested in the data sheet. i.e.
>
> cathode at DC ground
> grid at -60 V
> screen at +325 V
>
> I've also seen a design on the web which ties the screen and grid together, to
> use the tube as a triode, but again I want to avoid that.
>
> Are there any decent designs for a 2m amp with a conventional usage of
> voltages?
>
> I recall John Nelson, GW4FRX telling me the 4CX1000A presents more problems to
> hams than any other tube, so I don't think it's the best device ever made,
> but I
> happen to have quite a few sitting around collecting dust.
>
> Does anyone have a socket for one? I built the grounded screen sockets myself
> (had access to a workshop at the time). But I want to buy an Eimac socket,
> with
> the screen decoupling capacitor for this.
>
David,
The single 4CX1000A on 2m was published in the 1990 ARRL handbook and
does use grounded screen - its grounded at DC and RF with the cathode at
-ve screen voltage.
This approach avoids having to have a screen bypass capacitor, provides
very high isolation between anode and control grid and allows the tube
to work well at 144MHz without neutralising.
Several of these amplifiers were built in the mid 90's in the UK and
Mick G1EZF had one that would do 900W+ output with the drive from an IC202S!
I had the chassis for two amplifiers professionally built by a local
fabrication company at a cost of over £600 and have the hard to get
parts including the sockets, blowers, heater transformers and drive
train (SmallParts pulleys and belts) for the anode tune, etc. along with
a set of G3SEK Tetrode Boards that I may be willing to part with as I've
decided to change direction and go solid-state with a pair of BLF578
devices in a design that is a cross between Fred N1DPM's design and
F1JRD's design.
Contact me off-list if interested.
Mike G8TIC
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