Hello Richard,
I have a 1KW PEP output commercial linear using quad 4CX250B PA tubes,
built by an Australian company Codan in the mid-1970's. It was originally
designed for autotune operation from 2 - 15 MHz with a channelised exciter,
however I have been able to adapt it for use with an amateur transceiver on
the 80m - 20m bands (using the ALC to reduce output power to the legal
limit of course:-). I also use an identical 500W unit which uses only only
two PA tubes in parallel (more suited to our Po limit here). The only
significant difference between the 500W and the 1KW units are the Pi tank
circuit C1, L & C2 values to accomodate the different PA load matching
requirements. Otherwise everything is exactly the same except for the
component count and the 2 blanked holes in the 500 watter PA compartment.
These transmitters were originally designed as base stations for the Royal
Flying Docter Service in the Australian outback.
The PA tubes are connected in parallel, and are grid driven in class AB1
by a hi-mu class A pentode (Phillips E55L). They operate with 1800 VDC on
the anodes with regulated 300V - 350V on the screens, individual screen
voltage being adjustable on each tube for balancing zero sig Ic. In any
case, I try to fit tubes that are fairly close in emission (and mu) so the
screen voltages aren't all over the place. The grid bias has a single
adjustment pot for fine setting of the total zero sig Ic (60mA per tube)
after the tubes have been balanced with the screen pots. The 4CX250B's are
running very conservatively with this set-up.
The tubes are neutralised using a bridge configuration, and negative
feedback from PA anode to driver cathode is also used to minimise IMD. You
might note that the tube manufacturers usually recommend 100mA zero sig Ic
for linear service, so I suspect it is only because of the negative
feedback in these amps. that enable only 60mA without excessive IMD. IMD is
specified at better than 40db below PEP, and I have confirmed this to be
about right using a commercial distortion analyser.
Strangely, there is no parasitic suppression fitted to the PA anodes,
only
10 ohm resistors in the grid circuits. Nevertheless, the amps seem tame
enough. Perhaps its the effectiveness of the screen bypassing built into
the Eimac SK606 sockets that allows them to get away with it. If I was
building a new amp, I would put suppressors in as a matter of course. There
is plenty of info on this subject (often controversial) on this reflector.
Anyway, I hope this might encourage you to use your tubes in a homebrew
amp., and if you are seriously planning to go ahead, I'd be happy to send
you more info on the Codan design if it would help. I have complete
technical manuals here.
Regards
......................................................
Graham Jones (VK3BKG),
You wrote:
----------
> From: G3CWI@aol.com
> To: amps@contesting.com
> Subject: [AMPS] 4 off 4CX250BM Designs
To: <amps@contesting.com>
> Date: Thursday, March 25, 1999 4:07 AM
>
>
> I pick up 4 new 4CX250BM valves at a hamfest recently. Does anyone know a
> source of a design using 4 valves for the HF bands? Presumably they can
simply
> be operated in parallel but will I need independent bias adjustments for
each
> tube?
>
> 73
> Richard
> G3CWI
>
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