Subject: 2m linear using a pair of 4CX100A's- Help!!!
To: ham-homebrew@ucsd.edu
A friend and I got a couple of new Eimac 4CX1000A's cheaply, and deceided to design a linear amp for 2m around them. For those who don't know the 4CX1000'A, it's a 1000W, class AB1 only tetrode, designed for use to 110MHz, but many designs exist for use at 144MHz. Max current is 1A, max voltage is 3kV.
The amlifier is *supposed* to be is push-pull, but I'm not sure that they are, since the amplifier works rather poorly. Any suggestions would be appreciated. It was believed to be *almost finished* about two years ago, but has never worked properly.
Because of the cost of the bases, I fabricated a couple from brass and PTFE. To avoid having to make screen decoupling capacitors, the amplifier runs in gronded screen, similar to the design using a single tube in the ARRL manual. Hence:
Screen voltage 0V wrt earth
Cathode voltage -325V wrt earth
Grid Voltage -385V wrt earth
Anode voltage +2700V wrt earth
This gives the required 60V between cathode and grid and 325V between screen and cathode. Standing current is 250mA per valve (if my memory serves me correctly, since I haven't 'played' with it for a long time.)
DC conditions seem okay, and the ampifier seems stable, with no hint of output with no drive, no matter where the tuning knobs are. It is not neutrilised.
Applying RF drive, results in in output of about 100W, before the tubes start to draw grid current. Since the grid dissipation of these tubes is zero, it would not be safe (or linear) to run more output power than this. I have pushed the tubes to draw about 1mA of grid current each, where the output rises sharply, to about 600W. Since we were hoping for about 3200W output power, we clearly have some way to go!!!
The power supply is designed so that it is possible to cut one valve off, so that the standing current of each valve can be set precisely equal by slight adjustment of the individual grid voltage. Cutting one valve off, then applying drive, again gives at most a few hundred Watts from one tube.
One thing that has always bothered me, is that it is *poosible* that the tubes are being driven in push-pull (as I want), but the anode circuit is functioing in parallel (or visa-versa). Is there an easy way to check?? However, the fact it works poorly on one tube, with the other cut off, suggests this is not the problem.
The grids are two half-wave lines. The anode is a 'U' shaped arrangement, with quarter wave lines. The Valves are at the ends, and HT fed to the centre. The output was supposed to be link coupled, much like many amps using twin 4CX250B's.
I've virtually given up. Its cost nearly #2000 to build, but produces less output than a 4CX250B!! I can't see any way of getting it going properly. Any ideas?????????
By the way, we have a decent power meter and 3kW oil cooled load.
On Fri, 18 Mar 1994 20:02:00 GMT, Kenneth L Florence wrote:
>Hi all, I hv a Heath SB1000 (Ameritron AL80A), that I wud like to add QSK to.
>ALl the QSK kits are a bit expensive for my taste, but I was wondering. Is it
>just the speed of the internal relay that keeps this thing from being QSK? Afterreading Richard Measure's (SP?) article in QST, if I just replace the internal
>relay with a high speed vacum relay (Jennings) will I be able to run it in QSK
>or are there other restraints. Tnks for any help you can give.
>
>DE KA3PLS
You can qsk with the jennings relays but you need some control circuits to
prevent hot switching. If you find a diagram of an old alpha 77 that is
exactly the scheme they used. 73 Bill AA5ZQ
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Date: Thu, 24 Mar 1994 18:03:21 GMT
From: rit!sunsrvr6!jdc@cs.rochester.edu
Subject: Simple Tx/Rx?
To: ham-homebrew@ucsd.edu
Greg Segallis (gsegalli@ic1d.harris.com) wrote:
: I want to build an RF remote shutter release for a camera.
: I need a max range of about 50' and
: it would be nice to have the receiver fairly insensitive to
: unwanted noise.
: I have seen several designs for simple transmitters (CW, tone
: transmitters, etc.) They are simple (just a few parts, with
: coils I can wind myself), small and cheap to build. They seem
: to have adequate power for this application.
: What I havn't seen is a simple receiver circuit.
If you only need one (as opposed to manufacturing thousands), try
a radio-controlled car transmitter/receiver/servo. I use one for
kite aireal photography, and it works well. You only need one
servo, so a 2-channel radio is more than enough. Look for ads in RC
car/airplane magazines. Or better yet, buy a used RC car, and head
down to the local hobby store for one extra servo. The extra servo
goes on the camera, and you can move the receiver between the
