No.1
Amateur Radio
4CX400A
Using the Svetlana 4CX400A on 432 MHz
By: Fred Stefanik, N1DPM
Back in 1984, I decided that 100 watts of "brick" power wasn't enough for me on 432 so a home-brew project was in order. An article appeared in QST shortly before my decision that helped spark my decision to build an amplifier. This article was by Fred Merry, W2GN, and it was on a tetrode amplifier for the 70 cm band. This amplifier was based on the good ol' 4CX250B or you could use and 8930. I proceeded to "whip up" one of these especially since I already had most of the parts, and a power supply ready to go.
After building the amplifier the only difference was the tube that I was going to use was a 4CX350A/DX553 by Amperex since I had a few of these. These seemed to work OK on 432 although they did experience quite a bit of power drift. Even though, this amplifier would quite happily produce 350+ watts. It also proved to be quite reliable surviving over 10 years of contesting, etc. on the original tube. (just like a Timex!!)
About a year ago I heard about a new tube being built by the Russian manufacturer Svetlana, called a 4CX400A. This tube had full ratings to 500 MHz, and was virtually the same size as the popular 8930. I thought I'd give this one a try in the 432 amp. The only modifications that were made to the amplifier were a new plate line and top cover. These had to be made to allow for the larger size anode cooler on this tube than the 4CX350A that was in the amp already.
When I first fired the amp up a things became apparent very quickly. The gain seemed to be slightly less than the 4CX350A, and there was virtually NO power drift! The amplifier tuned right up easily and was stable. With a plate supply of 2500 VDC (no load) and screen voltage at 400 VDC, this amplifier would now produce 500 watts of stable, clean RF. The high voltage under key down conditions would sag to approximately 2000 VDC. The plate current was set to the recommended 160 mA at idle and would draw approximately 500 mA key down. This shows a plate efficiency of 50% with a dissipation of 500 watts, 100 watts over specification for C.C.S. type operation. I ran the amplifier this way during the 1996 January VHF Sweepstakes and it performed flawlessly.
Being the power hungry, spec ignoring, VHF'er, I thought that there was more to be had from this hot little tube since the grid current meter even at this power level was still not showing any current draw. So what's next, I said..."MORE POWER IGOR" and the little tube that could responded with..."YES MASTER!" I couldn't believe it! This tube just starts to draw control grid current at 625 watts output!! This is with 2000 VDC on the plate, and its rated for 2500 VDC. I do not recommend this as it really exceeds the maximum ratings on the tube, but it shows just what this thing will do. So I turned the drive back down to run this tube at the 500 watt level, gave me 500 mA plate current meter a rest, and hopefully this tube will provide at least as many years of trouble free service as the last in this amplifier.
Some closing comments. Everyone who has listened to this amplifier on the air has said it is very clean and splatter free. This includes a number of stations that are less than 25 miles away. I'm not exactly sure just what issue the original article appeared in, but it was back around 1982 or 1983. Don't forget to use plenty of cooling air, remember more is better. And finally, for all of you guys with either 2 X 8930's for 144, 222, or 432, or AM6154/6155 type amplifiers, when you need to re-tube them this is one to be considered as it ought to plug right in, and fire up easily.
**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.