No.5
Amateur Radio
4CX400A

Last year, Svetlana Electron Devices announced some economical new high power transmitting tubes for amateur use. One that caught my eye was the 4CX400A (400 Watts plate dissipation). A closer look revealed it had possibilities as a drop-in upgrade to the Eimac 8930 (250 Watts plate dissipation). We'll examine this upgrade to two popular VHF amplifiers, ARCOS (W2GN) 2 Meter Kilowatt, and the Fair Radio/FAA surplus AM-6155/GRT-22 222 MHz linear amplifier.

4CX400A Typical SSB Operation for a single tube (ICAS rating):

The 4CX400A plugs directly into a Svetlana SK2A or Eimac/Johnson SK-600 series socket. This should prove to be an excellent choice for new designs, and can be retro-fitted into existing amplifier designs quite easily. The anode size (1.984" - 2.031") is just slightly smaller than that of an 8930 (2.05" - 2.08"). It looks like all that needs to be done is make sure the fingerstock engages and re-adjust the grid bias! Let's try it, shall we?

The ARCOS 2M Kilowatt

This amplifier used a pair of 8930's. First thing to do is remove the RF deck top cover (Caution: Never work on an amplifier that's powered up!). Remove the 8930's one at a time by grabbing the cap at the top of each tube and wiggling them back and forth while pulling upwards. Once loose from the socket, you should be able to pull each tube straight up out of the finger-stock lined cutouts in the plate line. I took a vernier caliper and measured the anode OD of both the old 8930's were and the new 4CX400A's at this time. The Eimac 8930's were 2.0625" OD and the 4CX400A's were 2.01" OD, or about 50 mils difference. This gave me an idea of how much inward bending of the fingerstock should be done (0.025" inwards all the way around). I just used my finger to do a few tines at a time. Take your time, beryllium-copper is brittle. After you break the first one off, you'll know how much bending is too much. Don't worry, a few missing fingers won't affect performance that much. Plug in the new 4CX400A's by lining up the keyway notch on the socket with the key on the center pin of each tube base and push them in slowly. Replace the RF deck top cover.

Next remove the top cover of the powr supply (Caution: Make sure it has been unplugged a while and any residual voltages have discharged). You will see a vertical aluminum piece off to one side. On it are four stud mounted 10 Watt zener diodes. Three 110-130V zeners are in series to regulate the screen supply voltage. Determine which one is not, using an ohmmeter, and remove it. This is a 56V 10W common-anode stud zener. Replace it with a 24V 10W unit of the same type (ECG5198A or equivalent). Now you'll be able to adjust the grid bias from about - 60V down to - 24V. That's it, replace the cover. Hook everything up with a wattmeter in series with the load and power it up. Place the RF deck meter in the Plate current postion. After everything's warmed up, key the amplifier on SSB without audio and observe the plate idling current. Quickly adjust it to about 200 mA. using the bias adjust pot on the power supply. After retuning with RF drive, these were my results:

4CX400A Modified ARCOS 2m Kilowatt results (230 VAC operation)

Screen = 390V

650 mA x 2000V = 1300 W Plate input power

Output power = 750 W

Efficiency = 58 percent

RF drive = 7.5 W

Power gain = 20dB

On the air checks revealed a cleaner signal (improved IMD), and a 10 percent improvement in efficiency and power output over the old tubes. The exhaust air temperature was noticeably cooler also. This amplifier was now just loafing at 750 Watts. A bigger plate transformer could have made this a very nice 1000 Watt output amp.

The AM-6155/GRT-22 222MHz Amplifier:

This amplifier used either an Eimac 8930 or an Amperex DX393. My particular amplifier had the Amperex tube. Observing the previous cautions, unscrew the four screws holding the RF deck plug-in to the power supply unit. Pull the RF deck out and place it on the bench. Unscrew the square metal cover that forms the air exhaust outlet. Pull off the Teflon air chimney, and remove the tube using the wiggle and pull technique. You will need a tool like a small flat blade screwdriver with the tip bent at a right angle in order to bend the fingerstock. Carefully bend each tine inwards about 0.025". Work you way around until this is complete. Line up the notch in the socket with the key at the base of the 4CX400A and push it in. Replace the Teflon chimney and screw the exhaust cover bank on. Option: you may want to replace the directional coupler in series with the output coax with a N female barrel now. Most of these were rated for less than 100 Watts (which is what the FAA ran the amplifier at). Chances are very good it's blown now or will be very soon. Then it's only function will be to add insertion loss. Cut the wires off and insulate their ends if you choose to remove the unit. Plug the RF deck back into the power supply. Now the good news: you're done - the power supply bias circuit already has enough range to accomodate this tube!

After the amplifier warms up, key it with zero drive (SSB without audio). Adjust the Plate Current adjust pot through the hole in the power supply top cover in a CCW direction until you get about 10 percent of full-scale reading on the meter in the CUR-PLATE position. This read 5 on my 0-50 meter which equates to about 100 mA. of plate idling current. In any event, the idling current is 20 percent of the plate current at maximum RF output. Apply no more than 10 Watts of CW drive and start tuning, 10 seconds at a time. Start with the plate tuning and then peak the input tuning adjustments. If you don't see any output on the wattmeter right off, peak for maximum plate current first until you start seeing wattmeter deflection, then use it to finish tuning. The antenna coupling and plate tuning require consecutive retuning until full output is reached. Here are my results:

AM-6155/GRT-22 222 MHz Amplifier w/ 4CX400A results

Screen = 325V

450 mA x 1800V = 810 W Plate input power

Power out = 450 W (at 115 VAC - slightly more at 230 VAC)

Efficiency = 55 percent

RF drive = 15 Watts

Power gain = 15 dB

On the air checks revealed a very clean signal. N1DPM 35 miles away reported a 50 dB/S9 signal only 4 KHz wide. I noted 100 watts more output power and a vast improvement in stability over the old tube (it was probably soft). The amplifier has absolutely no tuning drift after initial warm-up now. An excellent improvement for contesting duty, and easily done!

-73, Ron WZ1V
Internet E-mail: klimas@uhavax.hartford.edu
Internet WWW: http://uhavax.hartford.edu/~klimas

**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.

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