KAWU Amplifiers In Use: (April, 2003)

222 Mhz -- HomeBrew 4CX250B -- 250 Watts+

On April 16, 2003 I finished this amplifier for 222Mhz. The amp is described in the 1971 ARRL Handbook (pg 440).

Wanting to run FSK441 (as well as SSB/CW) on 222 and being so happy with the 432 amp built around a 4" copper tubing cavity, I built another 4" Cavity. I liked this design in part for the ability of the cavity to sink heat during high duty cycle transmissions.

The amplifier was built around junk box parts. The 1000VCT plate transformer provides 1400Vdc plate voltage and by using a choke input filter off the center-tap, 315Vdc regulated is obtained for the screen.

The amplifier provides over 250watts output at better than 60% efficiency with only a couple watts drive from the DEM transverter. The rack panel is 8 3/4" high.

6 Meters -- Home Brew 8560A

On Feburary 13, 2002 I got this amplifier finished to the point where I could put it on the air. The amp with a "junkbox" tube is giving me good reports and about 200 watts output. It is built based upon a 1978 4CX250B ARRL Handbook design but using the 8560A instead.

The plate transformer's current capacity is lacking and the HV sags under load more than I would like. This amp, much like the 1296 amp has self-contained powersupplies and a variac for HV control. It drives very easily with the Tentec 1208 transverter. Thank you Roger KØMVJ for the meters that I rescaled and use to monitor plate, screen and grid current. Thank you Jim KBØCIM for the tube socket and heatsink for the 8560A.

2 Meters - A pair of 8560A's

The first week of January 2002, I became the proud owner of this "little" beauty. This is the amplifier chassis of a Motorola 155Mhz commercial two-way repeater. The amplifier had experienced some problems in it's past, but after modifying the plate, grid tuned circuits, bias voltage and some metering repairs, it is making power on 144Mhz.

The amp uses a pair of 8560A conduction cooled tubes. The access doors on the front panel allows tube replacement. With 1500 VDC on the plates at 430ma it provides about 400 watts output. The original power supply (which I am using) uses a constant voltage ferro-transformer for voltage control and does not allow a higher plate voltage to be used.

I used this amp for the January 2002 VHF contest and it ran very well.

2 meter 4CX250 :

The top cover has been removed where the plate line and grid compartment can be seen on the amplifier built in May 1967. It was described in the October 1956 QST. The amplifier is built in a "bathtub chassis 7" x 17". The plate line is constructed out of common 1.5" copper water pipe which is the same diameter as the anode of the 4CX250. Two copper straps to the left of the tube chimney can be seen, one clamps the tube anode to the plate line, and the second holds one disk of the plate tuning capacitor constructed out of copper disks. At the far left end of the plate line a square brass plate was soldered and is separated from the end of the chassis with 10mil Teflon for the plate bypass capacitor. It the left end, the grid compartment can be seen with the coil and capacitor. Input is link coupled from the exciter..

The amplifier has been in reliable service for almost 35yrs. It produces 250watts DC output with only a couple of watts drive with 1800VDC at 270ma on the plate.

432 MHZ 4CX250B Cavity

This photo shows one corner of the homebrew 432 transverter built in March 1972. Only the cavity amplifier is now used. It is built from a length of 4" copper water pipe that is now hard to find, at least here. Inside the cavity is a short length of that 1.5" copper pipe like the 2 meter amp. The internal construction is very similar to the 2 meter amp, only things are shorter. Teflon can be seen at the air exhaust opening at the top. The plate bypass is constructed the same as the 2 meter amp except the brass plate inside is round. You can see the ceramic feed troughs and nylon screws holding the bypass together. One screw is brass and is used to get B+ to the anode. This amplifier is described in the 1968 ARRL VHF Manual as well as various other ARRL publications around that time.

The amplifier is as reliable as the 2 meter amp. It does require a bit more drive but with less than 10watts drive it produces 250 watts DC output with 1500VDC at 320ma on the plate.

These amplifiers operate from separate HV supplies. The 300 volt + screen supplies are electronically regulated using a 5881 tube (rugged 6L6) series regulator and VR tube like reference. High voltage zeners were not dependable for me in my amplifiers.

1296 Mhz 2C39 (N6CA) Amplifier:

This amplifier was completed and put on the air in May 2001. It was described in the March 1985 QST and various other ARRL publications. The cavity is constructed out of aluminum heavy wall pipe, or can be machined.

I am driving the amp with the DEM 1296 transverter 3 watt version. I have variac control of the plate supply to in excess of 1100 VDC. I am starting with air cooling and hope to later construct a 2C39 intermediate driver amp and move to water cooling if it looks desirable. I am getting around 40 watts out, with a good 7289. Most of the time I operate it with 750 volts and around 20 watts is the normal output. I have seen 50 watts with 1100 volts and the resting current elevated.

The cavity and anode box can be seen at the far right in the photo next to the filament and bias supply transformers. The plate transformer, blower and antenna changeover can be seen mounted on the homemade chassis in this photo. The 5amp variac can be seen foreground right.

Front Panels:

(Left) 2 meters on top and 432 below.

(Right) 1296mhz

High voltage and cathode current metered.

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