A water cooled 4CW800 PA for 144Mc


After building several PA's with one or two 4CX250BM's I decided to change to water cooling.

The advantage of water cooling being the complete silence in the shack. (no blower noise, etc). In the beginning I used a closed circuit with the water reservoir, pump and cooling unit placed on the attic above the shack. Although this system worked OK the water slowly got polluted because of the electrolyses in the water. The water needed to be refreshed on a regular basis.

In later stage I installed a water outlet in the shack which was directly connected to the PA. The heated water went to the gutter of the roof.

The PA I build is based on an rf-grounded λ/4 design with a round messing pipe (the lecher) connected to the anode of the tube. The lecher tube contains the hoses for the water cooling. These hoses must be within the tube because in this way the are out of the rf-fields of the PA and have no influence on the tuning.


 The tuning and loading are done by sheets of Berrilium copper. They are hold in position by M6 threaded shafts. In the prototype the output C is pushed backwards by a piece of fiberglass (or etched PC board).

This has been replaced by a pull back mechanism using a nylon guitar wire.

Note the white Teflon on the lecher and the sheet of brown Teflon between the plates of the tuning C. Both are needed to prevent HT short circuiting.












A closer look at the cathode compartment.

The rf coupling to the grid can be seen on the right side of the tube socket.

The socket is a regular 4CX250B socket of which the center socket has been twisted to allow reception of the tube.









The lecher is grounded for RF by means of a double sandwich capacitor of approx. 2.5 nF.










The water in- and outlet. This is a (dusty) picture of a 432Mc version which unfortunately never produced any rf. The tube just would not like to be used on that QRG.

The spec's said so, but if you don't try you never know for sure....

The water hoses are approx. 30 cm long and connect between the HT of 3.5 kV and ground potential. They act as a water resistor of approx. 150 k Ohm. Care should be taken with the contamination of the water to prevent the resistance from becoming to low.

The inside of the lecher is covered with Teflon sheet in order to prevent the hoses from touching the lecher, which is on HT all the way. The voltage in the water decreases from HT to 0V so the voltage difference between the water and the lecher increases along the way.

The hose material can not withstand a voltage of approx. 3kV so additional insulation is needed, hence the Teflon sheet.

At the passage through the sandwich capacitor the hoses are also guided through a Teflon disk effectively holding the hoses in place and preventing them from touching the middle sandwich plate which carries HT voltage.