A 50 MHz Power Amplifier using a pair of 4CX250B tubes.


This amplifier was built during the early nineties, I was unable to find a reliable design for an amplifier that would be able to utilise the parts I had available in the junk box,so I decided that I would design something myself. As I already had a power supply that was capable of running an amplifier of this sort, I would only have to build the RF deck. For a chassis I used a redundant K2RIW 70cm amplifier, as I already had built two of these.6mpair250_2.jpg I first stripped out the Grid line and associated capacitors and then the plate line, plate tuning and loading controls as these would be of no use on 6m. The grid circuit was then fabricated out of Silver plated copper wire and a Jackson variable capacitor. With this configuration the amplifier requires about 4 watts of drive to achieve full output, which will be around 500 watts in AB1 or 700 watts in Class C. The tube sockets used are SK620 which is overkill on 6m, but if you change to SK610 you will need to space the tubes at a greater pitch because the flange is of a larger diameter on these bases. The input relay is an old Magnetic devices relay, the output relay is a Totshu CX520D.

The screen supply (330v stabilised) is supplied direct to the tube sockets, a MOV is soldered to each socket pin, this is done to ensure that, in the event of an HT to screen short the screen voltage will be clamped below 400v, otherwise damage to the screen bypass capacitors in the bases will effectively destroy them. The Grid bias is supplied through two paralleled resistors to provide the necessary grid swamping.



The plate compartment can be seen below, the tube chimneys are fashioned out of 0.25mm thick Teflon sheet and are held in place with clamps. The two tubes are connected together by a 10mm x 0.5 wide copper strap bent as shown and folded to form a clamp around the Tube Anodes. In the centre of the strap a hole is drilled and a Doorknob capacitor is connected between the strap and the Anode tune capacitor. 6mpair250_1.jpgThis capacitor is a wide spaced Jackson variable of about 25pf value. It must be wide spaced as there is a high RF voltage across this part. The plate circuit itself is a Pi-L circuit which was neccesary for two reasons, the first being spectral purity, second because the only load capacitor I had available did not have sufficient capacitance to provide sufficient load to the amplifier.

You will see the Load capacitor is actually two Jackson variables. The circuit needs around 140pf to correctly load the amplifier, the only available Capacitors were one of 100pf and one of 50pf, so they are both placed side by side and the smallest one remains closed all the time. The Main coil is 5 turns of 3.2mm dia Copper pipe, the L part of the cicuit is a 5 turn coil of around 1.5mm diameter enamelled copper wire. And is soldered between the load capapacitor and the output socket. The HT (2300V) is supplied though the the rf choke at  the top of the picture and is made up of a piece of 12mm diameter Teflon bar with around 70 turns of 28 swg enamelled copper wire.