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. 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. This 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.