|We got a TH327 TV amplifier cavity, we need to get it
working on 23cm. First thing was to modifiy the cavity, to work on 1296.
Click here to see how it has been done.|
The gain of this kind
of amps depends on the tube, and the way the DC voltages are set up. We chose
for a modern way to set up the different grids, in fact, we wanted to combine 'old'
tube technology, with modern solid state devices. Since the expected gain is
around 8 dB ( the more the better ), we need around 300 watt of driving power.
These days, mosfet devices are available to give this driving power. The
housekeeping of the tube amplifier is very important too. We wanted to get a
very stable UG2, a well protected amplifier circuit, and complete remote control
to the shack. Ian, G3SEK has described these matters in QEX magazine ( December
1997 ) and has made a circuit and board to feed and protect the Tetrode tubes.
This board has been implemented in the amplifier. The board does the 600 V UG2
supply using mosfet's, it does the UG1, and all nessesary amplifier housekeeping
tasks. The UA supply is the orriginal TV amplifier supply, using a 3 phase HV
transformer, and 3 phase rectifiers. The circuit has been changed a bit to meet
our needs. The HV supply is also being controlled by Ian's Tetrode Boards.