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2.
INTRODUCTION
The Challenger is a high quality RF linear amplifier which is designed around a single
GS 35 medium mu triode in grounded grid configuration. The amplifier uses a Pi
network in the output circuit and a tuned input circuit on each band to give maximum
rejection of harmonics. The amplifier is fan-cooled with an internal flatpack blower to
provide forced air cooling for the valve and also good circulation around the
transformer.
The GS 35 valve must be allowed to warm up properly hence a 2.5 min start-up
timing circuit is incorporated which disables the PTT operation until the time has
elapsed. There is also a soft-start circuit to prevent a high EHT surge on switch-on.
There are five LED status indicators READY, WAIT, GRID, TRIP and ON-AIR.
Two panel meters provide continuous indication of Plate current and a switch-selected
choice of Grid current or relative RF output. The latter should normally reside in the
Grid current position as it is most important to monitor the grid current at all times.
A nine position switch, one for each band 10, 12, 15, 17, 20, 30, 40, 80 and 160m,
selects band coverage.
The Tune and Load controls are attached to the capacitor using 6:1 epicyclic drives to
ensure silk-smooth tuning.
The power supply uses a specially designed 2.5kVA toroidal transformer, which
provides the three separate voltages needed in the amplifier:-
1.
HIGH VOLTAGE which is 3600V (voltage doubled from the transformer),
the main voltage applied to the valve anode.
2.
HEATER SUPPLY which is 13V at 4Amps.
3.
CONTROL VOLTAGE which is 12V used for pulling in the Tx/Rx relays and
gives the LEDs the control condition of the amplifier.
The cabinet chassis is made from Zintec steel plate for maximum strength but the
covers and front panel are aluminium to reduce weight.
