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RF Power Amp ZA1 MK-1

ZA1 Main
ZA1 Mk-1
ZA1 Mk-2
Collier and Beale
Circuit diagram
Internal views
ZA1 Mk1 being maintained

The ZA1 Mk-1 was housed in a wooden case similar in appearance to, but much smaller than, the  metal case of the ZC1. Two parallel 807's operating in class C were used as an RF amplifier producing 50 watts of RF.  Designed in an age when military communication was dominated by CW this unit was intended for CW use only.

Although the exact number built is not known today, the ZA1 was not produced in great quantity.


A pair of 807 transmitter valves in parallel forms the transmitter, with both the screen and control grids separated by resistors to reduce the risk of oscillation.  Pairs of heaters are in series to allow operation from 12v DC. The RF input is connected directly to the ZC1 aerial terminal and feeds the RF through a send/receive relay into the grid circuits. The input impedance would be essentially capacitive and high resistance, although it is not known how well the tuning unit in the ZC1 copes with this impedance over a wide frequency range.

The anode circuits are combined into a tapped Tank tuned circuit, with the aerial connection taking power from the tap via an RF current meter. The unit above has had its Tank circuit removed at sometime, and a substitute fitted in its place.

The built in power supply of the ZA1 Mk-1 provides approximately 500 volts using a vibrator and transformer, with the HT output rectified by a thermionic diode. This prevents fast changeover between transmit and receive, as the diode heaters are supplied from the HT transformer and will take some time to heat up. However, having operated many Army sets, a long wait between receive and transmit seems to have been a standard procedure! Note the RF amplifier valve heaters are powered all the time. The vibrator is a synchronous type, with the unused contacts paralleled to provide extra grunt for the primary drive.

A bank of buffer capacitors has been provided on the transformer HT secondary, with the required capacitance being selected in production test. Hence one capacitor is left unconnected in the sample model."


March 2009