Although the ZA1 Mk-1 was successful in increasing the RF power output of the ZC1, a necessity created by the huge distances being worked in the Pacific campaign, it was only suited to CW operation sending Morse code.
Operational experience showed the need for a more sophisticated version suitable for CW or AM Phone working. The speed of modern warfare was such that there was no time to encode and send CW messages requiring decoding at the receive point. More and more open AM transmissions became the norm.
To meet this demand a second version, the ZA1 MkII, shown above, was urgently designed, built and sent to the Frontline. It differed from the Mk-1 in that the parallel 807's were modulated by a second pair of 807's fed from a 6V6 phase inverter and a 6U7 microphone preamplifier. The ZA1 Mk-2 unit is a much larger and heavier unit housed in a modified metal ZC1 Mk-1 case.
For this unit the ZC1 acted only as an RF driver and keying or modulation are achieved via the ZA1 Mk-2 amplifier. The Mk-2 was a contemporary rather than replacement for the Mk-1 and both versions were manufactured around the same time in 1943.
The amplifier is built on a modified ZC1 Mk1 chassis reduced down to half height to accommodate the height of the four 807's in the ZC1 case.
The case is a modified Mk1 ZC1 one fitted with louvers at one end and with a slide cover at the opposite end allowing access to the final coil tapings for matching purposes. A slide cover on the top was included for ventilation purposes.
Opening up the top slide reveals the plate connections for the modulator, something O.S.H. (Ocupational Safety and Health) would go "bananas" over in this day and age, but of no consequence in 1943!
A dummy microphone lead connects the ZC1/ZA1 to key on the ZC1 when transmitting.
In summary, the major differences between the two versions is the addition in the ZA1 Mk-2 of an AM modulator, with associated microphone amplifiers, MCW oscillator, and a second built in vibrator power supply. On CW just one power supply is used, but when phone or MCW is needed, the extra PSU provides HT supply for the power modulator. The inclusion of this extra hardware considerably added to the weight and size of the Mk-2 version.
When a short mobile whip aerial was used, a ZA1 loading coil could be fitted to the case upper flange slide, to date no surviving example of this component has been found.
Both Marks of the ZA1 are believed to have only been made by Collier and Beale. Their design is reported as being a joint exercise between the Post Office Radio Section and Collier and Beale with the original design based on a circuit given in the RCA Transmitting Tube Handbook current at the time.