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RF Power Amp ZA1 MK-2

ZA1 Main
ZA1 Mk-1
ZA1 Mk-2
Collier and Beale
Circuit diagram
Internal views
ZA1 Mk-2 order

A second version, the ZA1 MkII, shown above, differed 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. 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.

March 2009