The four 807 version of the RF Amplifier was the MK.1 and extremely inefficient, so the design was re-worked to improve things. (Adding a cooling fan to the dynamotor and reducing the 807s to two, with corresponding circuit changes resulted in a rather more usable unit that could be operated continuously instead of a maximum of 15 minutes per hour.) I believe the WS22 amplifiers were conversions of the Amplifier, RF, No.2 Mk.3, with the addition of a meter for aerial current (using the existing metering boxes) and so on.
Chris.
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