Modifying the Tait T199 Power Module for Class AB1/(cont)
An added advantage offered by the T199 module is that it comes complete with a PIN-diode T/R switch between the SD1019/11 and the harmonic filter. It made sense to make use of this feature, making it possible to dispense with the large PCB relays previously used. In this configuration, the filter also serves to afford the receiver a modicum of protection from out-of-band interference.

As previously mentioned, this is essentially a no-tune amplifier. There is one tiny trimmer after the PA, but I elected not to touch it. The only adjustment required is the setting of the quiescent current for the two transistors. The easiest way to achieve this is by monitoring the combined collector currents of the two devices. Provided the output from each bias generator is below the base-emitter threshold, the current should be negligible. If it isn’t, something is wrong! I set the quiescent current for the SD1012 at 25-30mA and 45-55mA for the SD1019/1. (Available data regarding the two transistors is sparse, thus the final bias settings may require some tweaking following on-air tests, but these ought to be good starting points). Having previously set the output from the transverter board to 100mW, I applied some RF. With this level of drive, the combined peak PA collector current should be around 5A. It should not be necessary to go beyond 200mW in order to achieve 35W out.