Modifying the Tait T199 Power Module for Class AB1/(cont)
This photograph (left) shows the location of R502, R504, L503 and L504, who’s ‘earthy’ ends require to be desoldered from the ground-plane. The individual bias voltages are then applied to the (now free) ends of L503 (for the SD1012) and L504 (for the SD1019/11).
This photograph (right) simply serves to demonstrate the mounting of the 1uF chip capacitors used to DC-isolate resistors R502 and R504 from the ground-plane.
The photograph on the left shows the modied PA module installed along with the bias board described earlier. Note that the BD139 transistors are mounted on the underside of the board. This is to ensure good thermal conduct with the chassis/heatsink. As the transistors (RF) heat up they require less bias current to maintain the bias point. In turn, VBE for Q1 in each of the bias generators drops as the temperature rises, thus maintaining a stable bias point for each of the two RF transistors.