One of the discussions on the BITX20 IO list over recent weeks has been a variable power supply to control power out on different bands. This seemed like the wrong approach to addressing variable output of the µBITx to the editor of ubitx.net (where the problem of inadequate drive or inadequacies in the PA design should be addressed directly) until he spotted this circuit diagram drawn up by Walter W9KJO, based on earlier suggestions. It is simple and straight forward with the TIP142 device at the centre of the “buck” function. 24v input can be adjusted downwards to set the power output of the µBITx PA stage.
Walter says, “It controls voltage nicely. Really helps limit output power while working digital.”
However the TIP 142 really generates some heat. He has a heat sink on the device, but the heat sink will need to be much larger to be safe.