Light bulb, or light bulbs - the (now frowned upon) tungsten type. Pull-up the datasheet for the TIP147 and workout its Safe Operating Area and hence the maximum current at 40V. Then you need a D.C. power supply of more that 40V and whatever light bulb as appropriate to limit the current.
I assume the TIP147 will be on a heatsink?
You might also want to look at how you label the adjustment pot. As it is it appears to be labelled for the voltage present across this circuit, however, when you put this circuit into your final thing (transmitter cathode bias cct I assume), the sense of the pot is going to have the opposite effect on the bias current.