This circuit is from a 1975 RCA tube manual. It appears elsewhere on the site. R5 depends on the voltage of the amplifier with which it is used and can be changed.
The circuit shown in the figure is suggested by Micrel, based on the MIC2570 boost voltage...
This circuit can work with almost any JFET. The circuit is from an Electronics Now from April...
This circuit should only be used with incandescent lamps and the SCR should be fitted with a heat...