wrote:


And then there were those old 1B3 tubes in TV sets. Where did the 1 volt
heater voltage come from? (and why didn't they just stick with 6.3 volts
like other tubes?)


** Like all rectifier tubes, the cathode is the output point and in this case can be up to +26kv above ground.

The 6.3V heater chain used by other tubes cannot be used as a 26kV insulation barrier would stop all heat getting to the cathode. So the tube cathode is directly heated by a winding that floats at the HT voltage.

http://www.radiomuseum.org/tubes/tube_1b3.html

Since the load current is 50mA max, a low powered heater is enough - 1.25V and 200ma do the job.

The 1.25V needed is conveniently derived by a single turn around the HOT core, carried out with wire insulted to stand 20kV or more.


..... Phil