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RE: Cheap DC Filament PS

Posted by coffee-phil (A) on June 4, 2017 at 16:55:37

In Reply to: Cheap DC Filament PS posted by Triode_Kingdom on June 3, 2017 at 18:38:55:

Hi Triode Kingdom,

I like your approach. I have repurposed switchers for various purposes but I just get SMT resistors of the value which I need.

You may have to over size your switcher to start properly with a tube heater load. Tube heaters draw ~ 10X the normal current when cold.

I'm making a crystal pickup phono stage for a ~ 1946 78 changer I'm restoring. I was going to use a 12AU7 as a cathode follower. I have a sealed board mount 12V, 0.5 A switcher with which I was planning to light the tube connected for 12 volts. Since the switcher is potted I planed to just live with 12 volts. The 150 ma heater load caused the switcher to make several repeated attempts to start. Each try heated the heater somewhat so eventually it got there. This would work but seemed flakey to me. I was thinking of some active soft start circuit.

I found a cute little dual triode 6111 which I just had to use. This tube has a single 6.3 V, 300ma heater. I still want the 12 volts for a the cathode bias for the cathode follower. I tried the 6.3 Volt, 300 ma heater with a dropping resistor from the 12 V 500 ma switcher and it started on the first try.

Phil

Edits: 06/04/17 06/04/17

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