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RE: Yes, three 12AX7's ...

DC heating IS the way to go in a phono preamp.

3X 12AX7s require 900 mA. at "6" V. and 450 mA. at "12" V. IMO, the best way to build a DC supply for the heaters starting with a 6.3 VAC winding is via a Greinacher ("full wave") voltage doubler and a 7812 3 terminal rectifier. Use Schottky diodes and enormous 'lytics in the cap. stack. For 450 mA. of 12 VDC, a 2A. rated 6.3 VAC winding is needed. Nothing new here about TANSTAAFL applying.

Don't forget a substantial heat sink for the regulator IC.

It's cutting things too fine to bridge rectify 6.3 VAC and then regulate down. Maybe a low dropout regulator will have enough headroom, but the doubler definitely provides the headroom necessary to correct operation of an inexpensive 7812.


Eli D.

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