Freed Man 12AX7 wiring--no pin 9?

[ViperDoc]

I was looking at some internal photos online of a Freed Man Smallish Box Fifty and noticed he wires his preamp tube heaters individually on pins 4 and 5 and leaves the pin 9 heater center tap alone. Would that wire each triode pair out-of-phase? Why would you do that? Thanks.

[PaulD]

12AX7 heaters can be run in series on 12.6v (wired between pin 4 & 5 with pin 9 not used) or in parallel on 6.3v (wired between pin 9 and pins 4 & 5)

http://www.valvewizard.co.uk/heater.html

[M Fowler]

Pretty sure it's a regulated 12v DC filament supply.

Mark

[ViperDoc]

Cool, thx.