AC or DC filament

[pro_crip]

I was wondering, why is AC used for the filament heater when AC causes hum? You already go through the trouble of rectifying AC into DC, why not just use the DC for the heater? Just a curious newbie, thanks.

Rich

[EWBrown]

Probably just a matter of economics, if the manufacturers can save a few bucks, they will. DC filaments aren't really needed for a power amp like the ST-35, which handles relatively large signal voltages. Preamps, Linestages and Phono Stages really do need DC filaments, their higher gain and very low input signals make eliminating any extra self-generated noise much more of a priority.


Dynaco recognized this early on, their PAS series preamps use 4 12AX7s, the 12 VAC filament power is rectified/doubled to 24 VDC, the filaments are wired in two pairs in series, though the 12X4 rectifier for the B+ is powered with 12 VAC.

/ed brown in NH