by EWBrown » Tue Aug 03, 2004 11:27 am
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
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