by EWBrown » Thu Sep 27, 2007 8:29 am
That power trannie would barely be sufficient, and would not permit DC for the filaments.
I modified a Bottlehead "foreplay" linestage long ago, it originally had the same or a very similar AES (or Allied) hammond-made power trannie, to feed two 12AU7s, and it was just barely sufficient. I replaced it with a 125-0-125 25 mA and 6.3V, 1.2A trannie so that I could DC power the filaments and eliminate the magnetically-induced AC hum. The trannie was from Bottlehead, and was about $40, and it had magnetic shielding, and was over-built fo the power requirement. BTW, this is the exact same Bottlehead power trannie which is used in their "seduction" phono stage".
The "HV" secondary winding could be 125VAC, 15 to 25 mA should be more than enough, but the filament winding should have at least 6.3VAC @ 1 amp, or 12.6VAC @ 500 mA, if you plan on powering the filaments from DC, as the process of rectifying the AC voltage will increase the AC current demand to about 1.5X the actual DC filament current consumption.
This looks like the well tested back- to-back filament trannie scheme, and the fact that they are (more or less) shielded will help reduce AC magnetic-field induced hum.
HTH
(Instant Update)
The PA-211 trannie from Triode (intended for the PAS2 / 3 preamps) migne be an excellent choice here. The 330-0-330 HV winding is a bit high for this application, but just add in some extra CRC in the filtering for the B+ line, and it should be a winner! The filament winding is 12.6 VAC at around 800 mA, they doubled itup to 24VDC, and ran the four 12AX7s' filaments in a series-parallel arrangement, which drew 300 mA (and the 12X4 rectifier and pilot light filament ran straight off the 12.6VAC).
/ed B in NH
Last edited by
EWBrown on Thu Sep 27, 2007 9:01 am, edited 2 times in total.
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