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In Reply to: Re: 12sx7's in the new MA1 Silver Edition posted by Lew on October 27, 2004 at 10:30:09:
Cousin B, Lew,I feel that I can answer these questions..
The Silver Edition MA-1 has a DC filament supply @ 6.3VDC. The supply is derived with a full-wave rectifier (2 x 8A Schottky diodes) from the main dual-winding filament toroid. Complimentary leads from each winding are joined to form a virtual 'center tap', which serves as the negative (ground, if you will) side of the DC filament supply. The supply uses a simple pi-network filter, with three 2,200uf@16V caps, and a small (~0.16ohm) resistance between the poles of the filter. Schottky diodes are needed, as the voltage drop across standard diodes would be too large to create a useful pi filter; we need the small extra headroom that the schottkeys afford us.
I don't see any problem replacing the basic full-wave setup with a full-wave bridge rectifier, and deriving your 12.6V from that. As I mentioned, the caps are 16V devices, so will be fine to re-use for a 12.6V supply. One could also try a full-wave voltage doubler; though that type of circuit tends to be less reliable, and I don't suggest it.
A little experimentation will be required, to see if the bridge rectifier can be a standard type, or built from schottky devices; same reasons as stated earlier. Also, the resistance value between the filter poles will need to be recalculated.
Finally, and most daunting, two resistor values in the fixed-bias circuit will most likely need to be recalculated. The 12SX7 doesn't behave *exactly* like a 6SN7, so the grid voltages in the output section will probably be off. Measure them accurately with the stock 6SN7 configuration, then plug in new resistance values when the 12SX7 are in place, such that the output section grid voltages are the same as before.. roughly -66V in class A, and -92V in class AB.
Hope these comments are helpful. As always, this is my own personal opinion and does not reflect the views of anyone but myself. Your mileage may vary. Void where prohibited. You burn it, you bought it.. etc.
Bill
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Follow Ups
- Re: 12sx7's in the new MA1 Silver Edition - Legendre 15:59:35 10/27/04 (0)
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