Power Transformer for Tube Preamp Questions

[skipwave]

I've got a transformer here (actual a bunch of them) with the following:

PRI..........117V.......WHT-BLK
...............117V.......ORG-RED
SHIELD...................GRN-YLW
SEC.....263V 100ma.....YLW
...........15.3V 1A..........GRN

I want to use this for tube mike preamps, but a few things seem unusual about it.

-Two 117V primaries? Is that to allow series operation with 220V mains (234V?)?
-SHIELD? What?

-I want DC for the heaters, so what's the most efficient way to get 12VDC from 15.3VAC? I could just rectify then calculate a resistor to drop it.

Please forgive the newby questions. I just haven't encountered these things before.

 

[PRR]

> series operation with 220V mains (234V?)?

Yeah. They don't have that in Chicago.

110, 115, 117, 120, 220, 230, 240: all utility company voltages are approximate. I get 119V in good years, 108V in bad years. When I moved into my house I was getting 126V. The numbers are just so you don't connect to a totally-wrong voltage, not to say what the "right" voltage is. The secondary voltage will vary too, so don't take them too literally.

> -SHIELD? What?

Tie to chassis. When the elevator throws big spikes on the utility power, this helps block it from coming into the amp.

> what's the most efficient way to get 12VDC from 15.3VAC?

Several ways to skin the cat. Simplest is just a 100V 10A bridge rectifier and 3,300uFd 25V-35V cap. Expect 20VDC at 1.2Amps DC. Wire your 6V and 12V tubes up to 12V or 18V, use a dropping resistor and another 3,300uFd cap for extra filtering.

 

[skipwave]

Thanks PRR!

Yeah, we get a meager 107V here in Chi-town.

If anyone's interested I've got a bunch of these transformers and will be selling them cheap, see the Black Market. Thanks.