Author: Jim Doyle "jrdmedford" DIY Audio
URL: http://www.diyaudio.com/forums/tubes-valves/98705-use-45-drive-45-a-2.html
It's easy to explain. Transformers are power transfer devices. So, power in = power out. "70 V line" means that 70V is the maximum AC level allowed on the line.
P = V^2 / Z. So, if 2 W is going through the transformer, and 70V is the maximum AC signal on the primary then Zpri = 70*70/2 = 2450 ohms. On the 0.5W tap, Zpri = 4900 ohms. You can now figure out the turns ratio. With 2W coming out the 8 ohm tap and 2 W going in the primary 2W tap, n = sqrt(4900 / 8) = 25
With the RS #32-1031B the primary taps are: 5, 10, 2.5, 1.25, 0.62 W. Build a table Zpri and work the formula for Zpri = 70*70/Pin. The 5 W tap is about 1 K...
There are also 100V line transformers out there too. You can work the math. However, keep in mind the whole point of this is budget. If you start wanting to buy a $20 line transformer, maybe its best to just get the real thing.
-- Jim Doyle