Quote:
Originally Posted by

**00940**
- Transformers do a lot more than just removing the offset. They convert voltage to current. It's a big deal since tubes are mostly high voltage/low current devices.

I don't know that "convert voltage to current" is the best way to describe it. "Trade voltage for current" would perhaps be better. EDIT: on second thought, I don't think either of these really work and suggest people not think of transformers in that way.

Basically (and I'm not saying this specifically to you but just generally), transformers transform voltage by the turns ratio (the ratio of the number of turns of wire on the primary versus the number of turns on the secondary) and transforms impedance by the square of the turns ratio.

The loads we typically want to drive with tubes, such as headphones and loudspeakers have a much lower impedance than the tube wants to see and the output impedance of the tube is higher than we want to drive those loads with as well.

5,000 ohms isn't an uncommon load impedance for tubes. So if we want to drive a 50 ohm headphone load, that's a ratio of 100:1. And since transformers transform impedance by the square of the turns ratio, we need a turns ratio of 10:1 (10 being the square root of 100). So if we have 10,000 turns of wire on the primary, we'll need 1,000 turns on the secondary. Now with a 50 ohm load on the secondary, the tube will see 5,000 ohms at the primary. As well, the output impedance of the tube will be reduced by a factor of 100 so the headphones see a much lower impedance than they would without the transformer.

But as was said, transformers also transform voltage by the turns ratio. So in order to achieve a given voltage at the output, the tube will have to deliver 10 times that voltage at the primary. In other words, to get 1 volt out, you have to put 10 volts in.

This is all a consequence of transformers being passive devices so due to that nasty law of conservation, power out can never be greater than power in (and in real world devices where there will always be some losses, power out will always be slightly less than power in). One volt into 50 ohms is 20 milliwatts. 10 volts into 5,000 ohms is also 20 milliwatts.

Hopefully this will take some of the mystery out of transformers.

se

Edited by Steve Eddy - 3/14/14 at 9:35am