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I'll link you to an article

Posted by Russ57 (A) on May 19, 2006 at 10:10:15

In Reply to: Re: The answer to all your questions posted by radiance on May 18, 2006 at 15:16:55:

It should be pointed out that transformers don't have an impedance in the way that we are talking about the word impedance. What they have is a turns ratio and impedance is a function of the square of the turns ratio.

For example lets say you look up a given tube and determine you need a 5K transformer. What that really means is that when you hook up a load, like your speakers (we will assume 8 ohms) the tube(s) will see a 5,000 ohm load. So knowing that impedance is related to turns ratio squared we can apply some math. We have 5,000 ohms=(turns ratio)squared times 8 ohms. Or turns ratio=the square root of (5,000/8) which equals 25. So the side of the transformer connected to the tubes needs 25 turns for evey turn on the side of the transformer connected to the speaker. The most important part to get is that the transformer has no inherent impedance but only a turns ratio and we wouldn't get 5K if we didn't use 8 ohms.

All transformer have this property and it is just as valid to look at a power transformer in this way. Say you have a 120 volt outlet and need a transformer to mate with your 20 volt doorbell. You need a turns ratio of 6. It will give an impedance ratio of 36. The voltage and current are a function of the turns ratio. So the voltage gets stepped down by a factor of 6 from 120 to 20 volts. But the current does as well. If the 120volt side was drawing 1 amp the 20 volt side would have to be drawing 6 amps. In the audio transformer example above our turns ratio was 25 so if the tubes put out 250volts the speaker would see 10 volts. If the tubes put out 1 amp the speaker would be drawing 25 amps. But the impedance matching would be 25 squared or 625 which is of course our 5,000 ohms/8 ohms.

The link I have provided will help explain why certain transmission lines must have a particular impedance. That is why you needed matching impedances in your console. Often this is done with what we call termination resistors.

For triodes that load I advisded is a compromise between power and distortion. Experience has shown it is what sounds best. The best way to get a superior sounding amp is to operate only in class A and not to load the tube(s) too much. But this also gives you much less wattage so it is not commonly done in commercial amps (but all SET's have to be class A at least).

Russ

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