Steve, please explain why tube amps use output transformers and why they typically have a few output taps? Intuitively, I've always assumed that the transformer was there to "match" the impedance of the amp to the loudspeaker. So, you'd use the 4 ohm tap when driving a "4 ohm" speaker. But, I really do not understand the benefits of matching the impedance.
Sure.
One reason output transformers are used is because generally a tube can't supply sufficient current into a low impedance load to get any appreciable power. That's largely due to the fact that tubes generally run at very high voltages; a couple hundred volts and more.
For example, to deliver 1 watt of power into 8 ohms, you need to deliver about 350mA of current. Let's say the tube's running at 300 volts. 350mA times 300 volts is over 100 watts.
But let's say the load is 2,000 ohms instead of 8 ohms. 1 watt of power into 2,000 ohms is only about 22mA and at 300 volts, you're only looking at about 6 watts.
So, how can we take that 1 watt we've got into the 2,000 ohm load and transfer it to our 8 ohm speaker load?
We can use a transformer.
To get that 1 watt into 2,000 ohms, the tube is swinging 44 volts. And in order to get that down to 2.83 volts for our 8 ohm load, we can use a transformer with a step-down ratio of about 16 to 1.
And while transformers will step voltages up or down as a function of their turns ratio, they reflect impedances by the square of their turns ratio. So for a transformer with a 16:1 voltage/turns ratio, impedances get reflected by a factor of 16 squared or about 256.
And if we multiply 8 ohms by 256, we get about... 2,000 ohms.
In a typical tube amp, say a single-ended triode amp, the tube's output (the plate in this case) is connected to one end of the output transformers primary and the other end of the transformer's primary is connected to the power supply (or B+). So the load the tube sees is really the loudspeaker's impedance as reflected to the transformer's primary, or in this example, 2,000 ohms.
The reason for the multiple taps on the transformer is because a given tube works most linearly into a given load. So in order for the tube to see the roughly same load whether you're using a 4, 8, or 16 ohm speaker etc., multiple taps are wound with different ratios.
For example, let's say the speaker load is 4 ohms instead of 8. To get the same 2,000 ohms reflected to the transformer's primary, we need a step-down ratio not of 16 to 1 as in the 8 ohm case, but about 22 to 1 which means the 4 ohm loudspeaker load is reflected back to the primary by a factor of 22 squared or about 500.
Of course this means that the 44 volts the tube is swinging into the 2,000 ohm primary load gets stepped down by a factor of 22, so instead of 2.83 volts across the loudspeaker, it's just 2 volts. And 2 volts into 4 ohms is... one watt. Same as for the 8 ohm tap with an 8 ohm speaker.
And that's why tube amps are typically rated for the same power into 4, 8, 16 ohms, etc.
This help?
se
