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I can answer that one- You have to lower the output impedance somehow. But before you go about that, it is useful to understand what is meant by Output Impedance (Zout). There seem to be two definitions, and depending on which you ultimately subscribe to is going to say a lot about your design. Here are the two definitions: 1) Zout is just that- the measure of the impedance of the output section. This is the definition that I prefer. -Or- 2) Zout is the measure of servo gain available to the output section to control the load. I don't like this definition because the numbers you wind up with don't tell you what loads the amp will actually drive. Plus you have to use feedback. Either way though, the number has to be low enough to allow the amp to drive your idea of a real world speaker. If you have a speaker that is 100 ohms, that may be all you need. But practically speaking, 8 ohms is a more realistic target. Even amps with output transformers don't do four ohms very well (I have said this a lot over the years- if you are investing in a tube amplifier, your investment is best served with an 8 ohm speaker, rather then 4, if all other things are equal, regardless of the tube amplifier technology), so if I were you I would not worry too much about 4 ohm loads. But you will want to be able to drive 8 ohms fine. I don't like feedback, as odd-ordered harmonic distortion above the 7th or 9th order can be enhanced by negative feedback. These harmonics are very noticable to the human ear in vanishingly small amounts. So I prefer more paralleled tube sections. If you have a clean design, the extra power thus gained does not hurt, even if you have a high efficiency speaker. Have fun...
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