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What about IGBTs? Are they worth using? No is the quick answer. For the amplifier designer, these devices encompass the worst of both technologies: MOSFET and transistor, but then they were not designed with audio in mind. The sad fact is that very few active devices were actually designed with audio in mind. The 300B and the 12AY7 were. As were the Hitachi lateral MOSFETS. But most discrete electronic devices whether tube or solid-state were designed for power supply, computer, motor, or radio use. Audio must live on the crumbs that fall off the technology table.
The Greedy and the Puritanical
One trap we should avoid is greed. Virtually every commercially made amplifier is optimized for power output. I have seen excellent 32 watt amplifiers ruined because the output wattage had to be increased to 40 watts to make the marketing department happy. In spite of the our new energy shortage (I live in California), in spite of the vestigial puritanical outlook that all Americans suffer from (even the most extravagant rakehells are burdened with a puritanical overlay), in spite of the universal masculine urge to maximize power (who wouldn't want 10 watts instead of 7 watts), I have learned to let go of potential wattage. To use a food analogy, which is better: a small serving of the most delicious dish in the world or a serving twice as large but only half as tasty? (If you did not choose the first choice, I do not understand why you are reading this journal.) Triodes with their large unusable spans of plate voltage are inefficient. Chokes, voltage regulators, and cathode biasing with their voltage drops rob potential watts. Class-A operation is anything but inefficient and current source loaded Class-A operation even less so. Yet, all of these add to a better sound. (My good friend, Glenn, is so convinced by experience that the inefficient sounds best that he is constantly on the outlook for ways to lower his amplifier's efficiency.)
So if we used 2 ohm emitter resistors rather than the usual .1 ohm resistors or if we used chokes in solid-state portion of the power supply, we would certainly lose many potential watts, but we might gain a much better sounding amplifier. Neither the energy conscious, nor the puritanical, nor the inferiority complex ridden who have 400 watt amplifiers in their cars, none will be happy with the diminished wattage, only your ears will be.
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Of course, if you design your hybrid amplifier for the best sound rather than for the highest efficiency, you should at least turnoff the lights and the refrigerator. (If not for energy conservation, then at least for better enjoyment of the music.)
Inverted cascode
One real advantage solid-state devices enjoy over the vacuum tube lies in having a "P" version of the FET, MOSFET, and transistor. Tube electronics would immeasurably benefit from a P version of the triode and pentode. In other words, a tube whose cathode received electrons from its plate. Such a tube is impossible, sadly. But by using P solid-state devices we can come up with topologies that would be impossible with just tubes.
The circuit below uses only one triode and is DC coupled. In function it is a cascode circuit. The triode is locked between the B+ connection and the MOSFET's source. As far as the triode is concerned its cathode and plate cannot move up or down. But its grid can. As the triode sees a varying grid voltage, the current conduction through the triode will vary in response. The varying of current through the 100 ohm resistor defines a varying voltage, which is in phase with the input, as the both the resistor and tube share the same current path. Roughly, the gain is equal the triode's Gm against the resistor's value.
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