If you want to save all of your inductance for the speakers you might want to play around with output transformerless tube output stages. They will pass all of the ultra-sonics just fine. 
To do a hybrid you could maybe use a bipolar preamp/VAS section with local feedback as needed. Since you are not swinging a lot of voltage they should be pretty benign. You could even use a high voltage FET for the split load phase inverter on an inverse Futterman (or go cyclotron if you are so inclined) output stage with a bank of high current triode strapped horizontal sweep tubes as the output.
With the Futterman design I bet you could have a relatively simple power supply compared to an all tube version since the transistor stages can easily operate off of the low voltages of a single side of the bipolar supply. Possibly use one side for the left channel and the other for the right to keep the load balanced.
Just some miscellaneous ramblings.
To do a hybrid you could maybe use a bipolar preamp/VAS section with local feedback as needed. Since you are not swinging a lot of voltage they should be pretty benign. You could even use a high voltage FET for the split load phase inverter on an inverse Futterman (or go cyclotron if you are so inclined) output stage with a bank of high current triode strapped horizontal sweep tubes as the output.
With the Futterman design I bet you could have a relatively simple power supply compared to an all tube version since the transistor stages can easily operate off of the low voltages of a single side of the bipolar supply. Possibly use one side for the left channel and the other for the right to keep the load balanced.
Just some miscellaneous ramblings.
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Hi Kees,
I'm sorry to hear about your friend. I lost a friend to lung cancer a couple of years back.
Good to see you have made your amplifier and are enjoying the hobby. It's quite a different schematic you ended up with finally. To me it looks very good, if I understand it properly you have a CCS loaded anode follower followed by a CCS loaded cathode follower driving lateral mosfets. A dc-servo and no global feedback. Nice.
How does it sound ?
I'm sorry to hear about your friend. I lost a friend to lung cancer a couple of years back.
Good to see you have made your amplifier and are enjoying the hobby. It's quite a different schematic you ended up with finally. To me it looks very good, if I understand it properly you have a CCS loaded anode follower followed by a CCS loaded cathode follower driving lateral mosfets. A dc-servo and no global feedback. Nice.
How does it sound ?
Yes Cancer is a world problem en a cure is not what we get if the pharmaceutical profits are so high with chemo what not work, 3 years ago I lost 2 friends on lungcancer, one was the wife of my friend who now has cancer. That I go on has a reason, death don,t exist, it is a longer sleep with a young wakeup. death is out friend, but psychically we lost a friend that is the only hard part, but I am always happy that a good friend got,s peace and no more suffering from cancer what can be very bad with lungcancer.
Oke now the amp, it sounds very nice, al have I the idea that lateral mosfets are better, I have no feedback, but feedback is not always bad if properly put in. the amp is fast, and impulses are very good to hear, high tones are silksoft and bas is good controlled because of the dc coupled mosfets, the russion tubes perform very well, but maybe it is better to get a EL34 in triode for driving the mosfets capacitance, it need a beefy driver, I have now the 6n6p double triode on 30 mA idle driving the fets, it works fine with 2 mosfets but not for a high power amp with multiple mosfets..
Thanks for your words about my friend, monday he now for shure maybe it is something else but I think it is bad.
Oke now the amp, it sounds very nice, al have I the idea that lateral mosfets are better, I have no feedback, but feedback is not always bad if properly put in. the amp is fast, and impulses are very good to hear, high tones are silksoft and bas is good controlled because of the dc coupled mosfets, the russion tubes perform very well, but maybe it is better to get a EL34 in triode for driving the mosfets capacitance, it need a beefy driver, I have now the 6n6p double triode on 30 mA idle driving the fets, it works fine with 2 mosfets but not for a high power amp with multiple mosfets..
Thanks for your words about my friend, monday he now for shure maybe it is something else but I think it is bad.
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Well I think that if they could make chemo that worked better, they would do it. They would do it because they are human and saving people is a very powerful thing. And even commercially, they would want to make a better chemo because they would sell more than their competitors. Let's keep hopeful about the promise of good science.
Can you say more about the sound of the amp in addition to 'sounds very nice' - how are the lows, the highs, the mids ? can you compare to other amplifiers ? (for example, my all-tube SET is less open in the highs compared to my all-SS amp but the highs still sound so much nicer, my friends chinese hybrid he has heavily modified has both attributes together).
I'm not sure I know enough to say if the mosfet gate drive is adequate or not with more mosfets, you are probably right.
One benefit is that you don't have global feedback. I believe gnf feedback amplifiers require high frequency open loop gain to suppress high order harmonics and prevent artifacts from limited slew rates. You don't have these worries.
Of course the maximum current you can discharge the fet gates is set by the CCS load on your driver - 30mA. And in your source follower configuration you don't have to charge full Cgs but more Cgd. By my reckonning this is not a limitation for your amplifier even with additional mosfets.
p.s. I am not a fan of CCS's and don't use them in my amplifiers. I believe a choke load is the best option. A SS option that might be better is Wavebourns gyrator. Just some thoughts for future experiments.
Can you say more about the sound of the amp in addition to 'sounds very nice' - how are the lows, the highs, the mids ? can you compare to other amplifiers ? (for example, my all-tube SET is less open in the highs compared to my all-SS amp but the highs still sound so much nicer, my friends chinese hybrid he has heavily modified has both attributes together).
I'm not sure I know enough to say if the mosfet gate drive is adequate or not with more mosfets, you are probably right.
One benefit is that you don't have global feedback. I believe gnf feedback amplifiers require high frequency open loop gain to suppress high order harmonics and prevent artifacts from limited slew rates. You don't have these worries.
Of course the maximum current you can discharge the fet gates is set by the CCS load on your driver - 30mA. And in your source follower configuration you don't have to charge full Cgs but more Cgd. By my reckonning this is not a limitation for your amplifier even with additional mosfets.
p.s. I am not a fan of CCS's and don't use them in my amplifiers. I believe a choke load is the best option. A SS option that might be better is Wavebourns gyrator. Just some thoughts for future experiments.
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I have listening a lot, it is very open but I think there is compression of audio of some kind, maybe the CCS has a to low impedance, your idee of a choke or gyrator principle is worth trying.
Also it is possible that the dual triode can not properly drive the mosfets, i have the feeling that the EL85 in triode mode did a much better job, and it sounded warmer.
But it is good start, if not to loud the sound is so open that a buch of people speaking al together can be followed so the speak al the voices have so much room in it, but I also discover that how better the amp how more bad mp3 and dvd sounds, impulses are very very good! and it is stable also I can give full trottle and remove the speakers etc, no kick and no oscilations..
I read the gyrator principle it is een very clever russian there haha, but the idee to change current for voltage to let the tube act so that it find his own current is not a bad idee, also the tube see now a very high impedance what ;et him swing freely and so liniair, I now something about this with the mustage from kimmel, I did a time ago put a capacitor from anode to gate with a mustage like design what put distortion in a single step a lot lower, the ccs has a way to follow the signal, you see that also in the kimmel mustages.
I am not as clever as real electronic designers, I now a lot and I combine most things what is known, much more is not possible, but we can combine to get nice results, and the modern pc has simulations so calculations real time, that is what I do, earn mony with good designs is not possible anymore there a already to much, but as hobby it is very satisfactory.
Also it is possible that the dual triode can not properly drive the mosfets, i have the feeling that the EL85 in triode mode did a much better job, and it sounded warmer.
But it is good start, if not to loud the sound is so open that a buch of people speaking al together can be followed so the speak al the voices have so much room in it, but I also discover that how better the amp how more bad mp3 and dvd sounds, impulses are very very good! and it is stable also I can give full trottle and remove the speakers etc, no kick and no oscilations..
I read the gyrator principle it is een very clever russian there haha, but the idee to change current for voltage to let the tube act so that it find his own current is not a bad idee, also the tube see now a very high impedance what ;et him swing freely and so liniair, I now something about this with the mustage from kimmel, I did a time ago put a capacitor from anode to gate with a mustage like design what put distortion in a single step a lot lower, the ccs has a way to follow the signal, you see that also in the kimmel mustages.
I am not as clever as real electronic designers, I now a lot and I combine most things what is known, much more is not possible, but we can combine to get nice results, and the modern pc has simulations so calculations real time, that is what I do, earn mony with good designs is not possible anymore there a already to much, but as hobby it is very satisfactory.
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I have read about gyrators, I like the idee behind it, but where to find calculations, I have did a small test with it on the schematic, distortion is a lot lower, so promissing, there is a intelligent russian lurk here around, can ask him how to calculate and of it is usable with a
cathode follower. .
cathode follower. .
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Good news!! my friend has no cancer, but a bacteria in his lungs, a agressive one but it is with antibiotics over soon, he keeps walking to long with symtoms, that is my friend, never say something, walks always until the sickness go out of hand.
I am go further with listening, it seems the laterals are a lot better in sound reproduction...
I am go further with listening, it seems the laterals are a lot better in sound reproduction...
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