Yes, that's where it's at. "Simpler" can be defined in different ways. I think one ways is to use less gain stages and more followers with current gain. This would allow one to use less tubes and more silicon and making a simpler, cheaper, cooler (in temperature), and more compact amp.
Most everything that can be done has been done. One thing that I think hasn't been done is to combine a large amplification tube like a 12ax7 or similar voltage gain stage that is capacitively coupled to a high voltage mosfet concertina. Then have mosfet followers driven by the concertina. That would keep the mosfet concertina from becoming unbalanced on high drive signals. And though you would lost the traditional direct coupling between gain stage and concertina you would then have direct coupling "after" the concertina to compensate.
Finally, couple the mosfet followers directly to the output tubes using Tubelab's power drive scheme. It seems like it would work. You would have to have a separate high voltage supply going to the concertina because the theoretical maximum signal voltage would be determined by the voltage applied to that stage. The theoretical peak signal voltage out could only be a bit less than half of that voltage. That's how concertinas work.
It would look complicated in its final form but most of the complication would come in the form of silicon, which is cheap. And the silicon wouldn't detract from the sound because it wouldn't be used for voltage gain. And you would have perfectly symmetrical push pull drive because it would never become unbalanced because no current would flow after the concertina through the gates of the mosfet followers that it feeds.
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The question I have is... The cascode (like a pentode) will have very high output impedance, and will also (like a pentode) be not-so-linear.
Does the cascode's high output impedance cause a bit too much loss of high frequencies?
Does the resulting amplifier need NFB to play clean sound?
One big added benefit is that since you would operate in A2 bias for the final stage you could probably get mucho power depending on the output tubes. You wouldn't have to go with parallel output tubes to get it either.
I never said that this is the BEST amplifier in the WORLD.
BUT, it is really excellent and easy to build with readily available components.
The valves are commonly available and the Hammond 1650H transformers are available from DigiKey.
In my original build thread I described how I got around the B+ / Cathode / Heater transformer voltages. You could always get a custom transformer built but I used common 2020 components.
The hum problem could well be down to my using a balanced pre-amp in unbalanced mode originally. I've now converted the amp to balanced input and it is brilliant.
BUT, it is really excellent and easy to build with readily available components.
The valves are commonly available and the Hammond 1650H transformers are available from DigiKey.
In my original build thread I described how I got around the B+ / Cathode / Heater transformer voltages. You could always get a custom transformer built but I used common 2020 components.
The hum problem could well be down to my using a balanced pre-amp in unbalanced mode originally. I've now converted the amp to balanced input and it is brilliant.
Allen has worked this circuit a lot. In preamp form FR is into the many hundreds.
The output impedance is high, and power suppy has to be well regulated.
More here: Schematics
And the book that explains everything is invaluable.
dave
IIRC the same circuit in the phono preamp gives 50dB gain.
The circuit drives the EL34s really well, and the 300B amp with the same front end is regarded as one of the best in the world.
dave
I think one reason the gain looks lower than expected is those unbypassed 470 ohm cathode load resistors.
Also, I modeled it as an LTP phase splitter, not as a differential driver. The phase splitter means only one input gets the input signal directly, so half the gain at each output. It looks like the original circuit was designed with the expectation that it would be driven from a balanced source.
Maybe a 10k:10k input transformer could be used to split phase...
Thanks for nudging me into modeling that circuit. It looks surprisingly good. I'll have to investigate more of Mr. Wright's circuits.
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Also, I modeled it as an LTP phase splitter, not as a differential driver. The phase splitter means only one input gets the input signal directly, so half the gain at each output. It looks like the original circuit was designed with the expectation that it would be driven from a balanced source.
Maybe a 10k:10k input transformer could be used to split phase...
Thanks for nudging me into modeling that circuit. It looks surprisingly good. I'll have to investigate more of Mr. Wright's circuits.
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me too, get turned off when i see too many coupling caps in the signal path...
OSchade, partial feedback, for the usual audio power tubes..... and very few gain stages.....a 6C33 and similar tubes....
I re-iterate my post.
This is an easy to build excellent amp. Easily repeatable with readily available components.
I never said that this is the BEST amplifier in the WORLD.
The valves are commonly available and the Hammond 1650H transformers are available from DigiKey.
In my original build thread I described how I got around the B+ / Cathode / Heater transformer voltages. You could always get a custom transformer built but I used common 2020 components.
The hum problem could well be down to my using a balanced pre-amp in unbalanced mode originally. I've now converted the amp to balanced input and it is brilliant.
For once, the Chinese have got it right and it's easy to build with a ready populated PCB.
The original schematic shows 5K OPTs which may be better but mine is fine with the Hammond 1650H which is 6K. I'm pairing it with B&W CM8s2's which are slightly lower than 8Ohms which would make a difference.
This is an easy to build excellent amp. Easily repeatable with readily available components.
I never said that this is the BEST amplifier in the WORLD.
The valves are commonly available and the Hammond 1650H transformers are available from DigiKey.
In my original build thread I described how I got around the B+ / Cathode / Heater transformer voltages. You could always get a custom transformer built but I used common 2020 components.
The hum problem could well be down to my using a balanced pre-amp in unbalanced mode originally. I've now converted the amp to balanced input and it is brilliant.
For once, the Chinese have got it right and it's easy to build with a ready populated PCB.
The original schematic shows 5K OPTs which may be better but mine is fine with the Hammond 1650H which is 6K. I'm pairing it with B&W CM8s2's which are slightly lower than 8Ohms which would make a difference.
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