If this is really a single-ended OTL of conventional design, the 'pencil tube' is the voltage amplifier/driver tube, as it is a dual triode tube capable of driving the input capacitance of the 6C33 grids without loss of bandwidth.
The power supplies appear to be Switch Mode Power supplies, hence the wide range of AC line input frequency and voltage.
The real issue is how the tubes are coupled to the loudspeaker. The problem is that if a parafeed method is used, significant output power will be lost (I can see from internal photos that there are electrolytic capacitors connected to the speaker terminals, so this could well be the coupling means). If the speaker is placed in series with the power tubes, significant DC current will be seen through the load, causing a woofer offset (which will increase distortion in the woofer and limit excursion). A third (brilliant) method of course was developed by David Berning, although not a true OTL in the traditional sense, does allow one to built a compact amplifier with a small (air-core) "output" transformer. There were similar RF switching circuits that go all the way back to WW2; it may be that this company has found a way around the Berning patent by mining the prior art. Its all conjecture of course- the company website is severely lacking in technical information as are any of its promotional materials.
Given that the amp is operating in single-ended mode, it is also a given that it is operating class A. There is significant heat associated with this move, and the sockets appear to be ceramic, so they will have a limited service life due to the heating issues. The English text I have seen does not make sense and seems in need of better translation- it suggests that the amp makes considerably more power than it can possibly do, while in fact the dual-tube version seems to only make about 10 watts, which makes more sense.
Nice exercise in layout and the use of SMPSs to reduce weight and size. The pencil tubes might get difficult to find down the road, as they have not been made in decades. I suspect though that a 12AU7 could be made to work without much difficulty.
The power supplies appear to be Switch Mode Power supplies, hence the wide range of AC line input frequency and voltage.
The real issue is how the tubes are coupled to the loudspeaker. The problem is that if a parafeed method is used, significant output power will be lost (I can see from internal photos that there are electrolytic capacitors connected to the speaker terminals, so this could well be the coupling means). If the speaker is placed in series with the power tubes, significant DC current will be seen through the load, causing a woofer offset (which will increase distortion in the woofer and limit excursion). A third (brilliant) method of course was developed by David Berning, although not a true OTL in the traditional sense, does allow one to built a compact amplifier with a small (air-core) "output" transformer. There were similar RF switching circuits that go all the way back to WW2; it may be that this company has found a way around the Berning patent by mining the prior art. Its all conjecture of course- the company website is severely lacking in technical information as are any of its promotional materials.
Given that the amp is operating in single-ended mode, it is also a given that it is operating class A. There is significant heat associated with this move, and the sockets appear to be ceramic, so they will have a limited service life due to the heating issues. The English text I have seen does not make sense and seems in need of better translation- it suggests that the amp makes considerably more power than it can possibly do, while in fact the dual-tube version seems to only make about 10 watts, which makes more sense.
Nice exercise in layout and the use of SMPSs to reduce weight and size. The pencil tubes might get difficult to find down the road, as they have not been made in decades. I suspect though that a 12AU7 could be made to work without much difficulty.
![ACUHORN%206c33c%20power%20amplifier[1].jpg](https://www.whatsbestforum.com/data/attachments/11/11897-1afe02fd224c6524f305d9e46f7afa4f.jpg "ACUHORN%206c33c%20power%20amplifier[1].jpg")
![Acuhorn%20S2%20THD+N%20vs%20Generator%20Level[1].jpg](https://www.whatsbestforum.com/data/attachments/11/11898-d3dc8739bc2464fb393ecb8205241f39.jpg "Acuhorn%20S2%20THD+N%20vs%20Generator%20Level[1].jpg")
![Acuhorn%20S2%20Stepped%20Level%20Sweep[1].jpg](https://www.whatsbestforum.com/data/attachments/11/11899-21670922a9e8f0167290012dbb03d484.jpg "Acuhorn%20S2%20Stepped%20Level%20Sweep[1].jpg")
![Acuhorn%20S2%20Frequency%20Response[1].jpg](https://www.whatsbestforum.com/data/attachments/11/11900-fca16da6ede952d7b12563718b26b63e.jpg "Acuhorn%20S2%20Frequency%20Response[1].jpg")
