Thought on OTL tube amps

10-27-15: Mapman
Chalice Audio Grail amps are interesting to me. I didn't realize a SET amp could produce 50 w/ch into 8 ohm. How do they do that?

Mapman, it uses a parallel pair of type 845 tubes, which I believe were originally developed for transmitting applications, and have much greater power capability than the 300B, 45, and 2A3 tubes you are probably more used to seeing in SET amplifiers. There are a number of other 845-based SETs out there. The 211 is another such high powered tube that is sometimes used in SETs. Both types require about 32 watts just to heat the filaments of each tube(!), and run at very high B+ voltages (I believe often in the vicinity of 1000 volts), as well as handling substantial amounts of current. All of which presents much greater challenge when it comes to design of the output and power transformers than in the case of the lower powered SETs.

BTW, A'gon member Ait has described in past threads how he designed and built for himself a SET providing 200 watts per channel, which utilizes an 833C transmitting triode operated at 2500 volts(!), and having output transformers weighing 62 pounds each. Not an OTL of course, but quite a remarkable tour de force!

Psag, thanks for the reference to the Berning Quadrature Z, which I hadn't been aware of previously. Quite an innovative and remarkable design, providing 200 watts of tube power per 32 pound monoblock. But of course it is neither a SET nor an OTL, although its output transformer operates at low RF frequencies rather than at audio frequencies.

Best regards,
-- Al

Makes sense. Very heavy duty!

Thanks Al.

Al, check Berning's website. He clearly states that it is an OTL. Check the picture, you will see that there is no output transformer. Looks like one, sounds like one...

Hi Psag,

I had looked at the writeup at his site, and also at the patent which it references. There is a transformer between the outputs of the power tubes and the output of the amplifier, along with some solid state switching devices. As I read the writeup, what he said is that there is no AUDIO output transformer, but "the high-voltage, low-current tube impedance-plane is re-mapped to the high-current speaker impedance-plane through a special transformer at a constant RF carrier frequency of 250 kHz."

So while his very innovative approach avoids the use of an AUDIO output transformer, the design is not Output Transformerless (OTL).

Best regards,
-- Al

The fact that the Berning amplifier (while an outstanding and very innovative design) does not fall within the definition of an OTL is something that has been put to bed many times. While I respect David as one of the top designers in the audio world, I do find it inexplicable that his website makes this claim.

OTLs by definition do not have an output transformer; that much should be pretty clear. 'OTL' refers simply to 'Output TransformerLess. Over time (the last 50 or 60 years) the acronym generally has referred to a *tube* amplifier without an output transformer; in that regard having semiconductors in the output section would not qualify the amplifier as an OTL. And of course the Berning amps all include semiconductors as an essential part of the output circuit.