Thought on OTL tube amps

If you want power, quite often "paralleling tubes is the only option" regardless of OTL or not.

No paralleling of tubes in David Berning's Quadrature. 200 W into 8 ohms.

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?

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.