Circlotron amps

[cuallito]

I find this circuit topology interesting, but howcome it's not used more?

[fdegrove]

Hi,

Mainly because of cost IMO.

Cheers,

[SY]

Complex power supplies. Very high drive requirements. Nonetheless, it's a circuit that I think deserves more attention.

[runeight]

I'm building two 80W circlotron monoblocks. At some point I'll report on them.

I don't think they are more complicated than SRPP output stages (futtermans). In fact, I think that they can be simpler, depending on the specific design. For one thing, they only need one bias supply, not two. And they don't suffer from the unbalanced drive issue.

Depending on the design, they can need more drive signal because the degenerative feedback is active on both sets of cathodes.

Their native Zo is a little higher than a properly designed Futterman, but in cases where NFB is applied (and this is probably most cases), this gap is reduced to being almost negligible.

[SY]

Heh, a perfect example of the same text meaning two things to two different people! I interpreted the question in light of transformer coupling, e.g., the classic EV Circlotron.

[fdegrove]

Hi,

I don't think they are more complicated than SRPP output stages (futtermans).

I suppose you mean SEPP, not SRPP?

I interpreted the question in light of transformer coupling, e.g., the classic EV Circlotron.

So do I but it's still the same thing IMO...

Cheers,

[smoking-amp]

Practical problems of the Circlotron include the high drive signal requirement due to the 50% cathode drive, lack of suitable off the shelf transformers with low primary impedance, and the need for dual floating plate power supplies with low common mode capacitance to ground and to each other (in addition to a third plate supply for the front end section).

Most of the practical difficulties of a Circlotron can be overcome with the following circuit variations (Elliptron):

Elliptron Schematic

Using 40% taps gets the cathode drive down to 28%, reducing drive requirement from very difficult to just challenging. The plate impedance per tube is (1 + 0.4)/2 squared, times the end to end impedance rating of the transformer.
The Hammond 1650T is a reasonable choice for the Elliptron configuration. (By the way, I generally derate Hammond P-P transformers by 50% power wise to get the mag. flux density level down into the linear 7500 gauss max. region for HIFI. The given Hammond ratings appear to be near max saturation level for guitar amp. use.) I use two, cheap, "dual bobbin" industrial step up xfmrs. for the low capacitance floating supplies, with separate filter inductors mounted on insulating standoffs. For the conventional single supply version I use a small, dual 240V winding, industrial step up xfmr for the center tapped cathode inductor.

Don

[A3rd.Zero]

http://www.audiodesignguide.com/index_nobanner.html

is a great sight by Andrea Ciuffoli, where he talks about Circlotron amps in a good bit of detail. Specifically in the OTL section.

Milo

edit: I found it hard to find my way back through the website so here is the actual page
http://www.audiodesignguide.com/otl/