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The Covi Mark II Circlotron Amp

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#1
I am going to build «The Covi Mark II Circlotron Amp», but I’m a newcomer in the tube sound. I have some questions:
1) Does this schematics work? Has it any mistakes?
2) What advantages does the circlotron have? (in comparison with the variation on the Futterman)
Thanks!
 
#2
Does this schematics work? Has it any mistakes?
Click to expand...

As far as I have heard from people who have built it, it is a good design with high reliability.

2) What advantages does the circlotron have? (in comparison with the variation on the Futterman)
Click to expand...

Very minor if any! One advantage that is normally presented is that as the Circlotron is a symmetric design it should have better PSRR compared to any SEPP variant ala Futterman. In a SEPP the PSRR is different for the upper and the lower tube so the total PSRR is not that good. However there are many different kind of tricks that can be applied in order to improve PSRR in a SEPP so in practise this is not a problem. My own OTL which is a inverted Futterman ala Technics have about 0.2mV Pk-Pk noise on the output which is sufficiently low for any purpose, it is equivalent to about 106dB below max output power, this is a lower noise level than a Circlotron without the same kind of tricks.

Other than this I can't see any advantages, some say that a Circlotron should have lower distortion as it is balanced but that is not at all true, any push-pull amplifier have cancellation of even order distortion and it is not different in a Circlotron than any other type, my own OTL have a 2nd order distortion of -55dB before applying feedback, a Circlotron with the same tubes can't do better than that.

Instead I see dis-advantages when using a Circlotron design, one thing is the higher output impedance which can be a marginal effect when using low mu tubes but it is still there, the result is that a Circlotron need to use more tubes for the same performance which will affect reliability in a negative way. The other thing is that both output terminals have a DC potential to ground which feels un-comfortable when using headphones, a minor point but still.

Regards Hans
 
#3
And how does it sound? Have you heard this device?

[QUOTE
Instead I see dis-advantages when using a Circlotron design, one thing is the higher output impedance which can be a marginal effect when using low mu tubes but it is still there, the result is that a Circlotron need to use more tubes for the same performance [/QUOTE]

http://members.aol.com/aria3/otlpaper/otlhist.htm
In the article it’s said that with only 10dB of NFB an output impedans is less than 1ohm.
Now I have OTL by A. Ciuffoli (version 8.1 cheap & easy), but only one canal. I don’t like the deep NFB and it has more split capacitors than the circlotron one. Do you think it’s better for me to make another canal and forget about circlotron? :
 
#4
Hi,
Your experience seems similar to mine - I had a ,breadboard, version already made of the A.C OTL. Conversion to Hans' is quite straightforward - PSU and O/P stages are practically the same so can be easily modified - the main difference is in the Input and driver stages which are also straightforward. I have no experience of the Circlatron but have built nearly everything else (PP and SE)over many years! Don't waste time! build Hans' design - it's remarkable in it's absolute clarity of reproduction. Han's is a very experienced designer and builder of OTL (and other things I should think!) You can trust his judgement without question! and is very helpfull (as you are aware already) That is my 'tuppenceworth' anyway.

Kind regards,

David.
 
#6
And how does it sound? Have you heard this device?
Click to expand...

No, I haven't but I imagine ot wouldn't sound much different than a inverted Futterman using the same tubes at more or less the same operating points. In reality there are very small differences between a Circlotron and a Futterman variant, the differences there are are explained very clearly in the paper describing the Cov Circlotron as you refer to.

In the article it’s said that with only 10dB of NFB an output impedans is less than 1ohm.
Click to expand...

That is incorrect and probabaly a misprint or a simple mistake, according to fig 7 in the same articl;e a Circlotron with 4 6C33C tubes would have an output impedance of 10.5 ohm, using 10dB feedback which is equal to 3.16 times would then reduce the output impedance to 10.5/3.16 = 3.32 ohm. I think they made a simple mistake and that the feedback actually is 20dB equal to 10 times, then the output impedance would be 1.05 ohm.

Using the same 4 tubes in a inverted Futterman as described in fig 6 would give an output impedance of 6.8ohm without feedback, ~2.15 ohm with 10dB and 0.68 ohm with 20dB feedback.

If you for some reason do not like to use feedback the choice is therefore even clearer, the inverted Futterman always give the lowest impedance of any of the OTL designs that can be practically realised so it would also need the lowest amount of feedback for the same output impedance.

Regards Hans
 
#7
Hi Hans I can't find the shematics with the nominals of the componensts at your site🙁 Did you publised it? And I have the another question: is it possible to use 6n6p in your amp.
Thanks in advance!
Regards Vladimir🙂
 
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