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Hybrid Circlotron Amplifier with only 3 components on the signal path

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#182
tinitus said:
Originally Posted by pieter t
The modern circlotron is OTL, with tubes or semiconductors.


I just thought it was funny you connect the term OTL with semiconductors 😉
maybe its just 'semantics'
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I know, transformers are normally associated with tube amplifiers, but there are quite some exceptions: tube amplifiers without transformers (the well known OTL's), and transistor amplifiers with output transformers (or autoformers as used by McIntosh).
 
#183
pieter t said:
No comment 🙄
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Clearly this is where we part; to date we have never had email from Broskie regarding this amp he claimed to have (nor anyone who claimed to either be selling to him or buying from him, and in this business that is very telling due to the small size of the market- when someone is buying or selling, we hear about it), but the fact that he never made the measurements I mentioned speaks for itself.

Regardless of what you may *think*, based on no prior experience, the amp is indeed class A2 when driving 8 ohms. It is class AB2 when driving four; what we see here is that OTLs, like any other tube circuit, are sensitive to load. This is why there is so much care taken in the design of output transformers- to load the tubes to maximum advantage. The only difference with OTLs is that more or less becomes a recommendation.

In order to accomplish the A2 operation (essential as the A1 operation made almost no power at all), the driver has to be capable of running the tubes about 15V positive w/respect to the cathodes. We chose the 6AS7G because it has linearity when so driven; with 6C33s, EL509s and the like the saturation characteristics of the grids do not allow this. Other characteristics of the 6AS7G that made it suitable for this task were price and availability, plus filamentary and socket concerns.
 
#187
Comparative approach

Andrea
Just in terms of comparison, how do these amps compare against the 2A3 SE and OTLs of this world? perhaps you are better than anyone to judge since you have build them all. Are they really better?
Trusting your opinion i am really tempted to build them.🙂
 
#188
pieter t said:
I have a building instruction for the Atma-Sphere MA60II here (by John Harper).
He measures an output resistance of 10 ohms; sounds believable with 8 6AS7G's.
Harper measures low THD (mostly even order) up to 10 watts; above 10 watts distortion sharply rises with odd orders coming in; IMO switching from class A to class B at around 10 watts.
So we have PP OTL with 23,67 ohms Ri into 14,22 ohms load, giving 6,3 watts class A.
We have Circlotron OTL with 10 ohms Ri into 8 ohms.
Would it give 60 watts class A? I think not (with 550mA of idle current it will be some 5 watts pure class A).
A well constructed MA60 will be an excellent sounding class AB amplifier, no doubt regarding the excellent reviews.
I propose end of discussion in order not to hijack this thread any further.
When our member Atmasphere likes to comment, my suggestion is to open a new thread.
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Harper's article is pretty old; there are a lot of changes to the circuit since then that got rid of a lot of distortion, most of the changes were in the driver circuit. If the amp is working correctly it will not have any even orders at low power, only the 3rd harmonic....

We believe there was also a measurement error (and the following comment is relevant to this thread):

If you forget to lift the ground for the oscilloscope AC power (or for the distortion analyzer, which may be further complicating things as many distortion analyzers have one of the input terminals at ground), with **any** Circlotron you will get squirrely distortion readings as the drive to the output section will be asymmetrical with one speaker terminal at ground. There is a French review of the Atma-Sphere S-30 where this measurement error occurred.

This might be a good thread to continue such conversation:
http://www.diyaudio.com/forums/tubes-valves/161112-what-tubes-tube-amp.html
 
#190
Very nice design Yuriy, your "Tsirklotrona" is very elegant :up:

It just goes to show that in the world of audio and often designs are developed more than once by different people at different times.

I like the last variant, using the self-bias of the driver. It would make it easier for people to build this if there was an off-the-shelf interstage transformer recommended ? (sorry if I missed that you did this already)
 

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#191
Thank you, Bigun!
Unfortunately, I can not recommend transformers, because they do not know very well their types (as Lundahl Transformers, etc.). We use a self-made interstage transformers for this project.
Because they are no worse, and often even better than Lundahl 🙂
 
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#193
Tyimo said:
Hi Andrea!
Is the 42Vdc not too much for the 2SK1058?
How many amps going through them?
How much is the output power of the circlotron?
Is it possible to use only +24Vdc for this amplifier?
Greets:
Tyimno
Click to expand...

Is the 42Vdc not too much for the 2SK1058? no, this is perfect to get 50w

How many amps going through them? what you mean ?

How much is the output power of the circlotron? 50W

Is it possible to use only +24Vdc for this amplifier? yes, but the power is low

Amplifier End by Andrea Ciuffoli
 
#198
Any chance to use center taped transformer to these project successful to power the mosfets?
I have two nice good quality 420VA 30-0-30V transformer.
May be I will have to open up at the center tap to take a look if I can separate it to get 2X30V.
Greetings Gabor
 
#200
Thank you Andrea
That is what I thought but secretly I hoped to have some type of solution.
I made some calculation, even the VA rating would be way to low (almost half the req). I have to check the store where I got these transformers if they have some more, I would need two piece more like these.
Greetings Gabor
 
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