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midmoe 6th February 2013 06:24 PM

medium power OTL circlotron amplifier
I am making significant progress in designing an OTL circlotron monoblock with 6 output tubes, easily and cheaply built. See my thread at "impedance measurement" page two for details. A completed schematic will be posted soon. Thanks, midmoe.

azazello 7th February 2013 07:49 AM

With 6 triods or 3 double triods?
You will need 3 or 4 PSs....

midmoe 7th February 2013 12:46 PM

I'm not using triodes at all. Am using six 6550 pentode power output tubes, standard equipment in many guitar and stereo amp units...see impedance measurement thread for details....nothing of intrest happens until page 2, then I shift from impedance related to circlotron related. Not sure what "3 or 4 PSs" you mean "3 or 4 power supplies" ? If so, am using 4 supplies. The standard amount for circlotron topology, 2 B+, 2 BIAS.....Have been thinking about using a series string filament supply as filament transformer getting hot. Plus keeping down weight and saving space would be good. A rectified 120V AC would also allow filtering in the preamp/driver section giving DC to those filaments, while outputs would be on seperate string using unfiltered pulsating DC, as current requirements would be too high to try to filter.
Thanks for your intrest..midmoe

the_manta 7th February 2013 02:34 PM

I'm a bit curious: Why did you use 6550 (technology of the late 1940s) and not some frame grid tubes which can deliver more current and have higher gm for an OTL ?

artosalo 7th February 2013 02:53 PM

....just like PL519 / JJ EL509 / 6P45S.

midmoe 7th February 2013 03:40 PM

I used tubes that I have lying around the house, plus I wanted to use commonly available, inexpensive tubes, and not too many of them. I didn't want to have to enlist the help of a team of volunteers to move it, or require an air conditioner to cool it....and I'm not even sure what a "frame grid" tube is....please explain. Perhaps if these frame grid tubes can be used in my setup, which is a maximum of 3X3 output tubes in push-pull, then I will persue the matter. I was skeptical that the wierd looking circlotron diagrams would actually work. To my pleasant suprise, they do and I'm sitting here right now listening to Colin James and the big band 2. Swing music, jump blues, big band stuff...the new amp sounds so much better than the mac 275, I'm thinking about selling it to pay for my trip to europe...please see the "impedance measurement" thread of mine for details...I will switch over to this thread for further updates since my main concern is no longer related to the impedance of the circlotron output, but a investigation of the properties of the output topology...I'm using high impedance speakers, so the impedance issue is fading at least for me. I've rewired my bose 901s in series (64 ohms), put both systems in series (128 ohms) and now they are a close enough impedance match to wire directly to the cathodes, no matching xfmr. required. I estimate the amp's impedance output to be around 200 ohms using crude assessment techniques. Am working daily on getting impedance down, and still have a few tricks to try. The subs are pulled off the plates, using a 120 ohm 3 watt resistor in series to increase dc resistance....suprisingly high output from this, thinking I'd still have to use a xfmr for the subs. l'll be looking for another set of bose 901s to rewire, and any high end 18" woofers with a 16 ohm or higher impedance. Anyone know of 18"ers with a higher than 8 ohm impedance ? Thanks, midmoe.

midmoe 7th February 2013 03:41 PM

artosalo..thanks, I was typing previous thread when your latest reply showed up...will check out tube numbers....midmoe

azazello 7th February 2013 05:20 PM • View topic - OTL amplifier
You can think......cheep amplifier....
I used 6AS7G double triod- about 6-12$.
+U fase splitter must be 2 and more time than +U output tubes for driving them, that have bias -/80 - 100/ v.

cnpope 7th February 2013 06:11 PM


Originally Posted by midmoe (
The subs are pulled off the plates, using a 120 ohm 3 watt resistor in series to increase dc resistance....suprisingly high output from this, thinking I'd still have to use a xfmr for the subs.midmoe.

I'm not sure quite what you mean by this. Seems like they would then be at B+ voltage? And as far as the audio path is concerned, I wouldn't think it should matter whether you hook them to the plates or the cathodes, since the power supplies are (in principle, at least) more or less dead shorts to audio frequencies. So wouldn't it be better to hook them up to the cathodes, just like the other speakers?

It would be interesting to see your schematic, certainly.


midmoe 7th February 2013 07:27 PM

there seems to be a power-sharing thing going on between the plates and the cathodes...yes the sub is a +170, no big deal on that for me as am still in prototype phase. Identical information seems to be expressed at both cathode and plate rails...and if I present a high freq. load to the plates, it reduces highs expressed at the cathodes...leaving me to presume that if you were to remove low freq energy from the plates (with sub and associated cross.) then it would be reduced in the cathodes in direct proportion to removal from plates....hence the natural crossover effect. At least that seems to what's happening. Will do further tests tommorow about this. I'm getting the bias contols re-set today as I added a 100K between grid and plate rails for more neg. feedback and it threw my bias off beyond the control's range to compensate. Several diagrams show a 100K resistor, so I use a 2.2 meg in there now and it made quite a difference in the stage's behavior in a positive way, and within the bias conrol setting range. See entry in page two of "impedance measurement" thread for details. All posts related to the circlotron should have been made here, sorry for any confusion. Still a newbie about certain computer tasks, such as uploading pics. My friend Russel is coming up tommorow to help me out, and a functional schematic should be available in the evening. I will post new schematics as improvemets are made, but the basics shouldn't change. Thank you for your intrest. midmoe

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