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Zero Cool 4th June 2008 01:31 PM

FS: Rare Atma-Sphere Circlotron tube amp Prototype
FS: Rare Atma-Sphere Circlotron tube amp Prototype

I will be posting more information and pictures soon. But this is your chance to own one of the FIRST atma-sphere prototype amps! This is a circlotron OTL amp that uses a 6922 input tube with 12AU7 cascode, that feed 6CG7 driver tubes that run 8x 6AS7G output tubes per channel! This amp will produce somewhere around 40-60 watts per channel. I have never measured it but that is about the range for these amps with this config.

The amp is fully balanced differential from input to output and works best with 8 ohm high eff speakers but i have had no troubles running the various average sets of home speakers with it.

The sound is fantastic! good bass extension, very liquid sounding.

However. this IS A PROTOTYPE! and it is built as such using left over bench parts here and there. the design changed several times over the life of the amp and parts and have been cut and swapped etc. so there is a LOT of room for improvements and clean up. As this amp is now 30++ years old i would highly recommend it in fact! This would make a great chassis/test bed to roll your own circlotron amp!

Own a piece of history in more ways then one with this amp!

I am the 2nd owner of this amp. the first was a good friend of mine that just passed away that has an extensive history in the music biz. It was sold to him from Ralph Karston of Atma himself! I obtained it and had Ralph power it up and check it out and made sure it was running ok. then my father used it everyday for the last year without problems and loved it.

The ONLY reason im selling it is because of the rare opportunity to purchase Prototype number 2 from the guys at Atma! and i just cant keep both!

I am considering the asking price for the amp. I over paid a bit for it but it was a bit of a charitable purchase at the time.

I would consider $300 for it without tubes $500 for it with Electro Harmonix pre tubes NOS GE 6CG7 Driver tubes and RCA JAN 6AS7G output tubes. This amp runs better with Russian 6N13's but the RCA's seem to run just fine where as most US 6AS7G tubes dont!

email me at for more info or to discuss purchase

cscmc1 4th June 2008 04:46 PM

email sent!

Zero Cool 5th June 2008 03:59 PM

Additional photos now available!!1

Zero Cool 6th June 2008 03:26 PM

There have been a lot of emails about this amp! Thank you to everyone that inquired.

This is NOT a tube amp for beginners. I will only sell it to someone that is familiar with working on tube amps and is comfortable working around high voltages. Granted the voltages in this amp are lower then most! still it is best suited for someone with solid HV practices etc.



Arcolette 9th July 2008 05:35 PM

How is the biasing solved? One Pot for adjusting every 6AS7? And which capacitors are used for coupling between driver stage and power stage? Does this amp have any speaker protection?

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