[FullRangeMan]

It is the Tim Mellow OTL  as published on AudioXpress Feb2010
http://www.audioxpress.com/magsdirx/backissues/BISAXM.htm
For two 6C33 (or 6C41) to 25W and an impressive low output impedance=0.25 ohms according this DIYer bellow>
I have built three OTL's; the designs by Hans Beijner and Tim Mellow, both with 6C33C output tubes, and also a somewhat more complicated one by Alan Kimmel, which is a circlotron using EL609 sweep tubes. The Tim Mellow design has a slight merit over the Hans Beijner design of not needing multiple floating grid-bias supplies. All three are pretty low output impedance; 0.25 ohm (Mellow); 0.6 ohm (Beijner); 1 ohm (Kimmel), and all three sound excellent. I would say the sound is very clean and crisp, in comparison to a tube amp with output transformers. The Beijner and Mellow amps are really pretty simple to build.
If Google images ''tim mellow otl'' will appear some circuits. If anyone will made this project please keep us updated.
Regards, Gustavo

P.S.> Someone knows what are the input tubes in this project?? It can be changed??

> Never go to a psychiatrist, adopt a cat or dog from the street. On the streets pets live only two years average.

[FullRangeMan]

Someone knows what are the input tubes in this project?? It can be changed??

[JohnR]

From the schematic they appear to be ECC83.

[FullRangeMan]

Thanks John. The 6C33 model use ECC83/12AX7, and the 6C41 model use 6DJ8/6922.

[FullRangeMan]

The 6C33 model use two ECC83/12AX7, that have a 100x gain, so 100 x 100 = 10K gain. Looks a smart desing.
Could this amp have hi self noise??

[JakeJ]

I can't go to your link ("Link appears to be broken" message) but isn't the noise rating in with the rest of the specs in the article?  I would think it would be measured and listed.

[FullRangeMan]

The link is not great info, just index to the mag:
http://www.audioxpress.com/magsdirx/backissues/BISAXM.htm

[FullRangeMan]

This is a rare bird today a new OTL circuit:
the Tim Mellow OTL circuit as published on AudioXpress magazine Feb2010.
I can not read it, but maybe someone who can read it, can tell me what kind of output circuit it has (PushPull, SE Parallel, etc)??
Or even any other comment is welcome. Thanks in advance.

[JohnR]

Hi Gustavo, I gave it a quick scan and found it very interesting. I'd like to try adapting it for other tubes. However too many projects. I will try to find it again and give it a more careful read. It is a push-pull circuit. These OTL circuits can be very subtle and at the limits of my ability to understand them but I will give it a try.

[Steve]

This is a rare bird today a new OTL circuit:
the Tim Mellow OTL circuit as published on AudioXpress magazine Feb2010.
I can not read it, but maybe someone who can read it, can tell me what kind of output circuit it has (PushPull, SE Parallel, etc)??
Or even any other comment is welcome. Thanks in advance.

Well it appears that in order to obtain any kind of high frequency response, lots of feedback is needed as open loop frequency response is poor. 
However, the global feedback is variable (R3) vs the volume control setting (about 4 to 1 ratio). So distortion measurements, output impedance, frequency response varies depending upon the volume control setting, as well as output level from the source component.

Just an observation.

Cheers.

[FullRangeMan]

Thanks boys for your support. I just got the Audioxpress article, it say it is PP indeed, and NFB have two options, 26dB and 11dB(removing some capacitos maybe C3 and C4.
Input tube is one 12AX7, drive are two EF86 Pentodes, output are two 6C33.
It is suggested balanced input is better, with 3dB gain.

Do you think this is a Hi-End tube amp??
I wonder how is the sound quality with a 8 Ohms fullrange driver??
Thanks, Gustavo

[lo_tse]

Gustavo,

If you want more info, go to DIYAUDIO and search for 6C33 OTL.  With no output transformers, OTL amps are great sounding.  Generally speaking, there are 2 basic designs for OTL amps - Futterman and Circlotron.  The Tim Mellow design is a "pseudo Futterman".  Han's design is truer to the original Futterman.   As Chris (the guy who wrote the message that you quoted on your first post) had commented, Han's design is even simpler than Tim's  but the power supply sections were quite elaborate and you need 4 transformers (if I remember correctly) to complete it.  It is on my future project list. By the way, all the commercial OTL amps cost a bundle e.g. Transcendence, Graff.  Hope these info help.

Lo-tse

[FullRangeMan]

Lo-Tse,
Thanks for your opinion, I already read the DIY Threads on Tim Mellow.
The parts list is the smalest I see to date in a good tube amp, so seems a low price amp with usable 25W.

But now I in doubt it has great sound, as it use 11 or 26dB of NFB and it is PP design, that favor the odd order harmonic distortion.
Do you already listen this TimMellow OTL?? How is the sound quality??
Regards, Gustavo

[JohnR]

Um 

[lo_tse]

Gustavo,

Sorry for the late response.  I was out of town.  No, I have not got a chance to listen to these amps.  However, everyone that claimed (on the forum) has built one likes the sound.  The fact that the output transformer is absent is a big plus on the sound - better frequency response, wider bandwidth, much lower output impedence.....  Of course one can use those high perfomance OT (e.g. Tamura, Tango etc.) to minimse thess short comings but we all know how expensive those are.

I would not worry about the amps are PP design - there are many good sounding PP amps around, OTL or not.  Check out the Atmasphere website for more info on OTL - very informative.  Take care!


Lo-tse

[FullRangeMan]

LO-TSE:
Very Thanks for your personal opinion on this amps.
Iam more aware now of what sound it can do.
Regards, Gustavo

[zjj_wwa]

Well it appears that in order to obtain any kind of high frequency response, lots of feedback is needed as open loop frequency response is poor. 

Please kindly explain why to you believe that the open loop frequency response is poor ?
The driver pentodes have very small Miller Capacitance and are effectively working as current sources for the 100K resistors.
Under such circumstances, I believe that the open loop frequency response should be very good.
Furthermore, I believe that the closed loop frequency response should be outstandingly good.
Cheers,
Ziggy.   

[zjj_wwa]

Do you think this is a Hi-End tube amp??
I wonder how is the sound quality with a 8 Ohms fullrange driver??

Yes, I believe it should turn out to be a Hi-End tube amp. Even with 8 ohm speakers.   But I think that even better results should be achievable with 16, 32 or 64 ohm speakers.
I have build an Inverted Futterman OTL based on a pair of 6s233s (6c33c) tubes and definitely confirm that it is a sound is outstanding. A sound to die for. A sound well worth the electricity bill, which may prove to be an eye opener at first. The level of transparency and detail is breathtaking. Just take care not to fry the tubes, or even melt them. 
This is Russian MIG Fighter plane technology and they can take a fair amount of abuse, but still, try to stay within reasonable idle currents and peak current limits.
Anyways, for "normal listening levels" in not too exaggerated listening room surface areas, the OTL on a pair of the 6c33c is a Darling. 
For bigger rooms and/or louder listening, consider a split system, with dual amplifiers.  Use an OTL as above to drive the tweeter and midrange, and use a standard transformer based PP to provide the lowest bass slam.
Cheers,
Ziggy. 

[zjj_wwa]

But now I in doubt it has great sound, as it use 11 or 26dB of NFB and it is PP design, that favor the odd order harmonic distortion.
Do you already listen this TimMellow OTL?? How is the sound quality??

Do not be in doubt.  The sound should be OK. Although this Tim Mellow design is bold and innovative, different from the stuff that I have build, it looks like a good concept.
I have practical experiences with the Technics / Inverted Futterman configuration, something vaguely similar to Hans design and I am very happy with it.
The sound is very good. The PP is not an issue. If you are worried about the odd order 3rd harmonic distortion - please bear in mind that by adjusting the idle current,
you can quite easily shift from a cold AB1 to a very hot AB1 class.  The hot AB1 will essentially sound like nothing that you have ever listened before. Close to a "pure class A" but without OPT. 
As for the feedback issue - feedback is GOOD.  The more feedback - the better. There is NO SUCH THING as too much feedback.
Besides, if you want the output impedance of the OTL to be low and achieve a good damping factor, you NEED a high amount of feedback.
The good news is, that .... lacking the phase shift of the output transformer, you can go for an immense amount of open loop gain, and then essentially kill most of it by applying a huge amount of NFB.

The next project that I am thinking about is a a single differential Super-Voltage-Gain-Stage with mu-follower outputs, with a single coupling cap into current source configured cathode follower pentodes driving a pair of  {6s33s/Toshiba HV NPN BJT} Super-Tubes. Either the Toshiba's, or even better, some lateral mosfet technology. We shall see.
Anyways, the design will strive to achieve 80 dB open loop gain, with 40dB NFB, leaving 40dB of closed loop gain.
With the 40dB NFB, I am hoping to see the other side of the rainbow, i.e. the other side of the so called Baxandall's Hump.

Cheers,
Ziggy.

[FullRangeMan]

Thanks Ziggy for your detailed replies, I appreciated.
Your amps seems new interestings design and even more to listen.
Any sound impressions or photos?
Cheers