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Old 19th May 2011, 10:59 PM   #11
cnpope is offline cnpope  United States
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Quote:
Originally Posted by Alastair E View Post
Here's a scheme Ive designed and built. Its Very simple, with MOSFET loaded gain-stage, has no global NFB, has Auto-Bias--so no worrying about drifting tubes and all that nonsense, and has a superb, powerful open sound.

Also, the power-supply is very straightforward, a simple bi-polar supply for the O/P stage and a 350V rail, No fiddly 'bias' rails or pots needing re-adjusting periodically......

Since built it has proved utterly reliable, probably well over 500 hours on it so far...

Just build and enjoy.....
That looks like an interesting design! Do you have figures for distortion and output impedance?

Chris
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Old 20th May 2011, 12:35 AM   #12
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Originally Posted by knoberik View Post
I want to make clear that I am not related to Hartung Amps in any way, I just asked some questions for my curiosity. So I can not speak for them. And to share or not their schematics in public, it's their desicion. I think they must show schematics sooner or later, if they want to sell amp kits. Today they were ready to show schematic only for 50W 4 x 6C33C OTL amp. They have something interesting more in the sleeve they said, not to be shown, if not buying an amp kit... OTL made on GU50 (ГУ-50) tubes. I really can't imagine it...
There is an e-mail on the site, so please address all questions to their speaker person mr. Plamen Doynov, hartung.amps@gmail.com Mr. Doynov seems nice person and of course will satisfy all your curiosity.

Of course I can share my own experience about building the OTL amp freely, as I did in some other branch here. Just search for "6C33C" and you will find all reading about OTL amps also.
Thanks for your input. Four 6C33 is too much heat for me and 50W is excessive too.
Iam more interessed in the Tim Mellow with just two 6C33, I need 15 to 25W only.
THanks, Gustavo
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Old 20th May 2011, 07:56 AM   #13
siang is offline siang  Taiwan
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Hi Alastair,

nice work. can I ask to see the PS also?

Siang
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Old 20th May 2011, 08:45 AM   #14
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cnpope-

I'm sorry, I havent any figures of distortion, as I havent anything 'good' enough to test for distortion accurately. I did an O/P imp test when I first built it and got something around 0.2 ohm, but have altered the original design into what you see above since, removing loop-feedback so guess its somewhat higher in its current format.
I would love someone to do such tests accurately if they built it, be very interesting.

siang-

TBH, I havent put the PSU in schematic form, but is very simple, a Plus, Minus 150V derived from a 220V 'isolating transformer' and bridge-rec, that can supply around an amp or so.
I'm using 3,300uF 200V caps for filtering these supplies to the O/P stage.

The 350V again is just a simple CLC supply, nothing special...
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Old 20th May 2011, 09:20 AM   #15
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An inverted Futterman with 2 6C33C always give an output impedance of ~11ohm without global feedback, it should do that acording to theory ,
(Rp/(2*(1+))) and it always measure like that if the tubes are fresh enough. The resulting output impedance with global feedback is just a result of the amount of feedback so if you add 20dB of feedback = 10 times the output impedance will be 1.1ohm, it is just that simple. Other Futterman variants or Circlotrons or with other tubes can be calculated acording to theory and gives practical results very close to theory.

The easiest way to determine output impedance is to measure output voltage while changing the load impedance, just make sure that the amplifier doesn,t clip or is close to clipping.

BR Hans

Last edited by tubetvr; 20th May 2011 at 09:26 AM.
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Old 20th May 2011, 10:32 AM   #16
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Yes, Originally when I measured the Imp roughly, there was two gain-stages and it had GNFB...

I completely re-designed the 'front-end' to one gain-stage and removed the global, so you're right--Its O/P imp will be in the order of 11 ohms...

It sounds though much better --to me--in its current state than it did with the two gain-stages with GNFB....
(The clarity, soundstage seemed to open up and just sounded 'better' The bass control is still very good, no boomynes/woolynesss with my speakers...)
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Old 20th May 2011, 05:47 PM   #17
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Alastair E View Post
Here's a scheme Ive designed and built. Its Very simple, with MOSFET loaded gain-stage, has no global NFB, has Auto-Bias--so no worrying about drifting tubes and all that nonsense, and has a superb, powerful open sound.

Also, the power-supply is very straightforward, a simple bi-polar supply for the O/P stage and a 350V rail, No fiddly 'bias' rails or pots needing re-adjusting periodically......

Since built it has proved utterly reliable, probably well over 500 hours on it so far...

Just build and enjoy.....
Alastair,

What is the power out and load capability?

Regards
M. Gregg
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Old 20th May 2011, 05:54 PM   #18
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Hi Gregg,
Around 20W at clipping, and into 8 ohms when tested, although my speakers are only 6 ohm, which it handles well...
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Old 20th May 2011, 06:03 PM   #19
M Gregg is offline M Gregg  United Kingdom
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Quote:
Originally Posted by Alastair E View Post
Hi Gregg,
Around 20W at clipping, and into 8 ohms when tested, although my speakers are only 6 ohm, which it handles well...
What is the power in (Mains current?)<< I know stupid question LOL.

Genuinely interested in mains current "power in"!

Do you have a photo?

I think everyones waiting for a PSU schematic..LOL

It sounds very interesting 20W is good for OTL.

Regards
M. Gregg
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Last edited by M Gregg; 20th May 2011 at 06:26 PM.
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Old 20th May 2011, 06:23 PM   #20
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I genuinely havent a clue as to its current draw from the mains--Never measured it....

Will obviously vary with output power delivered...

I have a 3.15 A/S fuse in the supply line live, so its under that!
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