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Vacuum Tube OTL power amp!!

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Wavebourn

Wavebourn

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Joined 2006
wavebourn.com
It would be a good idea to remember young age of tube electronics and construct an active speaker with triode SE amp loaded on an electro-magnet coil of the speaker. The motor has already a non-magnetic gap, so this coil can be used as well as an output transformer that feeds another coil that is right on the cone.
 
B

banat

Member
Joined 2009
Matthewong said:
Hi, would like to ask whether the vacuum tube OTL power amp worth to build? How is the sound performance if compare to normal output transformer type?? Does anyone has any good schematic diagram for OTL power amp, can share out, thinking to try to make one.
Click to expand...

Atmasphere M60 OTL Circlotron power amp is just OK for DIY .

Sound of this OTL Amp is clean , neutral , non colored and extremely fast .

Here is the DIY topic for you : http://www.diyaudio.com/forums/tubes-valves/161112-what-tubes-tube-amp.html

Best Regards
 
knoberik

knoberik

Member
Joined 2011
knob.planet.ee/
Matthewong said:
Hi, would like to ask whether the vacuum tube OTL power amp worth to build? How is the sound performance if compare to normal output transformer type?
Click to expand...
It is very much worth to build the OTL amp if you feel the time has come. It changed my view on the whole tube amplification thing.
There is quite a lot of OTL amps schematics available nowadays, but I think the best ones are based on 6C33C tubes in output stage. This tube is capable to drive real current, needed for low-ohm speakers.
Since now I tried Hans Beijner's "tubetvr" solution and I'm ultimately satisfied with result. It is the best sounding amp I ever made in my 30+ years of DIY'er career.

About circlotrons: they tend to have somewhat higher output impedance and I do not like it. Also they have direct coupling with speakers, so those need some protecting devices for possible tube failure.
About Tim Mellow OTL: it is not 100% triode solution, but I'd like to keep the amp fully triode based. Also the schematic is a bit (only) much complicated.
About some japanese design: they may look a bit over-transistorized.
I also contacted with people in Hartung Amplification, who are now coming out with OTL amp kits, based on 6C33C. They kindly showed their OTL schematic, it's a circlotron, nicely engineered. Their 6C33C kits are in good price area for my view, if you do not fancy to do everything from scratch. (As I mentioned earlier, circlotron is not 100% in my taste.)
I think you'll find your way, when you look a bit around.

How is the sound performance if compare to normal output transformer type??
Click to expand...
The whole sound quality took a remarquable leap from any OT amp I heard/built. Tubetvr's amp has power, is fast, very detailed, sound is open, stereo panoramic stage room feels really live. This amp has very low output resistance for OTL amp which I see positive. OTL amp is extremly linear in very wide spectrum. The OTL amp reminds mostly a wire with amplification, the THICK one :) It does not color sound, it plays thru what it will be feed with, meaning it shows out all crap put on records. In the other hands it is capable to open for you all marvels put on records.
I'll never build any OT amp for home use, let those be for musicians.
 
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C

cnpope

Member
Joined 2009
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.

Chris
 
knoberik

knoberik

Member
Joined 2011
knob.planet.ee/
FullRangeMan said:
However its site is few informative, there is only 1 small photo, do you have any info about the 6C33 OTL?? as price, number of tubes etc...
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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.
 
Alastair E

Alastair E

Member
Joined 2004
doctordiesel.co.uk
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...:D

Just build and enjoy.....;)
 

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Alastair E

Alastair E

Member
Joined 2004
doctordiesel.co.uk
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...
 
tubetvr

tubetvr

Member
Joined 2003
qtron.se
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+µ))) 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
 
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Alastair E

Alastair E

Member
Joined 2004
doctordiesel.co.uk
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...)
 
M Gregg

M Gregg

Disabled Account
Joined 2010
Alastair E said:
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...:D

Just build and enjoy.....;)
Click to expand...

Alastair,

What is the power out and load capability?

Regards
M. Gregg
 
M Gregg

M Gregg

Disabled Account
Joined 2010
Alastair E said:
Hi Gregg,
Around 20W at clipping, and into 8 ohms when tested, although my speakers are only 6 ohm, which it handles well...
Click to expand...

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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