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OTL "a la Browsky"? - Food for Brain

77.43.10.3
Posted on October 27, 2009 at 02:33:00
6AS7_6SN7
Audiophile

Posts: 98
Joined: September 10, 2009



Sometime ago I remember that J. Browsky of www.tubecad.com reviewed several OTL variants with output caps.
I have preliminary explored this one (I do not know if it fits exactly with one of the mentioned Browsky's variants).
This particular one requires one +300V HT only and the standard negative voltage for fixed bias.
To my knowledge it might be one of the simplest OTL for DIY people.
In parallel with the speaker there is a 1:1 transformer for the 6 dB differential feedback that is applied back to the driver.
The performance (see simulated output spectrum @ 650 mA peak current, with predominant third harmonic) appears better than the novel circlotron just posted. And probabily there is still room for some improvement.

Best Regards
Luca Cellai


ecc230
Simulated Output Spectrum, posted on October 27, 2009 at 02:39:58
6AS7_6SN7
Audiophile

Posts: 98
Joined: September 10, 2009



Output Spectrum @ 650 mA peak current.
ecc230
Sorry, I meant predominant second harmonic...., posted on October 27, 2009 at 02:44:34
6AS7_6SN7
Audiophile

Posts: 98
Joined: September 10, 2009
 The third harmonic is the predominat one of the 'novel circlotron'.
ecc230
RE: Sorry, I meant predominant second harmonic...., posted on October 29, 2009 at 06:26:01
tubetvr
 Predominant 2nd order means that there is an unbalance. All push-pull derivates reduce 2nd order if made correctly, (well balanced). It would be interesting to know which level your spectrum shots are taken at, dependng on how close to clipping distortion levels will vary very much, even more so for higher order. In order to make fair comparison I always define output power at clipping as the power level at 1% THD, I also compare distortion levels at 1dB below clipping. What is your criteria?

I have to think it over, posted on October 31, 2009 at 04:51:07
6AS7_6SN7
Audiophile

Posts: 98
Joined: September 10, 2009
 Unfortunately I have not yet elaborated a criteria.

But your point is worth further investigation.

Thank you.

Luca
ecc230
RE: I have to think it over, posted on November 1, 2009 at 00:59:59
tubetvr
 Ok,
But I ask again, which level are your simulation plots made at? Is it 1dB below clipping or 10dB below clipping or what?

Clipping, how could we possibly define clipping for OTL?, posted on November 4, 2009 at 03:28:11
6AS7_6SN7
Audiophile

Posts: 98
Joined: September 10, 2009
 This is my point of view on the matter.
Clipping definition of OTL tube amplifiers depends on tubes performance, that is, how much current the tubes can deliver into the load.
Tube performance depends on tube manufacturer, year of manufacturing, tube ageing and the like.
In my experience Sylvania 6080 or Russian 6AS7 tubes are the best, as they can deliver (for a short time) more than 500 mA peak (as a matter of fact if I intentionally overdrive my OTL with four 6080 per channel the HT 2A retarded fuses blow).
But in order to maximize the life of OTL tubes is better to keep a reasonable safety margin.
For 6AS7/6080, whoever is the manufacturer or their ultimate performance, I think that they should never deliver more than 400 mA peak per triode.
This would put a practical 'soft clipping' definition at 400 mA /tube and 'hard clipping definition' at 500 mA / tube.

A final sad remark: unfortunately I do not think this approach will ever help to build better tube amplifiers.

Best Regards
Luca Cellai

ecc230
RE: Clipping, how could we possibly define clipping for OTL?, posted on November 11, 2009 at 20:58:00
Tre'
Industry Professional

Posts: 4845
Joined: February 9, 2002
 But what about cutoff. Those tubes will clip at cutoff long before they clip at saturation. I'm talking about the tubes in each phase of the push pull, not the output to the speaker.

If the tubes are biased to idle at 75ma. (each triode) And if they are linear tubes, they will clip, because of cutoff, at the same time 150ma. is reached on the tubes on the other phase.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

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