AudioNervosa
Systemic Development => Amplification and Preamplification => Tubes => Topic started by: Emil on May 07, 2016, 05:33:51 AM
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Why aren't all tube amplifiers autobiasing? Seems like a no brainer
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Emil,
A few reasons, the most important is that sound quality should be better with fixed bias (less complicated).
With fixed bias, you have to reset the bias every once in a while as tubes age and it is reported to sound better. You don't need a large cathode bypass cap (sometimes a crappy electrolytic) in the AC current path (signal path) as used in Auto Bias (cathode bias). Some tubes require auto bias since they're not stable (like a lot of us, LOL!) using fixed bias and can cause catastrophic destruction of the output circuit. Auto bias can be off +/- 20% as well!
Also, the newer production tubes of the classic Triodes do not have the same quality of the NOS tubes from Western Electric, RCA, Sylvania, etc. These newer tubes are inconsistent and can't hold their bias. This can be a problem with auto-bias, since they can run too cold or too hot. Amps with adjustable bias, that can measure current, can make these new production variations unimportant.
My $0.02,
Pete
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Pete,
Aren't the Russian/Eastern Block tubes supposed to be of very good quality? Maybe not like the old American stuff, but a close second. Attached is a Youtube video of a factory in Czech Republic. It looks to be well made stuff. https://youtu.be/8n4WVRKkmww
I think we all know the Chinese stuff is garbage. :duh
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I think we all know the Chinese stuff is garbage. :duh
Not all. Some of the best tubes are Chinese, hot from the communist owned Shuguang factory, like Black Treasure tubes. Grant/Psvane is also supposedly very good, but I've not yet tried them, by former Black Treasure designers.
The best Russian tubes now come from an American owned factory, New Sensor. The biggest Russian owned competitor SED (Svetlana) is fading out. Smaller eastern-european tube makers vary in quality.
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How do you know what the lyrics are using Chinese Tubes? Do I have to turn on sub titles?
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Pete,
Aren't the Russian/Eastern Block tubes supposed to be of very good quality? Maybe not like the old American stuff, but a close second. Attached is a Youtube video of a factory in Czech Republic. It looks to be well made stuff. https://youtu.be/8n4WVRKkmww
I think we all know the Chinese stuff is garbage. :duh
The quality I'm referring to is the ability to hold their bias tolerances... the newer tubes just can't hold their bias tolerances like the NOS tubes... That applies to ALL new manufacturers... If you have "Adjustable Bias", there is really nothing to worry about...
My $0.02,
Pete
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I prefer to run cathode bias/autobias exclusively for several reasons.
1) If fixed bias supply fails, tube then becomes a fire works display, which do enjoy, but only for those tubes I do not like.
2) Self bias provides increased stability. This is evidenced by the ability to increase the grid resistor without fireworks. For instance, a 6L6 tube allows for 500k grid resistor with cathode bias VS 100k for fixed bias. Think of it in terms of a DC negative feedback phenom.
3) The lone problem is the use of a cathode bypass capacitor. This problem is not so severe as first appears as the impedance is low. However, it is not perfect, but there are ways of lessening any negative effects.
Cheers
Steve