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Old 21st December 2005, 02:37 AM   #1
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Default Dynaco Mk IV question

Completely new to this, and tried search but came up empty. What should I bias these amps to wired in pentode? Thanks
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Old 21st December 2005, 05:09 AM   #2
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You can run them pretty low, say 35-40mA per tube (70-80mA total per channel).
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Old 21st December 2005, 05:18 AM   #3
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The Mark IV if stock is designed to bias to 100 ma at 1.36V (not the 1.56 of the ST 70) thus you would bias to 1V to achieve 40ma per tube. I generally Bias it at 1.3V (95ma total-It measured as the sweet spot for distortion at 1k on an Audio Precision setup).

Sean
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Old 21st December 2005, 05:37 AM   #4
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Awesome, thanks.
Like I mentioned, I am new to tubes, I've owned a lot of audio gear, and and very picky about the sound, but really limited in electronics itself.
These are the first tube amps I've owned, and now that I have a taste, there'll likely be no end to this new path.
What is the adverse affect to the equipment or tube life if biased too high or too low? Thanks again for your help
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Old 21st December 2005, 05:39 AM   #5
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If idle current is too high, the tubes will get quite hot, shortening their life and the life of nearby components. If idle current is too low, you can get a nasty distortion near the zero-crossing.

Just to be clear, were you asking about pentode or ultralinear mode?
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Old 21st December 2005, 04:38 PM   #6
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I was asking about the pentode mode. Thanks for the reply
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Old 21st December 2005, 06:00 PM   #7
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I assume these mkiv are modified as they never came wired for pentode mode from the factory, default configuration for factory or kit units was ultra-linear. (Hallmark of all Hafler designed amplifiers afaik.)

They are frequently modified for triode connection with the screens tied to the plates through small resistors of about 100 ohms. Power so connected would be about 18W.
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