Tubes Asylum

Posted by robster (A) on April 29, 2007 at 19:03:58

Fellow inmates..

I dropped in new tubes over the last few days into my
Cary SLI-80 and was womdering if biasing is proper under
an input volume or not? I've dialed in the recommended
75mA bias but noticed that as I turned up the volume the
75mA bias flutuated as the volume was turned up.

Probably a dumb question and probably covered off in earlier
posts...but was wondering where bias vs volume should be
considered? Thanks..

• Re: Manual biasing with volume or none? - lipmanl 12:06:07 04/30/07 (0)
  In Reply to: Manual biasing with volume or none? posted by robster on April 29, 2007 at 19:03:58:

  If this is a class AB amp, when the volume is turned up into the class AB range (the first bit is class A), the AVERAGE current through the tube drops (it's spending part of the time in cutoff). Maybe your meter is averaging, and is simply reporting the truth under normal operation.
  
  

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• Normal settings are.... - doodlebug 22:16:08 04/29/07 (0)
  In Reply to: Manual biasing with volume or none? posted by robster on April 29, 2007 at 19:03:58:

  to have the inputs shorted. Of course, specific manufacturers like Cary will have a recommended method. Generally, though, I like to have the inputs shorted to ensure there's no induced noise to cause my settings to be upset in some way.

  I'd suggest you contact Cary regarding their specific procedures.

  As for monitoring bias voltages while operating the amp, you should expect the bias to change under these conditions - there's audio being passed! If you monitored the bias with a scope you would see audio waveforms.

  Cheers,

  David
  
  

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• No input signal when biasing is a standard requirement. NT - Chuck Y 19:58:26 04/29/07 (1)
  In Reply to: Manual biasing with volume or none? posted by robster on April 29, 2007 at 19:03:58:

  .
  
  

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  • Re: No input signal when biasing is a standard requirement. NT - robster 20:19:29 04/29/07 (0)
    In Reply to: No input signal when biasing is a standard requirement. NT posted by Chuck Y on April 29, 2007 at 19:58:26:

    Thanks for the insight...cheers!
    
    

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