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Old 24th February 2007, 10:24 PM   #1
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Default bias voltage vs bias current

I'm pretty new to tubes, although learning fast.

I have a question though:

From the books, sites and so on, I see bias being referred to. This as I understand it refers to the difference in voltage between the grid and cathode (ie grid to ground vs. cathode to ground measurement). I see how this allows for the grid to control the flow from the cathode - keeping it simple with say a triode valve first.


Then I read a mag like stereophile where john atkinson, and he refers to bias current, which he measured in the mA range (think it was something like 35mA or so) for an amp he was checking out.

So what current does his measurement refer to? is this something he works out from V=IR?


thanks for the education in advance!

Fran
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Old 24th February 2007, 10:58 PM   #2
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Grrrr, this particular bit of sloppiness really bothers me, and this is why: it's confusing to people like Woodturner who are trying to understand this stuff and gets in the way of clear thinking. It's also universal...

So, let me try to unconfuse another poor victim of people who should know better. "Idle current" is the DC current running through a tube with no signal present. "Bias voltage" is the voltage at the control grid with respect to the cathode that is required to get the desired idle current.

When people talk about "bias current," they nearly always mean "idle current."
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Old 24th February 2007, 11:07 PM   #3
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Thanks SY,

I knew it must be something to do with the bias voltage, but just couldn't figure it out. Couldn't see it mentioned anywhere in morgan jones and the internet threw up nothing. Idle current makes complete sense. One more Q - I see bias often represented by "Vgk" is there a similar notation for idle current (eg Ika )? or is this the same as anode current?


to my rescue yet again! BTW that chinese amp with the hum,is working very nicely although I'm still waiting on some milspec tubes. biggest diff to hum was made by adding bigger caps to the voltage doubler. Hum now down to <1mV and unmeasurable on headphones outs although I can hear it on HD650s but not AKG501s.


thanks again,

Fran
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Old 24th February 2007, 11:24 PM   #4
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It's the same as anode current for a triode. A pentode is a little more ambiguous, and strictly, we should specify Ia or Ik for the idle current; they differ by Ig2.
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