If it's too low to drive an LED add a current mirror.

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In Reply to: Re: grid current posted by BBeck on December 29, 2005 at 06:49:54:

If the grid current is too low to light an LED you could always add a Widlar current mirror with emitter degeneration resistors chosen to give say 10:1 current step-up. You would need a DC source for the second side of the Widlar mirror but if you use DC filament supply that would do.
From my observations the distortion increase at 200 μA grid currenton my 211s is trivial, considering the grid current is only pulled on signal peaks anyway. I just use it as an "onset of overload warning". For real music I make sure that the LED doesn't light on orchestral tutti. For amplified music I let it beat time with the bass if I'm feeling like listening loud.
Mark Kelly

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Follow Ups Full Thread

Topic - grid current - BillyJack 18:19:00 12/28/05 (5)

Re: grid current - BBeck 06:49:54 12/29/05 (1)

If it's too low to drive an LED add a current mirror. - Mark Kelly 15:53:09 12/29/05 (0)

Grid current indicator - Mark Kelly 18:36:55 12/28/05 (2)

Re: Grid current indicator - BillyJack 18:46:49 12/28/05 (1)

High efficiency LEDs - Mark Kelly 02:01:46 12/29/05 (0)

Follow Ups

If it's too low to drive an LED add a current mirror. - Mark Kelly 15:53:09 12/29/05 (0)

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