by Ty_Bower » Thu Mar 05, 2009 7:44 am
Dave,
I'd assume you could simply change the 470k ohm resistor (circled on the schematic) to a lower value. I wouldn't expect the 7199 to mind at all. I've seen maximum permissible values specified for the grid leak resistor, but never seen a minimum value. It's perfectly safe for the tube even if you dead short the control grid directly to ground. Of course, you won't get any audio signal into the tube that way. :)
For what it's worth, the
RCA datasheet for the 7199 seems to suggest a 270k ohm resistor for grid No.1, with a maximum value of 1 meg if you're running the pentode section with cathode biasing.
Just out of curiosity, exactly what kind of upstream device are you using that doesn't like a high impedance load? Generally speaking, preamps prefer high impedances...
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