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RE: grid resistors first stage of amp

Posted by Jim McShane on June 24, 2012 at 13:16:33:
Even at 100K the "shunting" is minuscule as Chris wrote.

The value you use is a tradeoff:

a) a higher value is easier to drive for the component upstream

b) a lower value is (all else being equal) quieter.

You need to know the output impedance of the device that is going to be plugged into the amp, and ensure the resistor value you use is large enough to prevent problems upstream. But there is no need to use a larger value than needed to meet condition "a", since that may increase the noise (whether the increase is audible depends on a lot of things).

There is an old rule of thumb that says you need an input impedance that is a minimum of 10X the output impedance of the driving device. It's a pretty good rule of thumb. And of course the value should be in spec for the tube you use.