RE: Why is the grid leak resistor for 6sl7 amps 470k or higher?

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In Reply to: Why is the grid leak resistor for 6sl7 amps 470k or higher? posted by Iczerman on April 21, 2014 at 08:48:42:

Because it can be.
The max. grid resistor value for a given tube is based on the tube's grid leakage current.
The higher the grid leakage current for the tube type, the lower the max. allowable grid resistor value.
The data sheet for tubes with higher grid leakage current usually give the max allowable grid resistor value.
For example, a 2a3 with fixed bias can have a grid resistor value of up to 50k ohms and for cathode biased operation, up to 500k ohms.
I can't find a data sheet with this info for a 6sl7 but there are examples (on the data sheet) of operation with as high as 10meg ohms being used.
It should be understood, the lower the value of the grid resistor, the harder it is for the previous stage to drive.
That is to say, the lower the value the grid resistor is, the more vertical the load line for the plate of the previous stage will be.
This will cause less gain from that stage and more harmonic distortion.
BTW Low value grid resistors have nothing to do with the loss of information per se. As long as the stage driving the grid resistor value is fully capable of driving that value load.
Most line stage preamps use a volume pot in front of a tube stage.
When the volume pot is turned down the "grid resistor" value is very low.
There is no loss of information WRT that low grid resistor value.
A lot of phono preamps use a 12ax7 for the first stage. The standard 47k ohm grid resistor causes no information loss problems for the 12ax7 and is the suggested match for a MM cartridge.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

Edits: 04/21/14

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Follow Ups Full Thread

Topic - Why is the grid leak resistor for 6sl7 amps 470k or higher? - Iczerman 08:48:42 04/21/14 (5)

RE: Why is the grid leak resistor for 6sl7 amps 470k or higher? - shane. 00:41:49 04/22/14 (0)

RE: Why is the grid leak resistor for 6sl7 amps 470k or higher? - Tre' 11:16:18 04/21/14 (0)

In some designs, they are 10Meg or more NT - Alpha Al 10:51:38 04/21/14 (0)

RE: Why is the grid leak resistor for 6sl7 amps 470k or higher? - drlowmu 09:17:26 04/21/14 (1)

Micro dynamics - Caucasian Blackplate 15:18:53 04/22/14 (0)

Follow Ups

RE: Why is the grid leak resistor for 6sl7 amps 470k or higher? - Tre' 11:16:18 04/21/14 (0)

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