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Old 7th May 2010, 03:10 AM   #1
phrarod is offline phrarod  United States
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Default Why do you need a grid to ground resistor on a line stage?

I'm playing with a linestage design and saw Glass Tube audio had a circuit for a 12AT7 which showed a 220K grid to ground resistor in parallel with the volume pot.

I have played with putting it in and I do get more dynamic but also more noise by the nature of having another resistor in there.

Its a simple circuit using a 5689 single half with output transformer. If the 50K pot is loading the grid why would the circuit need another resistor in parallel?

I usually don't see this.
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Old 7th May 2010, 03:25 AM   #2
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In almost all cases the resistor is there in case the control goes open on the ground end (or wiper) leaving the grid floating. That's a rare event but it can happen. If noise increases, perhaps try a better resistor.
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Old 7th May 2010, 03:28 AM   #3
phrarod is offline phrarod  United States
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I using a caddock which is about the quietest I know of. Would it be a fair assumption to say that with a 50K and a 210K grid to ground my effective load on the grid is 40K since they are essentially in parallel?
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Old 7th May 2010, 03:29 AM   #4
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Yup. In the wide open position.
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Old 7th May 2010, 03:30 AM   #5
phrarod is offline phrarod  United States
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Then isn't that defeating the purpose of a 50K or say 100K pot?
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Old 7th May 2010, 03:33 AM   #6
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Depends on what that pot's purpose is. More commonly, the "safety" resistor will be much larger than the pot. Its noise is irrelevant since the pot is shunting it.
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Old 7th May 2010, 03:34 AM   #7
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Everything in electronics is a compromise. It's called designer's choice. To get something, you usually have to give something.
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Old 7th May 2010, 03:35 AM   #8
phrarod is offline phrarod  United States
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Volume pot, pot. One thing I noticed is the dynamics increase when I put the grid to ground in but low level detail decreased. Its a trade off. If its just for safety I'll keep playing around. Thanks for the explanation.
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