Why do you need a grid to ground resistor on a line stage?

phrarod · 7th May 2010, 02:10 AM

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.

HollowState · 7th May 2010, 02:25 AM

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.

phrarod · 7th May 2010, 02:28 AM

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?

HollowState · 7th May 2010, 02:29 AM

Yup. In the wide open position.

phrarod · 7th May 2010, 02:30 AM

Then isn't that defeating the purpose of a 50K or say 100K pot?

SY · 7th May 2010, 02:33 AM

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.

HollowState · 7th May 2010, 02:34 AM

Everything in electronics is a compromise. It's called designer's choice. To get something, you usually have to give something.

phrarod · 7th May 2010, 02:35 AM

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.

7th May 2010, 02:10 AM	#1
phrarod diyAudio Member Join Date: Aug 2005	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.

7th May 2010, 02:25 AM	#2
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	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. __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

7th May 2010, 02:29 AM	#4
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	Yup. In the wide open position. __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

7th May 2010, 02:33 AM	#6
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	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. __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

7th May 2010, 02:34 AM	#7
HollowState diyAudio Member Join Date: Aug 2006 Location: Taxland, New Jersey	Everything in electronics is a compromise. It's called designer's choice. To get something, you usually have to give something. __________________ "The supercomputer is technologically impossible. It would take all of the water that flows over Niagara Falls to cool the heat generated by the number of vacuum tubes required." ~ Professor of Electrical Engineering, New York University

7th May 2010, 02:28 AM	#3
phrarod diyAudio Member Join Date: Aug 2005	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?

7th May 2010, 02:30 AM	#5
phrarod diyAudio Member Join Date: Aug 2005	Then isn't that defeating the purpose of a 50K or say 100K pot?

7th May 2010, 02:35 AM	#8
phrarod diyAudio Member Join Date: Aug 2005	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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