Resistor after volume pot

gottlieb · 12th May 2009, 12:00 PM

Hi, this is my first post here. I'm studying electronics and I want to ask what's what's the purpose of the series resistor after the volume pot, what are the effects of changing its value, what are the recommended values, etc.

I've seen this configuration in many places but I'm not sure how it works. Thank you.

SY · 12th May 2009, 12:13 PM

That diagram looks wrong.

Generally, a resistor connected in series with the grid is put there for RF suppression or to prevent oscillation. It's known as a "grid-stopper."

azazello · 12th May 2009, 01:36 PM

1 kohm.

ray_moth · 12th May 2009, 01:52 PM

Gottlieb,

As SY said, that diagram is wrong. There HAS to be a path to ground from the grid. That o.1uF capacitor is preventing there from being such a path. The tube will be unbiased and will take excessive plate current,

gottlieb · 12th May 2009, 03:01 PM

Sorry, my mistake. Here's the complete circuit. It's a cathode follower configuration. I more or less understand everything except the resistor in red.

Wavebourn · 12th May 2009, 03:25 PM

You can assume a loop between cathode and the ground of the pot. If it's resistance is low it's inductance may cause oscillations. I saw such a problem in Ampeg bass guitar tube amps, it caused HF oscillations.

gottlieb · 13th May 2009, 08:51 AM

Thanks Wavebourn.

Is there a remedy for that problem you describe? Thank you.

12th May 2009, 12:00 PM	#1
gottlieb diyAudio Member Join Date: May 2009	Resistor after volume pot Hi, this is my first post here. I'm studying electronics and I want to ask what's what's the purpose of the series resistor after the volume pot, what are the effects of changing its value, what are the recommended values, etc. I've seen this configuration in many places but I'm not sure how it works. Thank you.

12th May 2009, 12:13 PM	#2
SY diyAudio Moderator Join Date: Oct 2002 Location: Austin, TX Blog Entries: 1	That diagram looks wrong. Generally, a resistor connected in series with the grid is put there for RF suppression or to prevent oscillation. It's known as a "grid-stopper." __________________ “Listening to records is like ****ing a picture of Brigitte Bardot.” - Sergiu Celibidache

12th May 2009, 03:25 PM	#6
Wavebourn diyAudio Member Join Date: Sep 2006 Location: Pleasant Hill, CA	You can assume a loop between cathode and the ground of the pot. If it's resistance is low it's inductance may cause oscillations. I saw such a problem in Ampeg bass guitar tube amps, it caused HF oscillations. __________________ The devil is not so terrible as his mathematical model! Wavebourn: We Create Creativity!

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12th May 2009, 01:36 PM	#3
azazello diyAudio Member Join Date: Aug 2008 Location: Bulgaria	1 kohm.

12th May 2009, 01:52 PM	#4
ray_moth diyAudio Moderator Emeritus Join Date: Jan 2004 Location: Jakarta	Gottlieb, As SY said, that diagram is wrong. There HAS to be a path to ground from the grid. That o.1uF capacitor is preventing there from being such a path. The tube will be unbiased and will take excessive plate current,

12th May 2009, 03:01 PM	#5
gottlieb diyAudio Member Join Date: May 2009	Sorry, my mistake. Here's the complete circuit. It's a cathode follower configuration. I more or less understand everything except the resistor in red.

13th May 2009, 08:51 AM	#7
gottlieb diyAudio Member Join Date: May 2009	Thanks Wavebourn. Is there a remedy for that problem you describe? Thank you.

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