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Old 12th May 2009, 12:00 PM   #1
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Default 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.

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Old 12th May 2009, 12:13 PM   #2
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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."
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Old 12th May 2009, 01:36 PM   #3
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Old 12th May 2009, 01:52 PM   #4
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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,
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Old 12th May 2009, 03:01 PM   #5
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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.

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Old 12th May 2009, 03:25 PM   #6
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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.
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Old 13th May 2009, 08:51 AM   #7
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Thanks Wavebourn.

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