Master volume drop

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lentzell
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Master volume drop

Post by lentzell »

Hello all, been lurking around for a long time, this is my first question.
What I have is: 12AX7 anode to 47N/400V cap to 1meg master volume to 5k6 grid stopper to 6L6 grid.
The master volume works right up until about 8, but then the volume drops a little instead of going up more and stays that way. The pot seems to be OK checking with a multimeter.
Any ideas? Thanks in advance.
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Bieworm
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Re: Master volume drop

Post by Bieworm »

lentzell wrote:
Sun 03/06/22 3:55 pm
Hello all, been lurking around for a long time, this is my first question.
What I have is: 12AX7 anode to 47N/400V cap to 1meg master volume to 5k6 grid stopper to 6L6 grid.
The master volume works right up until about 8, but then the volume drops a little instead of going up more and stays that way. The pot seems to be OK checking with a multimeter.
Any ideas? Thanks in advance.
A schematic of the amp and on how you wired it would be convenient.
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JMPGuitars
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Re: Master volume drop

Post by JMPGuitars »

Did you check with a multimeter in or out of circuit? Do you see a resistance change after 8?
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lentzell
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Re: Master volume drop

Post by lentzell »

Thanks to both of you. I actually solved the problem (without finding out the cause) by adding a 82K resistor at the top of the potentiometer, so the wiper never reaches full power, if that makes any sense.
I measured resistance at the point when the volume dropped and it was around 220K from the wiper to the top (the "1" lug), so I started with that and the problem went away: the full range of the pot is usable. But then gradually reduced the value of this resistor and it still worked. I stopped at 82K, I guess there's no point in making even smaller though I believe it would still work.
So that's that. Mind you, I'm not a newbie (well, I am a bit regarding tube amps) as I've been an electronics engineer for more than 40 years. Always learning.
Again, thanks. English is not my main language, you may have guessed.
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Bieworm
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Re: Master volume drop

Post by Bieworm »

'Might' be solved... but you might also have a hidden oscillation going on, which can damage the output transformer. Always solve problems at the source..
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