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Old 9th January 2005, 10:22 PM   #1
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Default help with input pot on el84 PP amp

Hello,
I added an Alps stepped dual 20k pot to the input of my PP EL84 amp today. Input signal to input of pot, ground of pot to ground of amp, output of pot to input stage of the amp (6CG7).
Of the 21 steps in the Alps pot, the volume is at max after the 5th click. Is there a way for me to change this so the pot is useful for all 21 clicks? Is a possible solution to make the pot a shunt to ground with a fixed resistor going to the input stage? If so, how would I go about determining the resistor value I would need? Thanks!

Mark
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Old 10th January 2005, 12:06 AM   #2
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What you've got there is a linear taper potentiometer, i.e., one whose divider ratio is linear with rotation. What you want/need is an audio taper potentiometer, one whose divider ratio is logarithmic with rotation. If you can arrange a swap with your supplier...

Probably not. You soldered to it, you own it. So, do the second-best thing and use a so-called law-faking resistor. This is a resistor tied between wiper and ground, sized to have the pot turn on more slowly. This does carry disadvantages, most notably a reduced input impedance. And you still might have to reduce your preamp gain to get the lower range to work OK- it's at best a poor approximation. For a 20K pot, use a 15-18K resistor.
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Old 10th January 2005, 12:17 AM   #3
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Quote:
Originally posted by SY
What you've got there is a linear taper potentiometer, i.e., one whose divider ratio is linear with rotation. What you want/need is an audio taper potentiometer, one whose divider ratio is logarithmic with rotation.
The pot is marked "20ka", which I thought for Alps was a log taper (audio)? Is that not correct?
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Old 10th January 2005, 12:29 AM   #4
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I don't know about Alps's marking scheme, but it's easy to verify with an ohmmeter. If it is indeed an audio taper pot, then you need to knock down the gain of the succeeding stage.
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Old 10th January 2005, 04:59 AM   #5
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usually "a" stands for linear and if there's a "b" it stands for logarithmic
so 20ka is 20 kohm, linear.
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