replacing a pot

[Pelliott321]

I have a power amp I am getting ready to sell that has a volume control on the input.
Originally I removed it and replaced the input cable in hopes of a sonic improvement.
To get it ready to sell I want to return it to stock and discovered that the original pot is really poor quality and the two channels are not even close to being equal.
How to I know whether this is a linear or audio taper or does it matter. It is marked 250k pot. one side is 200k the other 220k

[Roscoe Primrose]

Volume controls are almost universally audio taper.

Roscoe

[SoundMods]

I have a power amp I am getting ready to sell that has a volume control on the input.
Originally I removed it and replaced the input cable in hopes of a sonic improvement.
To get it ready to sell I want to return it to stock and discovered that the original pot is really poor quality and the two channels are not even close to being equal.
How to I know whether this is a linear or audio taper or does it matter. It is marked 250k pot. one side is 200k the other 220k

Typically volume pots for audio have an audio taper (Logarithmic) as opposed to linear so the volume adjustment is gradual rather than abrupt.
Here is a nice one for only $10.00.
https://www.ebay.com/itm/284294288300?_ ... BMztaX8MRf

[tomp]

Make sure the shaft diameter and mounting hole are the same size. If the shaft is longer you can always cut it to length.

[Pelliott321]

Yes Tom this will be a problem
I have to keep the original knob because there is a matching knob on the adjustable feedback control. These are very nice brass knobs but use friction fit. I have to decide should just leave this alone and just rewire the original control and let the next owner deal with it

[SoundMods]

Yes Tom this will be a problem
I have to keep the original knob because there is a matching knob on the adjustable feedback control. These are very nice brass knobs but use friction fit. I have to decide should just leave this alone and just rewire the original control and let the next owner deal with it

Like this one?
https://www.ebay.com/itm/292388446563?h ... SwUv5bWJlS

[David McGown]

Paul,

Chiming in here. The measured value across each of the two pot sections are not critical, this basically defines the input impedance of the amp (the resistance the input is working into) and the difference between 200K and 220K is negligible. What is more important is section-to-section tracking. A pot is a voltage divider, so the important thing is the resistance at the wiper in reference to the full pot value for each section. The %full value should be similar between each section (good tracking). Because this is a log (audio) taper, I would read the absolute values at the wiper on both sections at various positions, then perform the following calculation to convert it to dB attenuation:

Attn = 20 * log (Rw/Rt), where Rw = resistance at the wiper and Rt = full value resistance.

Set up a spreadsheet to record the values at each knob position.

Compare the results of both sections to see how well they track. If they are off by fractions of dB, I would not sweat it, they track well.

Now, if a pot is noisy, then I would use a fader cleaner/lube like DeOxit F5 or similar. That should restore a pot to like new condition.

David

P.S. Pots are going to track poorly at the low level, so do not be surprised if the tracking is decent through most of the range and really poor below 9:00 position.

[Pelliott321]

Ok thanks David

[DaveR]

When you listen to the amp is it obvious that the channels are not at the correct levels relative to each other? If not, shy not just sell the amp and don't worry about it? I'd be surprised if you get more money for it by putting in the effort to do the upgrade.

[Cogito]

Paul,
P.S. Pots are going to track poorly at the low level, so do not be surprised if the tracking is decent through most of the range and really poor below 9:00 position.

Rosenblitz says 95% of the signal is burned off when the volume control is at 9 O'Clock position. Could that be the reason for poor quality at low levels?