Potentiometer taper

[mlp-mx6]

Hi folks,

I started a discussion about this on Ampage, but wanted to ask specifically about the ODS schematics. In the diagrams we have, are the "A" pots log and the "B" pots linear?

Also - has anyone built an ODS using the fancy PEC pots? (or AB Type J?)

Thanks,
mp

[mlp-mx6]

Anyone?

[Normster]

are the "A" pots log and the "B" pots linear?

That's the way I interpret it. I ordered some PECs from MarVac but haven't received them yet. I'll let you know how the "feel."

[Robert]

I used the PEC pots from Digikey, ~$9-10 ea, for my express build. It was my first amp and I wanted no compromises. My gain control on the Express actually makes a grainy, static like sound when I turn the pot. Could be the pot, the circuit or the builder. Disappointing anyway! Since its a sealed pot theres no way to spray it with a cleaner. However, for my Dumble I used cheap Alpha pots from Mouser, ~$2 ea. At that price I can play with different values, tapers and throw away defects. In a blind "feel" test turning the knobs, I think there would be little if any perceived difference.

[glasman]

Hi folks,

I started a discussion about this on Ampage, but wanted to ask specifically about the ODS schematics. In the diagrams we have, are the "A" pots log and the "B" pots linear?

Also - has anyone built an ODS using the fancy PEC pots? (or AB Type J?)

Thanks,
mp

The answer is yes, if you have any Alphas laying around you will see that the linear pots are marks with a B before the value.

This is an industry convention, if you do a search on potentiometer taper you should find a few detailed lists that describes the various tapers and designators.

Gary

[glasman]

I used the PEC pots from Digikey, ~$9-10 ea, for my express build. It was my first amp and I wanted no compromises. My gain control on the Express actually makes a grainy, static like sound when I turn the pot. Could be the pot, the circuit or the builder. Disappointing anyway! Since its a sealed pot theres no way to spray it with a cleaner. However, for my Dumble I used cheap Alpha pots from Mouser, ~$2 ea. At that price I can play with different values, tapers and throw away defects. In a blind "feel" test turning the knobs, I think there would be little if any perceived difference.

The alpha's are not terribly bad. But make sure you measure and bin out your pots. Alphas can be WAY off as far as value is concerned and this will effect the performance and tone of the amp. Especially in the OD section.

Gary

[ayan]

I used the PEC pots from Digikey, ~$9-10 ea, for my express build. It was my first amp and I wanted no compromises. My gain control on the Express actually makes a grainy, static like sound when I turn the pot. Could be the pot, the circuit or the builder. Disappointing anyway! Since its a sealed pot theres no way to spray it with a cleaner. However, for my Dumble I used cheap Alpha pots from Mouser, ~$2 ea. At that price I can play with different values, tapers and throw away defects. In a blind "feel" test turning the knobs, I think there would be little if any perceived difference.

Robert, it may not necessarily be the pot. It could be the tube leaking DC to the grid. Try another tube in that position and see.

Gil

[mlp-mx6]

I was going to say the same thing - DC on a pot = scratchy sound when turned (only when turned). Check to see if there is DC on the pot and you'll know if that is the cause of the scratchiness.

[pedro]

Ive had this too....

DC on the pot will cause the scratching - just like an old Rangemaster ( it was a feature ! )

On a couple of my builds it turned out to be a leaky coupling cap feeding the control in question.

check any cap feeding your pot and swap it for another - 9/10 times this cures the problem.

cheers
Pedro