Noisy pots

[bsco]

Just a basic question......Noisy pots....usually caused by excessive were and tear.....right?? especially in older amps from the 60's and 70's.....After replacing the volume pot, I did notice that when turning the control, there is scratchy noise between 7 and 9 on the dial.....the rest of the pot travel is fine....the original pot was extremely bad.........so am I still dealing with a bad pot even though it is new......or do I have another contender in the ring.....
Cheers

[Leo_Gnardo]

Two main causes: worn track and/or DC on the pot. Even a little bit of DC, say 30 millivolts or so, can make a pot sound scratchy and awful. So make sure you don't have any DC first. Sometimes I dissect an old pot and can make it work better by carefully cleaning the track and conductor ring, a drop of Caig D-5 may help, but often that's a waste of time and replacement is in order.

I always love hearing from customers who think their pots are "dusty", like they've got dust bunnies stuck in 'em. "My pots sound dusty, spray 'em out, OK?"

[bsco]

Two main causes: worn track and/or DC on the pot. Even a little bit of DC, say 30 millivolts or so, can make a pot sound scratchy and awful. So make sure you don't have any DC first. Sometimes I dissect an old pot and can make it work better by carefully cleaning the track and conductor ring, a drop of Caig D-5 may help, but often that's a waste of time and replacement is in order.

I always love hearing from customers who think their pots are "dusty", like they've got dust bunnies stuck in 'em. "My pots sound dusty, spray 'em out, OK?"

Thanks Leo...This pot is new actually.....and only has noise between the 7 and 9 setting....so even if there is say 5 or 10 mV there it would cause noise when the shaft was turned?? SO ......no dc whatsoever..... I thought if there was dc there the pot would be noisy all the way through the rotation......

[Mick Bailey]

I've noticed quite a few pots are noisy from new - even CTS. Pretty annoying.

There's been the odd time I've been trying to track down a noise problem where there's no DC and no problem with the pot. I have a TL Audio 4-channel tube preamp right now where it scratches on every channel at exactly the same part of the pot rotation (about 8), but only if the pot is rotated at a certain speed. No DC and all pots good. Replaced one as a test, with the same results. 4 separate independant channels with identical 'faults' on each pot?

[Dave H]

I have a TL Audio 4-channel tube preamp right now where it scratches on every channel at exactly the same part of the pot rotation

Is it a 'log' pot made up of linear sections? Perhaps it is noisy at the junction of two sections which could explain why it is always at the same rotation.

[Mick Bailey]

Good point. They're 16mm A taper Alpha pots - pretty low end for an otherwise decent piece of equipment. A check would be to sub in a linear pot.

[J M Fahey]

Probably there is DC there ... just not the kind you expect to find, I mean often ignored grid leak current.

I find such problems in amps where the pot feeds straight the next tube grid , and is the only ground path return / reference for it.

That makes the slightest contact resistance irregularity way more noticeable.

Just to make sure:
* disconnect whatever is feeding that pot, simply to clear the field from any external influence ... still scratches?
* connect a 1M resistor from grid to where the cathode resistor is grounded .... still scratches?

Just for kicks, consider the pot dead anyway, inject some thick grease so besides lubrication, it mechanically damps the wiper sliding over the track.
I mean the old thick grease which was used on turret type TV tuners, some British equivalent of:
Electroquímica Delta - Tungrease®
Obsolete today, but some shops still carry it.

[bsco]

Thanks Mick and J M.......assuming it is dc, wouldn't the pot be noisy through the entire rotation? This one is noisy in one particular spot.....and it is brand new........an Alpha pot I believe....

[Dave H]

I've too much time on my hands. There were a few Alpha log pots out on the bench so I measured one. The x axis corresponds to the numbers on the Strat knob I was using.

[J M Fahey]

Dave H, thanks for taking the time to measure and post

That Alpha log pot is a perfect example:
* resistance on "5" is 10% of total resistance
* not true Log all the range (is possible, of course, but very expensive) but 2 linear tracks joined at the middle.
Works well enough in practice and is inexpensive.

[bsco]

Probably there is DC there ... just not the kind you expect to find, I mean often ignored grid leak current.

I find such problems in amps where the pot feeds straight the next tube grid , and is the only ground path return / reference for it.

That makes the slightest contact resistance irregularity way more noticeable.




Just to make sure:
* disconnect whatever is feeding that pot, simply to clear the field from any external influence ... still scratches?
* connect a 1M resistor from grid to where the cathode resistor is grounded .... still scratches?

Just for kicks, consider the pot dead anyway, inject some thick grease so besides lubrication, it mechanically damps the wiper sliding over the track.
I mean the old thick grease which was used on turret type TV tuners, some British equivalent of:
Electroquímica Delta - Tungrease®
Obsolete today, but some shops still carry it.

Ok. Checked that out.....Channel 1 volume control disconnected on the high side.....scratchy between 7 and 9.....1M ohm resistor connected to grid and cathode cap/resistor ground.....scratchy between 7 and 9...... same thing on channel two....

.I cleaned them again with a lubercating contact cleaner and DeOxit....no change....I injected a small amount of vasiline for lubercation and no change......I had ordered these pots new a couple of weeks ago...I took a couple of new ones and checked them on an analog meter...when the pot is in the uppermost range of its travel, and with the meter set to X1000, if you slightly wiggle the shaft of the pot, the needle will start to move downward...I tested two new ones and same thing......

so it is possible I have bad pots...all six of them....Just a note here...when playing a guitar and adjusting the controls you can't hear the scratchy noise...it is only when no signal is present and only between 7 and 9 on both pots.....

Subbed in a 500k linear pot......no scratchy sounds.....so I have a half dozen pots that came in from Antique Electronic Supply that appear to be bad.......well.......will have to go to the local parts store here tomorrow to see if the have something......even if I have to cut the shaft to the right length....as long as it is 1M audio taper and 24 mm in size.........
Thanks for the help guys......
Cheers

[J M Fahey]

Ok, thanks for the test.
Yes, it proves the pots bad, all of them and certainly the whole batch (which may easily be up to 1000 pots, or even more).

FWIW I had custom pots made, that is when we still had an Industry, and saw the process firsthand.
Even helped assemble some of mine once I was in a hurry

They take long strips of Pertinax (brown phenolic paper) and coat it with rollers soaked in resistive paint, basically half cured phenolic varnish mixed with very finely ground graphite.
The layer is very thin and even.
Then they cook it in a special oven.
Linear pots get a single uniform layer, Audio (or reverse Audio, it's the same) get a second half width coat of 1/10th resistivity paint, which gets a second pass through the oven.

Then they simply punch out the omega shaped strips which will become the tracks, punch the rivet holes and attach legs.

I guess your bad batch of pots got improperly coated between 7 and 8 , maybe the Pertinax was not uniform or there was a problem with the coating roller , and in that area very little or very much paint was applied.

Really they *should* replace those pots, it's a clear case of "Manufacturing defect" , also Alpha should be warned about it.

Almost forgot: pots as described hardly reach real "0" ohms at the ends, because there is always a little carbon between wiper and terminals, so it's common to spray/print some "silver paint" around rivets so wiper touches them at the ends of rotation.

It's a very simple part but, as they say, "the devil is in the details"

[bsco]

Ok, thanks for the test.
Yes, it proves the pots bad, all of them and certainly the whole batch (which may easily be up to 1000 pots, or even more).

FWIW I had custom pots made, that is when we still had an Industry, and saw the process firsthand.
Even helped assemble some of mine once I was in a hurry

They take long strips of Pertinax (brown phenolic paper) and coat it with rollers soaked in resistive paint, basically half cured phenolic varnish mixed with very finely ground graphite.
The layer is very thin and even.
Then they cook it in a special oven.
Linear pots get a single uniform layer, Audio (or reverse Audio, it's the same) get a second half width coat of 1/10th resistivity paint, which gets a second pass through the oven.

Then they simply punch out the omega shaped strips which will become the tracks, punch the rivet holes and attach legs.

I guess your bad batch of pots got improperly coated between 7 and 8 , maybe the Pertinax was not uniform or there was a problem with the coating roller , and in that area very little or very much paint was applied.

Really they *should* replace those pots, it's a clear case of "Manufacturing defect" , also Alpha should be warned about it.

Almost forgot: pots as described hardly reach real "0" ohms at the ends, because there is always a little carbon between wiper and terminals, so it's common to spray/print some "silver paint" around rivets so wiper touches them at the ends of rotation.

It's a very simple part but, as they say, "the devil is in the details"

The cost for me to ship six pots back to the US isn't worth the trouble.....However I don't think I will be buying any pots from this company again.....I will however send them a quick email to let them know........
Cheers,
Bernie

[The Dude]

The cost for me to ship six pots back to the US isn't worth the trouble.....However I don't think I will be buying any pots from this company again.....I will however send them a quick email to let them know........
Cheers,
Bernie

Yes, Bernie, definitely contact them. Often times with my vendors, they will issue a refund and not even ask for the defective parts back. Regardless, you shouldn't have to pay to send them back anyway. It's not your fault. On the rare occasion this happens to me, I simply ask for a refund and tell the company that they can send my a prepaid shipping label if they want their parts back. On something this small, they probably won't mess with it. The cost of shipping probably exceeds the parts cost.

[Justin Thomas]

+1!

Usually if I get bad or wrong parts, they send me new parts, free shipping. On a couple occasions, they send me the wrong tube, and say, "keep both!" Usually if you're respectful and not whiny, they're much happier to keep a customer happy, especially if you have a history of working together in the past. And by notifying them, you could be saving them oodles of $$$ in other dissatisfied customers who aren't quite as polite.

Justin