"bad" potentiometer

[tubenit]

Occasionally, we have someone (including myself) bring up the issue of a hum or buzz or some other unwanted noise. So here is my story.

I had already gotten the D'Mars OD Special quieter than the original Princeton Reverb I owned even with the OD engaged.  Having said that, I could hear a difference in floor noise between the OD (normally closed) and clean (normally open) with the OD noisier of those two.

On a scale of 0 to 10 (regarding floor noise), with 0 being the amp isn't turned on at all (dead quiet turned off) .......................  And anything above "5" is totally unacceptable floor noise.

The clean channel was 1.2 even with the pots set for a very loud volume.  
The OD channel was probably about a 2.5 to 3 with same pot settings. It was still a quiet amp but had more noise on OD.

I narrowed the "problem" down to the OD drive pot. I tested all continuity to and from pot and everything was OK. It was difficult for me to envision that the pot could be inducing the hum?

So, I decided to try another pot, and the issue is resolved.  I can NOT hear any difference in floor noise between the clean and OD channel. The OD channel is also a 1.2 in floor noise now. Again, the is with the pots set at a volume louder than I normally play at.

I wanted to share this because I have not really ever had a "noisy/bad" pot before.  I don't know if the pot was overheated at some point like when putting the buss wire on?  or ........ if a  piece of wire or solder dropped inside it or what?   But the pot induced some slight hum.

I hope someone else may benefit from this discovery.   With respect, Tubenit

[stratele52]

Thank's to share that.

But I wonder if your problem was not that the pot was rather badly ground. It is possible that the contact between the metal frame and the potentiometer is defective ( oxidation ) ?

Also, a bad solder on the potentiometer?

Because a potentiometer is shielded and could not theoretically induce hum.

Having replaced the potentiometer solves this problem because you redo the solder and ground with the chassis be repaired with this new installation.

What is you opinion ?

[tubenit]

No, I've worked on amps enough to know a good solder joint flow. They looked great and they tested perfect continuity. I'm confident that was not the issue.  Plus I had reflowed them a 2nd time prior to switching out pots and it made zero difference in noise.

With respect, Tubenit

[jojokeo]

In addition for pots to have mechanically caused noises, carbon elements, in particular, are prone to producing electrical noise. This noise is heard as a soft, steady hiss that can degrade sound recordings. The resistive materials have improved over the years, so newer pots are quieter than their ancestors. In a circuit, the ground serves as a reference point for other voltages, representing zero. It also serves as a return point for voltages in the circuit. An ungrounded or poorly grounded potentiometer can emit a residual, low-level hum that becomes worse when the ungrounded part of the circuit receives interference.

[kagliostro]

That is very strange to me

Many thanks for sharing the info

K

[tubenit]

An ungrounded or poorly grounded potentiometer can emit a residual, low-level hum that becomes worse when the ungrounded part of the circuit receives interference.

Totally agree with that. And maybe that was the case, but I honestly don't think so. The buss wire was soldered on so well that it would not budge at all pulling hard with pliars.  In fact, I had trouble getting it to give free even with the soldering iron & pulling it.  And the ground terminal had that very fluid shiney solder look & tested with good continuity with the buss wire.

I do think I may have overheated the pot soldering the buss wire on to it & perhaps that damaged something? 

Who knows?!  Anyway, replacing it worked fine ...... so I am happy with it.   

With respect, Tubenit

[jojokeo]

T - I wasn't referring to "your ground" just added it for general info for other's reading/knowledge. I was thinking more likely issue #1 - a bad or weak carbon track performing similar to an old carbon comp resistor.

I always take a small file or screwdriver edge and scrape the small pot area a bit, then add small dab of non-corrosive flux paste w/ a toothpick. Then I turn up the iron just a tad more and I barely have to touch the pots and it starts flowing smoothly. Lastly, I always turn my pots down/off completely when soldering them too.

[kagliostro]

One thing I would like do is to recover that pot, mark it and try to reuse in one other construction, looking to what will happen

K

[tubenit]

I always take a small file or screwdriver edge and scrape the small pot area a bit, then add small dab of non-corrosive flux paste w/ a toothpick. Then I turn up the iron just a tad more and I barely have to touch the pots and it starts flowing smoothly

Yeah, I typically use a dremel tool drum with 50-grit sandpaper. I etch it vertically and horizontally. No problem getting solder to stick to it.  The issue for me is how heavy/thick of a buss wire that I am using. I use copper wire like one would find inside the walls of a house.

Never thought about  turning the pot "off" prior to soldering.  Good tip.

With respect, Tubenit

[Willabe]

Never thought about  turning the pot "off" prior to soldering.

I put a small piece of blue painters tape over the pots opening (when I solder them up) that's just below it's solder tabs. I worrie about flux splater getting inside the pot onto it's resistive track.


                  Brad     

[EL34]

I put a small piece of blue painters tape over the pots opening (when I solder them up) that's just below it's solder tabs. I worrie about flux splater getting inside the pot onto it's resistive track.

This is exactly what most of the pot issues I have encountered have been.

Flux shoots off in all directions when soldering.
If it gets on the carbon trace and then the wiper spreads it out, it can cause problems

I used to spray some tuner cleaner into the pot first and move the wiper around
Then I soldered the pots with the amp face plate facing down (shafts pointing down vertically)

[tubenit]

I put a small piece of blue painters tape over the pots opening (when I solder them up) that's just below it's solder tabs. I worrie about flux splater getting inside the pot onto it's resistive track.

Brilliant idea!  Never thought of that before. Cool.

With respect, Tubenit

[Willabe]

Playing one of these          and eating crawfish etouffee on a regular basis helps to open the mind.


           Brad      

[mresistor]

Playing one of these          and eating crawfish etouffee on a regular basis helps to open the mind.


           Brad      

you mean like this.....  El Diamante - Ramon Ayala (Acordeon)

[Willabe]

Yes, a button accordion, not one with a keyboard.

But I was thinking cajun/creole type music. Their (diatonic) accordion only has 1 row of buttons/levers on the treble side. Hold down 1 lever, squeeze/push in the belows 1 note, same button/lever pull out different note. Only 2 bass buttons.

Sorry for the     


             Brad      

[Ed_Chambley]

Pots were a big problem for me.  I quit using cheap pots.  I also clamp a piece of wood an the inside of the chassis before drilling.  Front and back.  This does 2 things, 1 it keeps the burrs off the inside of the chassis and 2, makes a jig for soldering the pots buss wire.  I can then solder on the wood board and put the buss wire along the bottom.

[Colas LeGrippa]

I think that a pot shouldn't be grounded on itself but on a  terminal bolted solidly on the chassis with a lock nut / nylon nut. This assures a perfect ( or near ) resistance free ground to eliminate any hum.

Colas

[phsyconoodler]

I avoid using pots to attach ground busses for the most part,instead just running a ground buss wire above the pots and dangling the ground wires down to the pot terminals.

Here is pic of my latest Overdrive Rocket with it's ground buss.

[stratele52]

Very nice job psyconoodler