pots vs. resistors

[tubenit]

I have observed at times, that pots may induce hum at certain settings. For example,  between 7.5 - 9.5 there may be some hum. Below 7.5 or at 10 there is no hum.

I am wondering if fixed resistors emulating those same exact settings would still induce hum or is it something that is intrinsic in the nature of potentiometers.

Haven't had time to try this out for myself for a comparison and thought I'd ask if anyone here knew the answer?

With respect, Tubenit

[kagliostro]

I think that pot will be less prone to catch noise than usual resistor

this because pot casing acts like a shield and also usually we connect the casing to ground for this purpose

the noise you note setting the level 7.5 or higher (to me) is already present in the circuit, only at lower levels you didn't hear it

K

[Willabe]

I have no idea if this applies or of if it's got some truth to it but IIRC....

Ken Fisher wrote that you shouldn't leave the wiper of a pot with a cc resistive track set in any position other than fully on/off when the amp is not in use. He said the wiper will cause the R track to absorb humidity and will become noisy around the spot you leave the pots wiper set.


                     Brad        

[Ed_Chambley]

Neither should induce hum unless it is noise from a CC resistor or CC pot.  Like in a princeton.  When adding a mid pot and replacing the resistor on the bass and wiring to a pot, I have never noticed an increase in hum.  However, I will say that I have ended the use of Alpha pots as I am snakebit with them.  I found a great and cheap source for Allen Bradley NOS pots and I use PEC pots and some Bourns.

Interesting thought of the metal wiper causing humidity which can transfer to the CC track.

[Colas LeGrippa]

I have exactly the same problem with a Mesa/Boogie Caliber 50 on my bench, now. The volume between 7 and 9 induces white noise, wherever is set the master volume to. Under 7, no prob, at 10 no problem either.

Colas

[jojokeo]

My opinion:
What you are likely observing is humm induced oscillation or parasitic, that for this part of your circuit has become "the optimized bandwidth and/or the maximum gain within a frequency range" created by the pot's divider network which is frequency dependant through this range or setting. I hope I'm saying this clearly enough to get my thought accross?

Also, when you're using pots on the faceplate, their location is at a (sometimes great) distance away from the circuit board & components which then extends grid and plate wiring which then potentially creates issues where the wiring can pickup stray currents and induced noise much easier (especially if not shielded w/ longer runs). This will also affect to a greater extent or sensitivity to what I said in the earlier paragraph.

So there's several factors to consider regarding the pot's role for the given part of the circuit you're using it in. This is where layout design and lead dress becomes very important to minimize any of these effects. It can be affected by tone control settings at times but not always. Wiring also has capacitance. Capacitance together with pot's resistances create varying high & low pass circuits which create inherent varying frequency responses. Coupled with the tubes gain and frequency production and you can see why this phenomenon occurs. The controls of highest probability (or noticeable) with this are the gain and volume pots since the frequency dependant parts of the circuits created by these resistor/capacitor networks are amplified directly by them until heard or they negatively affect the circuit response. But this can also be out of our auditory range and show up as other symptoms like a decrease in or very little sustain for example since the note's upper frequencies (harmonics) are being canceled out.

You then have other parts of the circuit/amplifier which can affect the overall performance and/or earlier stages via coupling, induction, or reactances such as negative feedback networks, proximity to heater, B+, plate, cathode, and output transormer wiring.

[tubenit]

What you are likely observing is humm induced oscillation or parasitic, that for this part of your circuit has become "the optimized bandwidth and/or the maximum gain within a frequency range" created by the pot's divider network which is frequency dependant through this range or setting. I hope I'm saying this clearly enough to get my thought accross?

Also, when you're using pots on the faceplate, their location is at a (sometimes great) distance away from the circuit board & components which then extends grid and plate wiring which then potentially creates issues where the wiring can pickup stray currents and induced noise much easier (especially if not shielded w/ longer runs). This will also affect to a greater extent or sensitivity to what I said in the earlier paragraph.

So there's several factors to consider regarding the pot's role for the given part of the circuit you're using it in. This is where layout design and lead dress becomes very important to minimize any of these effects. The areas of highest probability with this is with the bass, treble, gain, and volume pots since the frequency dependant parts of the circuits are created by the resistor/capacitor networks. Wiring also has capacitance. These are by nature varying high & low pass circuits which create inherent varying frequency responses. Coupled with the tubes gain and frequency production and you can see why this phenomenon occurs.

Leave EVERYTHING the same ..........................   now simply substitute two resistors on the same exact place on the faceplate. Will the resistors not induce hum with the same settings as the pot does?   Pot  50k to ground/ 200k   VS.    50k resistor/200k resistor.

With respect, Tubenit

[jojokeo]

Yes, I think you should have the same gain at that exact frequency response...for better or worse. The circuit should behave and perform the same whether it's two resistors or a pot - all else being equal.

I think that pot will be less prone to catch noise than usual resistor this because pot casing acts like a shield and also usually we connect the casing to ground for this purpose

All the pot is shielding is a virtual connection or splice (being the pot itself) and does nothing for shielding purposes sake of the entire wire run. If the wiring leading to and from the panel is not also shielded, then it is not serving any practical shielding function at all. If you think about it, the point of shielding wire inside the amp is to protect the wiring from interferences from the other components within circuit itself. Not the amp as a whole. The entire chassis serves the purpose of shielding the entire amp's circuit, similar to an oversized effects pedal. :)

Ken Fisher wrote that you shouldn't leave the wiper of a pot with a cc resistive track set in any position other than fully on/off when the amp is not in use. He said the wiper will cause the R track to absorb humidity and will become noisy around the spot you leave the pots wiper set.

All this means is that this helps to be one of the contributing factors in the creation of "scratchy" pots necessitating the use of contact cleaner or if left in this position long enough, to cause a "corrosion" or premature wear point on the wiper track. It has nothing to do with contributing to oscillation frequency dependancy points though.

*I do like to be aware of this when I'm soldering my pots whether for use in an amp, guitar, or effect pedal. I turn all of the pots to the "OFF" position especially when I'm soldering the middle lug or the casings when it really heats them up the most. But a small file w/ a little dab of paste lessens contact time significantly.

[PRR]

> pots may induce hum at certain settings

Hum induces itself. There's relatively massive AC fields in most rooms, relative to circuit signal levels.

> pots may induce hum at certain settings

In most cases, when the pot is half-way _electrically_.

That is, when resistance from wiper to ends is maximum.

> between 7.5 - 9.5

Audio-taper pot.

First take a linear-taper pot. Short the two end-lugs together (to emulate the usual connection with one end grounded and the other end fed from a lower impedance source). Measure ohms from center lug (wiper) to the ends. Turn the pot.

For a 1Meg Linear, you get zero at each end, 250K with knob in center.

If there is any hum in the room, the zero and low resistance wiper settings divert it better than the high resistance (centered wiper) setting.

Now try with a typical Audio taper pot. From 0 to 5, all about the same. Low resistance here to give you an extended range of lower gain. Most of the total resistance in in the 5 to 10 zone. The highest wiper resistance (still 250K for a 1Meg pot) is in the 7 to 8 range.

In a real amp the top lug is not shorted but taken from some impedance. 40K tube plate, 220K mix resistor. You can compute the worst-case wiper resistance; but in practice the worst hum is still half of total resistance (including source) which is now a little higher than 7 or 8. Say 8 or 9, even a bit over 9 with really large source resistance.

The "advantage" of fixed resistors is you can keep the total area of the high-impedance section small, without long wires and extended pot-track. (OTOH metal pots do give some shielding.) ALSO... since you can't diddle the fix-resistor your attention is not called to the varying hum level and a knob which makes it worse.

[tubeswell]

I had the opposite problem with a hummy vol pot once. The pot would hum at '0' or '10' but was quiet around 6 or 7. I eventually tracked the fault to a faulty input jack ground tip switch.