OK, Bourns pot issue, maybe others too!

[rooster]

Just a IMO headsup to anyone listening to their amp. I am believing now that the material that is used for the carbon path in the Bourns pots, the ones that sell for $2-3, it is wrecking the tone of an amp.

And here's a simple experiment for those who have the interest. If you have an amp that has one of these pots in it, and your treble control pot is a 250K -1 meg value, then pull this pot and replace it with either a vintage version from the 'day', or a PEC pot. Then listen to the difference. I know I myself bought a boatload of these Bourns pots and have now stopped using them completely.

And today, so you understand my statements better, I will tell you a tale...

Years ago I built an amp, Plexi like for lack of a better comparison. I had replaced the volume pots with PECS some time ago, and before that I was using some newer version of a CTS pot in the amp - but I had tried the Weber pots, etc, everything I could find that was a new type pot.. The PEC pot improved the amp right off, BTW. And then I lived with it but pretty much put it in the 'way' back line for gigs, hardly using it. So then somebody expressed an interest in an amp like this and I thought, heck, I am in love with my Express so I could maybe sell it. ....And I dug into things trying to figure out where and what exactly it was that I didn't like about this amp. I went through everything, changing values, revoicing things a bit, etc. But eventually I ran out of ideas and yet there still remained a kind of edginess to the treble part of the amp. (And yes, I had added a 'cut' control by now, which helped a bit but didn't really get to the sound I was hearing.) ...And then I thought, screw it, ...and next started going through my pulled pots, vintage ones that I had replaced in amps over the years - and trust me - I rework most of them when I can for the customer. OK, so I found a red plastic shafted CTS pot that came out of a 70's Fender amp I think, something that used push on knobs (!!!), 1MA, that I could use. I cleaned it up and put it in the amp. WOW, there it was! The amp was transformed just like that and I am NOT exaggerating! And I can tell you that here, in this pot I used, the carbon trace is huge and thick, very unlike what is used in the Bourns, Weber, etc., that you see today.

Further, remember that in the case of a treble pot, don't forget that typically the amplified guitar's voice travels through it - it is definitely not like a bass pot or a mid pot in this regard.

So you might want to think about this if you have an edgy amp or something that doesn't sound quite right. When you buy the PEC pot, you get good carbon, just like in the day. When you buy something modern this is probably not the case.

[gearhead]

Plus Mil-Spec pots are sealed.

[Cliff Schecht]

Ohms are Ohms. If there is a tonal difference, it's not from the resistance itself. Perhaps the thicker carbon elements add another form of parasitics, although I can't name what it would be. Put them on an LCR meter and take some measurements (or I can do it when I get my box of pots out of storage).

[rooster]

Cliff - Ha, you are killin me here! But clearly I disagree. This is like saying that capacitance is capcitance. And here I don't expect to change your mind, but only to say that, IMO, like a particular capacitor CAN change the quality of the audio signal passing through it, I also think a particular resistive compound/element/ingredient can do the same thing.

And to qualify my focus, it would be the volume pots and the treble pot of most guitars amps I can think of. Which it to say that,IMO, if the pot's resistance is in the signal path, it is affecting the quality of the signal.

[PlinytheWelder]

I used PEC pots in my build and other than the shafts sticking way out, they've been stellar.

[VacuumVoodoo]

It is an inherent property of classic tone stack that each potentiometers absolute resistance value affects frequency response. This in contrast to James/Baxandall T/B tone controls where potentiometers are used in true potentiometric mode i.e. frequency response is defined by the ratio of resistances between wiper and end points.
I would suggest to measure and compare resistance of these different potentiometers.
To illustrate: I recently replaced a 1M audio potentiometer with a new Alpha pot. This changed the tone considerably. Secret property of the carbon track? No, the new pot measured only 830k - still within 20% spec - and midpoint was at 15% of resistance not 10% like the old pot. Occams razor. Look for simplest answers, they are usually the right ones.

[Structo]

Interesting Rooster.
Don't PEC pots use a conductive plastic trace verses carbon?

I've been trying to track down some high end hash on my D'lite for ever it seems.
I used all Alpha pots so I don't know if that could be the problem, but I could certainly sub in another treble pot to see if it helps.

Glad you gave me a heads up on the Bourns.
I was contemplating replacing all my Alpha's with Bourns after the recent taper thread.

It stands to reason that the treble pot does in fact affect the tone.
Just like the pots in guitars do, you are running a lot of signal through that trace depending where the wiper is on it.

Just to be clear, do you think it was this particular pot or have you tried other Bourns pots that you think sounded bad as well?

[Cliff Schecht]

Capacitance is capacitance as well. The dielectric betwixt the plates of a capacitor, the shape and size of the plates and the distance of the plates all of course makes a difference in sound. But you aren't measuring any of that when you put a cap on a capacitance meter, you are measuring capacitance!

[PlinytheWelder]

Interesting Rooster.
Don't PEC pots use a conductive plastic trace verses carbon?

The ones I use are the RV4 carbon available from Digi-Key and others. They
are really nice pots, but I wouldn't try to solder onto the backs.

http://dkc3.digikey.com/PDF/US2010/2046-2047.pdf

[rooster]

Alek, et all - Well, on one level I get that I should not be hearing this kind of change in the sound of the amp. I also found the pots to be very close in taper and overall value, 1.02M vs. 1.01M. Which leaves me with my conclusion, its the carbon path.

And since the treble pot is so crucial to the Express tone stack in that it passes guitar signal, and the fact that the 'edgy' treble sound I was hearing went away, well, I am done investigating. It may be that this 'edgy' sound is nothing anybody else can hear - except that by now I returned the amp to the potential customer for another shootout vs. his amps and my amp was easily the the best amp in the room. (Before this, on the first visit, it was at best a draw, and certainly not a 'point of sale' experience. ) Eh, and frankly, I am not at all clear now that I even want to sell the amp! Funny how that goes...

So, is this science? I think on some level it is, but for now I am just reporting something that I think very interesting. The bottom line for me is tone, feel, and using an amp that inspires you to play. I am suggesting that if any of you built a TW inspired amp and it fails to reward you with these things, considering strongly the idea of changing ANY pot that passes signal to something that KF used in the first place. Do any of you actually think Ken would be using a $3 Bourns/Weber/whatever pot in his amps?

[FYL]

So, is this science?

It's not. It could be if you did proper double blind tests.

Do any of you actually think Ken would be using a $3 Bourns/Weber/whatever pot in his amps?

KF used whatever parts were available and could build an outstanding sounding amp using what was in stock at RatShack or what was left in his parts bins - vide the Songwriter or the DLM for instance.

[JamesHealey]

FYL, I really like Ken's approach to things in that respect.

When I have a customer come to me with an amplifer in mind we start from stratch and I use what's available to me and what I have in stock I'm able to usually find something great sounding by the end of it.

I build handwired amps for other companies here in the UK and this gives me access to good price parts if/when needed.

[rooster]

OK, yes, I get that Ken could do some mighty mighty things with what he had on hand. ...But then look at the pots that were available at the time he started building amps - since I am focusing on just this part of the amp right now. They didn't even make pots from the materials they are using today, especially the cheaper ones. And then look at the Komet amp line, for that matter, all PEC with big carbon traces. He probably had a voice in this matter I would imagine? Eh, but maybe not...

So use whatever you like, sure, and make whatever adjustments you can to tune the circuit - you might get to something interesting and new.

On the other hand, since a lot of us here are copying a KF circuit, maybe the case can be made that - without the correct KF type pot - we are not going to succeed. Thinking of Glen and his clones, I find it interesting that he went to a lot of trouble to source the correct pots - and his amps sound a little different from each other, yes. But I wonder first: How 'bad' is 'different' in his case? And secondly: How much more different would they be if he used Bourns/whatever pots throughout? Now, for that matter, I would love to hear an original Express with the 'real' pots and then with the pots I am talking about, but this ain't gonna happen. Naw, who would donate that test amp? Glen?

[RJ Guitars]

I bought some Bourns pots when they showed up at a discount electronics place a couple years back. They seemed ideal because they had a long shank on them which allowed you to use a thick (1/8") wood faceplate and still have enough threads to allow you to get a nut on them. However out of the first ten I bought there were two that had dead spots on them. It must have been an internal shorted spot because the volume died right in the middle of the travel. Two out of ten was unacceptable to me and after I made an amp using five of them I threw away the two bad ones and gave away the other three. Could have been a fluke but I'll probably always avoid them. For the record, the amp I did build with 5 good working Bourns pots does everything it should and sounds dandy to me.

If we think about how most control pots work in guitars and amplifiers, they throw away some part of the signal we don't want. Does it make sense that if you have all the pots at zero resistance they add or subtract nothing to the signal? Now that might be unpleasant and unuseable but it should tell you a little something about what a pot is doing? There may be flaws in this theory but ultimately it seems reasonable that if a pot is doing bad things to your tone then those things should be less bad as the turn the resistance down...

I do like the thought that resistance is resistance, capacitiance is capacitance, inductance is inductance, etc.. There is no such thing as Mojo, but there are many things that are difficult to quantify and it's just a lot easier to refer to them as Mojo. Everything in a tube amp is constrained to the fundamental laws of physics and it is usually only our abiltiy to accurately measure things that limits our ability to understand and reproduce them.

I would agree that one pot could do something different to the sound of an amp than another might, but what I really want in addition to that piece of info is to know "why" so I can use that to accomplish good things and avoid bad things in the next amp(s) I build.

rj

[M Fowler]

I have drawers full of Bourns pots from Apexjr and have not had many that were bad. Therefore, I still have a bunch of Bourns pots to use up.

I have had one bad PEC I paid $10 for it and many of the shorter Alpha's that had spots that cut out as you turn the wiper, not soldered to either. No the business I got it from would not replace it either.

I started using Mojo's CTS and Mojo's custom Alpha pots 3/8 inch made to match the CTS because I find them smoother than the Bourns and better quality.