Making do with the wrong potentiometers

[Chipper Chapowski]

Hello there!
This is my first post at this place, in fact, its my first post anywhere so I'm very excited..... Oh yes....  

Thank you for this site Aron, its such a useful resource.  You get much respect from me for providing all this knowledge for free, off your own back, its great!  But anyway, the reason I have signed up for this insanity is that me and a mate are about to enter the world of building effects, see if we can do it, and then replace my entire rig with unique effects all made by us, and hopefully make a few bob along the way.

My problem is that being based in the UK, I can't use people like 'Mouser' and 'Smallbear' for components, and I am finding it difficult to find the potentiometer values that I need.  Maplin.co.uk seems to be quite good, but say I'm after a 250K pot, about the closest I can get is a 470K or an 100K

What effect will this have on the operation of my pedals, i.e. can I use them instead, or if not, how can I wire resistors either in parallel or series with the pots to bring them up/down to the correct value?  Thanks a lot dudes!

[RDV]

You can get a 250K at your local music store as it is very common in guitars. I've used a bunch of them. Just use guitar knobs on them also.

Regards

RDV

[Paul Marossy]

Welcome to Stomboxaholics Non-Anonymous... Hi, my name is Paul, and I'm an addict.  

Here's a trick: If you want a pot to be half of it's value, solder a resistor across the two outer lugs of the pot that is the same size as the pot.
So if you have a 100K pot and you want a 50K pot, solder a 47K or 50K resistor between the two outer lugs of the pot and you will have cut it's value approximately in half.

I have also used a pair of same value resistors in series with the two outside lugs of the pot to increase a pot's value. I did this to increase a 500 ohm pot to a 1K pot. It seems to work OK, but I'd much rather cut a pot value in half than try to increase it...

[Mark Hammer]

Welcome on board.

There are plenty of instances where having a pot of a different or "wrong" value will make little difference in the functioning of the device.  The listed value may well be a sot in the dark anyways, rather than anything meticulously worked out using formulae.  

Of course, the problem is that there are many instances where it *will* make a difference.  The tricky part is knowing when you're in the one circumstance, as opposed to the other.

Since a great many people here either see distortion boxes as their fundamental goal, or see such devices as a helpful way-station in their learning about effects design, we'll start with those.  

Here's one example where you can fudge a great deal.  In many instances, a pot is used to adjust the gain of something.  Quite often, that adjustment is made in the feedback loop or ground leg of an op-amp stage.  In the case of the MXR Distortion+, the DOD 250 Overdrive, and the Ross Distortion, a 500k to 1meg pot goes from the op-amp to ground and adjusts gain.  The smaller the actual resistance value of that pot, the higher the gain and the more distortion there is...in principle.

Bear in mind, though, that the final amount of distortion produced in these instances is aproduct of how much gain is created in the circuit, but also how hot a signal you feed it in the first place.  It may well be that a single-coil equipped instrument requires one to never turn the distortion control below the 2 o'clock position.  It may also be that turning up past the 10 o'clock positon gets you pretty much the same sound throughout the pot's rotation.

What these scenarios suggest is that, in the first one, you may not want to have the actual pot resistance go up past 100k, and in the second one you may not want the pot resistance to ever drop below 100k or so.  In the first instance, then, even though the schematic *calls* for a 1meg pot, anything from 100k through 1meg would be just fine, since all of them are capable of providing 0 ohms and are all capable of providing 100k as well.  You *could* go higher than 1meg if you only had something higher in value, bt that would waste much of the pots range, and also restrict the usable part to a few degrees rotation, which would make for a twitchy control.

In the second instance, you could also use just about any value you want...but with a 100k fixed resistor in series.  That way, you know that, no matter what pot you use or where you set it, you will always have at least 100k resistance in that path.

In some instances, it is actually smarter to use a fixed resistor and a lower value pot to provide "ranges".  I do this on my Anderton Tube Sound Fuzz.  I have an op-amp gain stage up front with two series resistors to ground that set gain.  I also have a fixed resistor in series with a pot on the output that sets level.  I use a DPDT switch to simultaneously shunt-bypass or unbypass those resistors.  If I shunt one of the series resistors on the input gain stage, that is the same as turning the resistance "down", thus increasing gain.  By unbypassing the resistor in series with the output pot, it's like using a slightly higher value pot and turning it down (e.g., a 10k pot with a 1k resistor on the input is like an 11k pot turned down 10%).  My bypass switch lets me go to a higher drive setting for more "hair" on my tone, but maintain constant volume, which I like.  By analogy, no reason why one couldn't use a 500k pot where a 1meg was called for, and switch a 470k-510k resistor in series with it, to achieve two different ranges.

I said that in some instances, value matters a great deal.  Here we come back to the Distortion+.  In this circuit, there is a polarized cap, and 10k resistor in series on the output of the op-amp, going to the output pot.  A pair of diodes goes to ground at the junction of the 10k resistor and pot.  Now the schematic *shows* a 10k pot and having traced one out in the late 70's I will vouch for that.  But, consider that the 10k resistor in series with the pot makes the pot behave like a 20k pot turned down halfway...even when turned up full blast.  True, the diodes are there, but from the circuit's perspective, that's a mere interruption or digression from the "resistance chain".

A common complaint about the Distortion+ is that it just isn't very loud.  If you replace that 10k pot with a higher value than shown, a funny thing happens - it gets much louder.  Again, consider that if the amount of attenuation at the output depends on the series resistor and pot, then a 100k pot, for example, in conjunction with the 10k series resistor, would be,  if turned up "full", like having a 110k pot turned down 10%.  Big difference.

So here is an instance where the pot value can make a huge difference in sound which is not easily compensated for by other means.  You'll learn more as time marches on, but it is safe to say that there will be many instances where you can make do with a smaller or larger value, until the parts arrive, or even forever.

[gez]

Nice to see the UK lads out in force!  If you haven't checked it out already, give www.esr.co.uk a go.  You're not going to find odd values as such, but they're better stocked, more reliable (delivery often next day) and cheaper than Maplins.

They sell cheap die-cast boxes and stomp switches too - I don't think they're in the catalogue so just phone them (up in Geordie land) and ask.

[Marcos - Munky]

Welcome to Stomboxaholics Non-Anonymous... Hi, my name is Paul, and I'm an addict.

Hey, cool! Welcome, man. Take a look at the stores in your city, they have it. If you don't find a 250K pot, ask for mini pots, they are cheaper and smaller, but I think the biggest value in mini pots is 500K.

[petemoore]

If you start with a smaller than called for value pot [say 100k is called and your using a 50k] the amount of range will be half.
 Often times this is enough to get you in the 'sweet spot', say for a gain knob [I usually have these set between 6-10 anyway], all you really need in a case like this is enough range to get on either side of the sweet spot.
  Say if the value of the pot sounds good at 80k ... if you setup a 50kpot/resistor combo that can range from say 60k-110k, you'ld have more than enough to adjust beyond both 'sides' of the 'desirable' range/sweet spot.

[Chipper Chapowski]

Thanks you very much for everyones impressively rapido replies!  They are very useful, but I'm guessing for certain applications its gonna be a case of buying a few different pots, wiring them up and having a go!  You never know I might stumble on something even better!



From: Paul Marossy

Here's a trick: If you want a pot to be half of it's value, solder a resistor across the two outer lugs of the pot that is the same size as the pot.

From what gather here, is this just the simple resistors in parallel rule?  I can see why it would be better here to buy a larger pot and bring it down.



From: Mark Hammer

since all of them are capable of providing 0 ohms and are all capable of providing 100k as well.

This is a very good point!  Thanks for explaining that, it makes a lot of sense.

Keep up the good work guys, many thanks for helping me out!!

[Ammscray]

Welcome to Stomboxaholics Non-Anonymous... Hi, my name is Paul, and I'm an addict.  

Here's a trick: If you want a pot to be half of it's value, solder a resistor across the two outer lugs of the pot that is the same size as the pot.
So if you have a 100K pot and you want a 50K pot, solder a 47K or 50K resistor between the two outer lugs of the pot and you will have cut it's value approximately in half.

I have also used a pair of same value resistors in series with the two outside lugs of the pot to increase a pot's value. I did this to increase a 500 ohm pot to a 1K pot. It seems to work OK, but I'd much rather cut a pot value in half than try to increase it...

I think you meant to say it would be a 100K cheater resistor across the 100K pot to make it 50K...

[Paul Marossy]

Duh. You are absolutely right. I meant to say a 100K resistor.  :oops:
Not enough sleep last night!

[Belanger]

Welcome to Stomboxaholics Non-Anonymous... Hi, my name is Paul, and I'm an addict. 

Here's a trick: If you want a pot to be half of it's value, solder a resistor across the two outer lugs of the pot that is the same size as the pot.
So if you have a 100K pot and you want a 50K pot, solder a 47K or 50K resistor between the two outer lugs of the pot and you will have cut it's value approximately in half.

I have also used a pair of same value resistors in series with the two outside lugs of the pot to increase a pot's value. I did this to increase a 500 ohm pot to a 1K pot. It seems to work OK, but I'd much rather cut a pot value in half than try to increase it...

isn't this the wrong way to do it?   shouldn't you put it between lug 3 and ground to extend the pot?   sorry to comment on an old topic but I'm a newbie and now I'm confused     if you solder a resistor to lug 1 and 3 it drops it's value. how exactly can you put a resistor in series with both lugs?  if I solder each end to the component that comes before it, wouldn't that be the same thing as soldering it across lug 1 and 3 directly lol   

I'm really confused obviously so excuse my ignorance

[merlinb]

Maplin.co.uk seems to be quite good, but say I'm after a 250K pot, about the closest I can get is a 470K or an 100K

Maplin are a waste of time and money. Go to bitsbox.co.uk or eBay.

[bluebunny]

They may have been a little better back in 2003!     (But not much...)

[ElectricDruid]

...and they were better still back in the 1980's when I were a lad! I bought some SSM filter/VCA chips from Maplin, when they still stocked the SSM2045 and SSM2047 (try getting those now without selling both kidneys).

I'd recommend Rapid Electronics for parts in the UK:

http://www.rapidonline.com

RS and Farnell are good, but massive. Rapid are a smaller operation but still big enough to have all the things you're likely to need in one place and in stock.

HTH,
Tom

[Fredjones32]

Duh. You are absolutely right. I meant to say a 100K resistor.  :oops:
Not enough sleep last night!

I know this is an old thread, but I wanted to say thank you.

Thank You!. I blew a pot on a variable power supply and the electronics store only had a 5k pot and I needed a 2.5k. Followed your advice and it works like a charm.

Also thanks to @Ammscray for catching the resistance need to match the pot's.

[samhay]

Seeing as this necro post has truly risen from the dead.

Maplin are a waste of time and money. Go to bitsbox.co.uk or eBay.

I would counter that one of the few things worth buying from Maplin is their pots.
They are good quality and their dual-gang pots in particular are actually quite good value.

While most of my pots come from Chain via eBay, I will pop into Maplins if I need a pot or two to finish something. Can't really say that about anything else they sell.