1 resistor parallel with pot VS 2 resistors with 1/2 value, lug 1-2 and lug 2-3?

[ericohman]

I want to make a B220k out of a B250k.

Solving for a parallel resistor (values in k ohms)
(250*x) / (250+x) = 220

x = 1833.33 or let's settle on 1.8 M

What is the difference between
a) soldering this 1.8 M resistor between lug 1 and lug 3
VS
b) soldering one 1.8 M / 2 = 900 k resistor between lug 1 and lug 2 as well as one 900 k between lug 2 and lug 3?

I read R.G. Keen's article The secret life of pots, Gauss Markov's resistor mods page and this Analog Alchemy site where they use 2 resistors http://www.diystompboxes.com/analogalchemy/emh/emh.html

So, are there any differences, load, taper?

[bluebunny]

I want to make a B220k out of a B250k.

Pots usually have 20% tolerance, so using your B250K will most likely make no difference at all.

[R.G.]

ACK!  Simulpost.

BB's correct. And a whole lot more readable. 

I want to make a B220k out of a B250k.

Strictly speaking, you can't.
Less strictly speaking, you probably don't need to.

The Secret Life of Pots glosses over one issue, that being that you can only simulate some external behaviors of the"ideal" pot, not perfectly simulate them all. To really change a B250K  pot to a B220K, you need to add conductor material to the conductive track on the wafer. That's the "strictly".

Less strictly, pots have much worse tolerances than resistors. Where 5% resistors are bog standard and 1% values are pretty common and cheap, 10% pots are pretty good and you'll get 20% tolerance if you're not careful. A +10% B220K pot is a B242K. A -10% B250K is a B225K. The two values have substantial over lap. Many pot makers recognize this fact by simply not offering values so close as a nominal 220K and a 250K, only one or the other.

Actually, that is probably why you're writing this. You can get B250K, but have a schematic that says to use a B220K, and can't find one.
If that's the case, put in the B250K and ignore the differences.

What is the difference between
a) soldering this 1.8 M resistor between lug 1 and lug 3
VS
b) soldering one 1.8 M / 2 = 900 k resistor between lug 1 and lug 2 as well as one 900 k between lug 2 and lug 3?

The resistor across the whole pot  is just a bit more loading on whatever drives the pot if you use the pot as a three terminal device (that is, two ends and a wiper) and has a complex, but miniscule effect if you are using it as a two-terminal variable resistor.

The split resistor from the ends to the wiper have the effect of warping the pot's three terminal response to be faster at the ends and relatively slower in the middle, compared to the absolutely-even linear variation of the un-changed pot. Again, such a small change in the base pot value may not be worth worrying about.

So, are there any differences, load, taper?

Yes. But they are tiny for such a small difference in base pot value. As a practical matter, unless you already KNOW that your application requires the accuracy, ignore it. If you don't already know this, post your application and I'll try to help you figure it out.

[rutabaga bob]

Have you measured the actual resistance of your 250k pot?

[rankot]

You must connect two equal resistors - one between lugs 1 and 2, and another between 2 and 3. But you if you want almost linear response and approx. half value at mid point, then you must use 20 times bigger resistors than the value of potentiometer.

[ericohman]

Wow, thanks for all the replies.

Yes, as R.G. mentioned, the scenario is that the schematic calls for B22k as well as B220k and I want to use alpha pots with solder lugs which are B25k and B250k
Schematic, not really a stompbox
http://www.blueguitar.org/new/schem/marshall/2550orig.gif

Yes, I've have the bad B22k lying here and it measures 1 ohm up to 20k ohms.

In the schematic there's odd values at
VR2 220k lin – Lead Master
VR4 220k lin – Treble
VR7 22k lin – Presence

I went ahead and ordered Alpha 24mm with solder lugs instead of Marshall PCB mounted direct replacements (I suspect the quality is bad because only these three odd value pots are bad, others are ALPHA and CTS)

[thermionix]

Silver Jubilee!  The 2550 model number was used to commemorate 25 of Marshall amplifiers (1962-1987) and 50 years of Jim Marshall being in the music business (he was a drummer, then a retailer).  I guess if Marshall is still around in 2037 (doubtful) they'll have to release a diamond-covered amp.

[bluebunny]

Yes, as R.G. mentioned, the scenario is that the schematic calls for B22k as well as B220k and I want to use alpha pots with solder lugs which are B25k and B250k

You'll find that pots in the US are predominantly available in a 1/2.5/5 series, whereas in the UK you tend to find 1/2.2/4.7.  Marshall, being British, would indeed have specified 22K and 220K.  Build a Murcan version with 25K and 250K!   

[PRR]

> there's odd values at

All of those can be 250K/22K nearest available value.

The gain and treble pots are true pots, their value is not critical, only where you set the wiper along that value.

The Presence is actually a 2-leg rheostat. Here the main thing is it goes to more than R20 4.7K. Whether that be 20K 22K or 25K makes no great difference. It is not a precision adjustment anyway. You basically have zero to a few-K (lots or some Presence), or "more" (little Presence).

[rankot]

The Presence is actually a 2-leg rheostat. Here the main thing is it goes to more than R20 4.7K. Whether that be 20K 22K or 25K makes no great difference. It is not a precision adjustment anyway. You basically have zero to a few-K (lots or some Presence), or "more" (little Presence).

Maybe a little of topic - but I would like to know and you're the only one I could ask - how Presence actually works? Band pass filter, or something else?

[ericohman]

Thanks!

Yes, a few weeks ago I had 3 silver jubilees at home.
Bought a 100W (2555) 5-6 years ago, and a few weeks ago I saw that someone locally was selling his 50W (2550). Living in a small town I felt I had to go and play it, and I did, and it sounded better than my 100W IMO.

So I bought his 2550 and put my 2555 for sale, same week I made an offer on a combo 2554 on ebay (from Finland, neighbour country so already prepared for 230V), he accepted.

Sadly both the 2550 and 2554 has bad pots, and the 2554 has some mods done to it.
My 2554 mods that I will remove https://youtu.be/xJzPp2N-T5A

So I will go over both of them in the near future. Great amps, here's right before I shipped out the 2555. I both loved having 3 jubs side by side, but also felt like I had gone insane