Here's why can't you find a modern 250K or 500k potentiometer.
 

Forum members often enquire about the availability of replacement potentiometers, resistors and capacitors in long since obsolete values, and wonder if they're not looking in the right places. It might be worth mentioning that for about sixty years, standard resistor, capacitor and inductor values have been in six ranges - the number of each range being indicated by its name. Thus, the E3 range has only 3 values, 1, 1.2 and 4.7, (and their decades) and at the other end of the scale the E96 range has 96 values and their decades. (Incidentally, 2.5 and 5 and their decades don't appear in any of the ranges, which is why there are no current production 250K or 500k pots, not that it matters - 220K and 470k are standard values and would be fine as replacements).

The ranges are E3, E6, E12, E24, E48 and E96. At first sight, the values in the ranges might seem a bit odd - why not just have round numbers? The answer is that the different values are spaced such that the top of the tolerance band of one value and the bottom of the tolerance band of the next one don't overlap.

The most common range of values that we come across in the hobby are those in the E12 range:

1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.1 and their decades.

The history of how these ranges came about is that in the post war years it became important to develop international standards, and over time, components evolved towards common values. The International Electrotechnical Commission (IEC) began work on an international standard in 1948. The first version of this 'IEC Publication 63' (IEC 63) was released in 1952. Later, IEC 63 was revised, amended, and renamed into the current version known as IEC 60063:2015

More about it here, if anyone is interested:

https://en.wikipedia.org/wiki/E_seri...ferred_numbers

And here:

https://www.electronics-notes.com/ar...24-e48-e96.php

Hope that's of interest.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

That's a nicely illuminating post David.
And I wonder if it's worth being used as a "sticky".
I'm thinking about the many queries we get from folk asking where they can get 0.25 coupling capacitors etc. If it helps a few its worth it and people could perhaps be pointed that way to help them understand a few things, maybe with an explanation of tolerance bands and why exact values aren't always that critical, depending on application?
Excellent post David :)

Andy.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

I've just clicked on the 'like' button.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

Hi David, very interesting, hate to be a pedant but 250K and 500k pots are still readily available as used in electric guitars, such as here and here

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

Indeed, and those aren't particularly "guitar pots" as most such pots have a knurled shaft for push on knobs. I think 250k and 500k pots have broken 'the rule' for a long while.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

It is interesting that despite the foregoing, for many years the Mullard C426/C437 ranges
of electrolytics used non-E values e.g

5, 8, 12.5, 20, 32, 50, 64, 80, 125, 160 uF etc

I speculate this might be due to RC time constant calculations.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

So why 3.3 in E6, E12, E24 ranges and not 3.2? It's the rounded value of square root of 10, or 3.16... which rounds to 3.2.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

Including 10, 25 and decimals, I think of this as the E10 series. I wonder if it was a historical thing that stuck in the capacitor world, starting as "E5" with 25% capacitance tolerance, then the intermediate values were filled in later?

There was a sort of variation on E6 with pre-war GEC/Marconi sets that ran 10, 15, 23, 35, 50 and 75- I expect that it was someone's particular judgement on rounding-up, though 75 and 100 end up a bit over-lapping with 20% spread.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

I'd wondered that and surmised that it might be just an artefact of preceding round-up approximation in E12 in that 32 would then have been a bit close to 27, so a hop to 33, extended to E6?

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

You can get a full range of values, both lin and log, including 250k and 500k from Blore Edwards

https://www.blore-ed.com/product-pag...-potentiometer
https://www.blore-ed.com/product-pag...-potentiometer

And even dual concentrics

https://www.blore-ed.com/product-pag...-potentiometer

Craig

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

Of all the possible components, potentiometers were the worst example to pick indeed, but if you read resistors or capacitors instead, the post is nice and clear for those who always wondered why.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

Definitely not looking in the right places - 2 mins with Google found many (big and small) suppliers of pots in those values in all sorts of shaft configurations including dual ganged pots and even some with switches.

These are new stock, not NOS or any other abbreviation thought to mean "old no longer produced items".

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

As I’ve seen lots of requests for sources of exact replacements of components in obsolete values, I thought in might be helpful to explain why, and how, the current E3 – E96 ranges came about. I was prompted to write this thread, having answered the request for information in the KB MR10 thread as outlined below. I didn’t want to take that thread off topic, by explaining the whys and wherefores of the E3 - E96 ranges, but thought it might have been worth another thread. Perhaps I should have tempered my enthusiasm to rush into print with discretion! :-)

When in this thread I said ‘Here’s why you can’t find a modern 250K or 500K Potentiometer’ I should have clarified that I meant "as replacements for either linear or log track pots with a double pole switch for use as tone controls or volume controls", but even by my standards, that would have been a tad verbose! I’d reiterate that I've yet to see an ‘off-the-shelf’ source of 250K or 500K log or linear pots with a double pole switch.

I did say that in any event, a 220K (20% low) or 470K (6% low) would be fine as replacements.

At post #147 of the thread below, I responded to this request by the OP in post 146 in which he stated:

Quote:

“I have looked around for an appropriate 250k with double pole switch but can't seem to find an exact match, specifically in relation to the d-shaped metal spindle”.

https://www.vintage-radio.net/forum/...=159373&page=8

End quote.

I responded thus:

Quote:

“You won't find a current production 250K potentiometer as for a long time, the standard value has been 220K, which would be fine, (in the same way that 470K has superseded 500K).The next value above 250K would be 270K, but you're unlikely to find one of those either. Most suppliers tend go from 100K to 220K, 470K, then 1 Meg, 2 Meg.

End quote.

In the KB MR10 thread, I did say that if the desire was for the exact value, same spindle diameter length, in steel and with a flat, Blore Edwards who may be able to help:

Quote.

You can however obtain a new switched pot made to order from Blore Edwards with a 1/4" metal shaft on request and if you wished, could ask if they could do a 250K linear track, but I doubt it, and as I said, 220K would be fine. Being a tone control, it will almost certainly have a linear track, but you can make sure by setting it to the middle of its travel and checking with an Ohm-Meter from the centre tag to each end, to check that it reads approximately 125K either side of the centre tag.

If you got to this link at Blore Edwards, you'll see that they state that their standard Type 45 potentiometer is supplied with a 6x50mm (two inch) plain Delrin spindle, but that metal spindles are available on request and are also available in both metric and imperial diameters. You'd have to ask them to add a flat to the shaft if you want to avoid having to file a flat on it yourself”.

The person to ask for is Brett, who is very helpful, as others will testify.

https://www.blore-ed.com/series45potentiometers

Phone No: 01685 882 043

End quote.

So the question remains, does anyone know of an off-the-shelf source of 250K or 500K linear and log pots with double pole switches?

Incidentally, Blore Edwards aren't 'the only show in town' when if comes to non-standard pots. Some years ago when I wanted two replacement anti-log 50k switched pots for a TRF Portadyne 'Princess' and a 'Noble' (identical set but different brand), Blore Edwards couldn't help, so I contacted OMEG, who actually manufactured the original pots. Yes, they could help, but had a minimum quantity of ten and a minimum order value. That made not financially viable.

https://www.omeg.co.uk/

Likewise the topic of tapped pots as well as antilog tracks crops up, as in this thread:

https://www.vintage-radio.net/forum/...adyne+Princess

No solution as far as I've seen.

I'll get my coat. :)

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

David thank you for your extensive research into this topic, all very interesting and informative.
I’ve used Blore Edwards and yes Brett is very helpful and a good contact, I had never heard of OMEG so thanks for that. Incidentally BE have no minimum order, I purchased two switched log pots , about £8 each, with no problem.
Cheers
John

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

Thumbs up from me too, for Blore Edwards. No connection other than as a satisfied customer!

In fact, from the thread title Here's why can't you find a modern 250K or 500k potentiometer which between us we've all managed to derail, the answer could have been 'Because you've been trying the wrong places.' And Dave's post #13 is then very helpful!

I do seem to remember another Forum member commissioning a batch of potentiometers from Omeg, years back.

And as for tapped potentiometers - no, I have no suggestions.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

BTW, a somewhat similar practice was followed with standard wattages for electric lamps.
These were round figures, but with each standard wattage being about one and a half times the previous wattage.

15, 25, 40, 60,100, and 150 watts.

Smaller graduations in wattage being considered needless for the everyday illumination of homes and workplaces.
For a small room or area, if a 50 watt lamp would in theory give just the right amount of light, the waste of fuel by use of a 60 watt lamp, or the slight reduction in lighting by use of a 40 watt lamp would be negligible.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

Thanks for reading the thread Terry and for your response.

As I said, this thread was prompted by the OP in the KB MR10 thread who had a need for a 250K linear pot with a dual gang mains switch. If you can point to the right place to look for an off-the-shelf new production (not NOS) pot which meet that spec it would be most helpful. Even better if it has a 1/4" (not 6mm) D-shaped metal shaft.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

I thought the whole idea of the values chosen for the various “ series” was that they DID overlap slightly. It’s a function of the tolerance, which is why there are more values per decade the closer the tolerance becomes.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

Hi David, I'll go back through my browser history and see what sites I ended up at.

I notice your link to Blore-Edwards has them, but I assume the value is 220k as you mention, as I haven't been able to find a list of the actual individual values they can supply.

Re: Here's why can't you find a modern 250K or 500k potentiometer.
 

One place that showed up was Alpha Taiwan, unfortunately, they do single pots with switches and dual pots without switches, which doesn't help, plus you can't buy direct in small quantities (but I believe they may have an Ebay shop).

As above, Blore-Edwards was another one, subject to the 220-250k discrepancy and I'm still looking for any others I saw (I have my browser set to clean up at shutdown, so I have to try and resurrect some of the temp files to find stuff).