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

[daviddeakin]

Originally Posted by kalee20

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.

There were letters to Wireless World in the 1940s that asked the very same question, and no one had a definitive answer even then. For all we know, some engineer in the distant past simply nudged his slide rule a little too far!

[David G4EBT]

I still don't think any suppliers are able to supply off-the-shelf 250k or 500k log or linear pots with a DPST switch, but Blore Edwards will indeed make them to customer requirements (250K & 500K as well as 220K & 470K) in a wide range of options which can be specified on their order form. (They can also supply reverse log tracks in ten values, though not the 50K needed for the Portadyne Princess. Maybe 100K with 100K resistor in parallel would suffice?). The shaft can be plastic or metal, imperial or metric, in three lengths up to 50mm, with either no flat, or a flat as specified by the customer on the order form, including the depth of the flat and the angle. T(he tracks can handle 1/2 Watt linear, 1/4Watt log).

They can even supply dual concentric pots, with or without swtch.

As to the switch options, there are six combinations, which sounds confusing until you look at the diagrams. For example, what we (me anyway!) would consider to be normal double pole on/off switch, is describe as 'OFF-ON OFF-ON'. I'd never thought of it that way, but there are two poles and each pole switches from Off to On. The poles can be in six different combinations, as depicted in the diagram below.

All for £7.25 for a single pot - what an outfit!

https://www.blore-ed.com/product-pag...er-metal-shaft

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

So, not quite 'off-the-shelf' - better than that 'made to measure'!

Interesting history:

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

Not a topic for discussion in this thread, but as an aside, I find it fascinating that Blore Edwards is tucked away in Aberdare - a small Welsh town with a population smaller than the village I live in. But then why not? As Spike Milligan once said: 'Everyone has to be somewhere'!

https://en.wikipedia.org/wiki/Aberdare

[Synchrodyne]

Originally Posted by turretslug

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?

I had also wondered about that given that in other fields, the E6 series was used in what might be called its “natural rounded” form, namely 10, 15, 22, 32, 46, 68 and 100. Thus, in electronics, both 33 and 47 were the apparent result of “wrong-way” rounding.

Judging by the attached Wireless World 1952 February article, “Why 47” (by Cathode Ray) (see second page, second column) this came about because the rounding was chosen to smooth out the E24 series, with the same numbers then carried over to the E12 and E6 series.

There were 8 cases where the actual electronic E24 series departed from the natural series, as shown:

“Natural” E24 series rounded to two significant digits:

10, 11, 13, 15, 16, 18, 20, 22, 24, 26, 29, 32, 35, 38, 42, 46, 51, 56, 62, 68, 75, 83, 91, 100

Actual electronic E24 series:

10, 11, 13, 15, 16, 18, 20, 22, 24, 27, 30, 33, 36, 39, 43, 47, 51, 56, 62, 68, 75, 82, 91, 100

Most of the deviations were a case of rounding up (or down) where rounding down (or up) would have been mathematically correct. But 30 and 36 were bigger deviations, the “natural” series numbers, to five significant digits being 28.723 and 34.807.


Cheers,

[Maarten]

Originally Posted by David G4EBT

I still don't think any suppliers are able to supply off-the-shelf 250k or 500k log or linear pots with a DPST switch

Piher T21 has those in the standard range. Standard multipliers for that series are 1.0 - 2.0 - 2.2 - 2.5 - 4.7 - 5.0 for resistances between 100 ohms an 5 megs. Lin, log, alog. Several switch types to choose from. Other values upon request, though I suppose custom versions might be more expensive than 7.25 pounds when you want only 1 piece.