KB 279 TRF radio from 1931

[Terry]

Resistors 'came of age' much earlier than capacitors which were still normally only available in a 1, 2, 5 sequence until the early 60s.

Early resistor tolerances must have been very wide so there wasn't much point in having lots of values but gradually, as it became possible to make closer tolerance resistors it became possible to produce resistors in the E6 series with the 'low' tolerance of only 20%, which became the standard tolerance for much usage even after the E12 series was introduced - although it did now mean that they were also available with a 10% tolerance if you paid a little more.

I don't know about today, but the E12 range became the standard for resistor stocks and remained so in my service department long after we switched to 5% carbon film which were also, of course, available in E24 values.

Of course, a lot of magazine projects were written by engineers working at places like Marconi who had ready access to the whole range so, if a quick bit of mental arithmetic suggested 5k for a collector load, they would specify 5.1k which, of course, most folk, including us, didn't stock. It always amused me when the local schoolboys trooped in with their latest copy of PW - or whatever - and asked for  the list of components for the circuit they wanted to build and which invariably included at least one 5.1k resistor. I would offer them a 4.7k instead and then point to the line at the bottom of their list which stated "All resistors are 20% tolerance"!

Apart from there being no such thing as a 5.1k 20% resistor as far as I know, if there was such a thing its value would range from 4.48k  to 6.12k  whereas my 4.7k 5% resistor 4.7k only had a range from 4.47k to 49.94k!

Here's a chart I produced some years ago:



and here's a post I wrote on the subject

[Crackle]

Hi Terry
Many of the slimmer resistors in this radio were measured to get the value because the marking were eligible or missing, and the figures were rounded to the nearest 10. But in the days of these radios, values seemed to be normally in factors of 5, 10 or 50 or even 100.
They seemed much simpler and neater than the odd values you see today in the E12 or E24 range, which look so messy to me. However saying that once the standard colour code came into common use the resistors did get a lot more colourful with the advent of the E series values.
Mike

[Terry]

... in the days of these radios, values seemed to be normally in factors of  5, 10 or 50 or even 100.

What I expected really, so I was puzzled to see those other odd values!

Thanks for telling me how they were derived, Mike.