A question going back to some 1920s wireless circuits

[AdrianH]

I like to read and will often print off old articles, I downloaded and printed off a Popular Wireless Valve Experimenters handbook.

But on reading the text and looking at the circuits I am greatly confused with circuits that are shown as 2 Ohm grid leak resistors. Now I am guessing that they should be 2 M Ohm (2,000,000) but suspect the M was typically dropped as it is not just one drawing but many?


Also seen in Wireless constructor of the same era.


So please tell me that I am correct in the assumption that these are Million Ohms and not 2 or fraction of Ohms?

Adrian

[Cobaltblue]

I think you will find that the Ω was used in may 1920's circuits to mean MegaOhm.

Cossor were using this in some documents even in the 1930's

For Ohms they used used ω which is the lower case version of Omega

Cheers

Mike T

[G6Tanuki]

The different terminologies can be deeply confusing; I've got a genuine RCA schematic for my AR88 receiver where they use "M" to signify Kilohms, and "Meg" for Megohms.

[I can only think that the use of M for Kilohms is because M is what the Romans used to signify 1000]

[AdrianH]

It is confusing when they can talk of 300 to 400 Ohm potentiometers in part of the circuit and then use 2 for 2,000,000 Ohms etc ( I am not sure how to use the symbol ). It sort of stops me in my tracks.

Adrian

[Junk Box Nick]

In my 1924 Boy’s Book of Wirelesss the Ω symbol doesn’t make an appearance; values are denoted using either ‘ohms’ and ‘megohms’.

In my 1930s Cassell’s book, rheostat values are followed by ‘ohms’. There is no value shown for a grid leak though they do show you dimensions for making one out of a length of pencil.

In my 1932 Everyman’s Wireless there are values such as 50 000 Ω and values of 1mΩ and .5mΩ (using a lower case ‘m’) so Ω would now appear to be as we understand it.

k for kilo doesn’t seem to have arrived at this point

Thus in your books as they write 300 ohms when notating potentiometers the convention would seem Ω for a megohm and ohms for smaller values.

[AdrianH]

I have a pdf of 'The Boys Wireless Annual 1925' and as you say no mention of Ω.

I am just used to the modern convention of
p for 1/1000,000,000,000
n for nano 1/1000,000,000
μ for 1/1000,000
m for milli 1/1000
etc. that this old convention threw me.

Adrian

[Junk Box Nick]

I remember n for nano turning up. The first time I saw nF I couldn’t figure out what was going on. I forget who told me the answer – probably the science teacher. No Google in those days!

In Practical Wireless and the usual books of the time I had been used to just pF and μF; and μF was sometimes mfd on some (new) components.

I had seen kpF on some circuit diagrams which was distinctly odd.

[Dave757]

Hi,

I don't understand why the HT- in Adrian's first circuit is taken to the
LT+ rather than to ground.

Re the AR88 Schematic use of M for Kilo I remember having some
AR88 potentiometers which were marked in this way.

Kind regards
Dave

[Cobaltblue]

I would expect that it would effectively make the grid slightly negative with respect to the HT -ve (about halt the filament supply) if it were connected to the other way around the grids would be slightly positive.

Cheers

Mike T

[kalee20]

I've seen ω for ohms, but I hadn't made the connection that Ω could be megohms in the terminology.

Originally Posted by G6Tanuki

I've got a genuine RCA schematic for my AR88 receiver where they use "M" to signify Kilohms, and "Meg" for Megohms.

LTSpice uses M and m for milli (it's not case sensitive) and Meg or meg for mega.

Originally Posted by Dave757

Hi,
I don't understand why the HT- in Adrian's first circuit is taken to the
LT+ rather than to ground.

You generally get better results when the grid leak is returned to filament +ve rather than filament -ve.

So what they've done is exactly that, and they went the whole hog and made filament +ve the ground line. And then they've taken HT -ve to ground (which is conventional!).

[Dave757]

Hi Mike,

Many thanks for explanation, - point taken!

Kind regards
Dave

[AdrianH]

https://worldradiohistory.com/UK/Pra...20Handbook.pdf is the download link at the moment for the one I am reading

Later images do show both HT- and LT- being common and ground, sometimes using a potentiometer to shift the grid leak potential between 0 and +2 Volts as in this image: -


At the end of half term break when the Mrs goes back to work, i hope to be playing with 2 Volt battery valves.

Adrian

[AdrianH]

Have been playing this week and have built a one valve receiver using regen or reactive feedback, which ever you call it.

Quite surprised with how well it works to be honest.




Next step is to add a LF valve and try and come up with a suitable high impedance speaker! and the odd LF transformer.

Adrian

[Herald1360]

That wiring should be 1/16" square section with neatly dressed right angle bends!


Whatever is a "star" point doing in a '20s reproduction?





Nice job, really!

[Junk Box Nick]

These little sets are great fun and can work very well.

I have rebuilt my HAC one valve kit I had as a boy in the mid 1960s. I still recall the magic of trawling the shortwave bands and hearing amateurs for the first time. Sadly, the broadcast bands are nothing like as busy as back then.

[AdrianH]

Originally Posted by Herald1360

That wiring should be 1/16" square section with neatly dressed right angle bends!


Whatever is a "star" point doing in a '20s reproduction?





Nice job, really!

It's not a star point, just three wires crossing at different heights, awaiting some better coils to get made.

Adrian