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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 28th Apr 2015, 9:49 pm   #1
FERNSEH
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Default "Miniature" octal valves.

Found these valves in a box full of other interesting types.
The metallised valve is a Marconi-Osram X73M. It's one of a series of 0.16amp valves for AC/DC receivers. The "Uniwatt" range. The two which have the missing metallising are marked NR49. The NR49 is the RN equivalent of the EF36 and I'm certain these valves are not pentodes, the internal elements look like the metallised X73M.
It's the small size of these octal valves which is interesting, 90mm (3 1/2 inches) from the top cap to the centre spigot.

DFWB.
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Old 29th Apr 2015, 11:38 am   #2
turretslug
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Default Re: "Miniature" octal valves.

That is a markedly small envelope for IO, yet they still appear to have a (admittedly also compact) pinch construction. It emphasises the size of the base compared to the emergent B7g etc. types. The period from the late 'thirties to the late 'forties seems to have been awash with intriguing transitional types, many of which were relatively short-lived- the odd little 717A springs to mind, rather like a 6AK5 structure mounted on its side on IO base. One could almost describe B8B/B9G as "transitional", being of similar footprint to IO and earlier types but developing the short lead/glass base techniques that came to the fore in later miniatures.
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Old 29th Apr 2015, 12:33 pm   #3
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Default Re: "Miniature" octal valves.

Closer examination of the X73M reveals that it is a pentagrid frequency changer. The signal grid, G4 is the top cap connection. So Grids 1 and 2 will perform the oscillator function.
The other two valves are pentodes.

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Old 26th Jun 2015, 6:43 pm   #4
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Default Re: "Miniature" octal valves.

As a follow up to this topic I've more small octal valves during the shop clear out. These are a metallised type A915A. All are in plain boxes. No makers name. Also printed on the box: part number 20049A

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