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In Reply to: Tonearm alignment for 78 rpm records posted by KWILLIS on December 27, 2007 at 07:20:05:
the British Standard (BS1928 of 1955 I think) is as follows for inner grooves:
33rpm inner diameter 4¾" I calculate the radius to be 60.325mm
45rpm inner diameter 4¼" I calculate the radius to be 53.975mm
78rpm inner diameter 3¾" I calculate the radius to be 47.625mm
there were earlier standards for radio transcriptions, but I don't have any data. It is possible that the major companies had reasonably close standards by the end of the 30s. I believe the BS standard above is identical to RIAA and IEC standards.
Many older records have a much smaller radius, I've just measured a 1900 Berliner and the inner groove radius is 37mm. That's not the smallest I have.
The outer diameters are:
12" outer diameter 11.5" I calculate the radius to be 146.05mm
10" outer diameter 9.5" I calculate the radius to be 120.65mm
7" outer diameter 6.594" I calculate the radius to be 83.74mm
all ±.254mm.
I am lazy and just use 60.325 which is usually fine, but as I use SME arms with adjustable overhangs you've got me thinking.
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Follow Ups
- Inner Radius of 60.325mm is only for 33pm LP records.. - richardz 12/27/0710:28:35 12/27/07 (0)