Tonearm Geometry for 10" 78's

[wer]

In the process changing cartridges to get my "new" (about 1975) Dual 1229 set up for recording. Adjusting the arm geometry according to standard procedure, the outer Null point does not make a lot of sense, since it is calculated for 12" discs. Is there a template or calculation available for 10"records?

[Marc Hildebrant]

Wer,

Is your question :

The previous tonearm was placed for a 12 inch record. I now want to change the placement for a 10 inch record ?

Regards,
Marc

[wer]

Hello Marc,

Sloppy diction - my speciality.

It should read "Adjusting the cartridge geometry", since the arm mount immovable. Most of the protractors i found have null points at 66 and 121 mm from the spindle, based on an inner groove at 60 and an outer of 146 mm for 12"records.
However,my average for 10" 78's is:
inner groove at 50 mm and outer at 121 mm, so extrapolated null points might be around 55 and 90 mm. Since that the pivot to spindle length is fixed at 200 mm, the angular offset is fixed and only the overhang can be adjusted to 24 +/- 3 mm, further consideration makes me doubt that such null points could ever be reached, short of taking a sledge hammer and modifying the arm.
I would still be interested in a protractor (or at least a calculation) for these conditions, since i dream of custom building a TT for 78's.

Regards,
Werner

[Marc Hildebrant]

Werner,

Do you have access to the "Radiotron 4th Edition" handbook ? If so, there are design equations that tell you where to locate the arm for different arm lengths and record sizes.

I'll need some time to find the equations is you do not have access to the handbook.

Regards,
Marc

[wer]

All settled

Thank you Marc,

With the help of a friendly soul at VE i got a calculator where you can set inner and outer groove distance and arm lenght, which then prints out a protractor with the 3 different adjustment types (Lofgren A and B as well as Stevenson) and shows a graph of tracking distortion along the radius of the record for all 3 types.
Luckily my Thorens TD 166 (with a better arm mountable without modifications, which allows the necessary greater variation of overhang) will fit the requirements. All i need to find now is a spindle adapter for 78 rpm and a cartridge


Just in case somebody is interested: http://www.vinylengine.com/overhang_...calculator.php
and The Custom Arc Template Generator http://www.conradhoffman.com/chsw.htm