Question on FR 66s

Chris,

The issue about P2S and overhang using SPUs is something I've also struggled with using my SME M2-12R. Especially aggravating is the small de facto variance among SPUs that purport to have the same overhang. I read surprisingly little conversation about this issue, which will wreak havoc if one's armboard and tonearm-mount are unmovable.

I do what Henry recommends, which is to adjust P2S so that the SPU aligns with my selected geometry. Fortunately, the SME has a sliding mount; otherwise I'd be out of luck. I use arc protractors from MintLP and Accutrak, and I'm able get the stylus to track the arc almost perfectly. The UNI-Pro, which I also have, doesn't really work in this instance because it is contingent on a prescribed P2S.

Bill

Wrm57

If you use an integrated headshell cartridge like the SPU then you can't adjust the offset angle.

The SME arm you have is designed for LofgrenA (Baerwald). It is also designed such that that alignment can be set by adjusting the base for whatever cartridge is mounted.

In other words the P2S distance is variable, the effective length is variable and the offset angle is fixed. If you used a headshell and separate cartridge with sufficient clearance in the holes or slots, then the cartridge can be twisted to allow other alignments to be achieved (although Lofgren B doesn't require a twist, merely an adjustment of the sliding base.)

You can use any standard two point protractor to set up, or a the Dennison which works for any unknown mounting distance. (The Uni protractor uses the same principle, and doesn't require you to know or set the P2S, but it has to be Lofgren A/Baerwald.)

Unless you know the exact actual effective length, an arc protractor is not going to help, and if you did know it, it would only apply with LofgrenA/Baerwald.

If you have the wrong E.L, then the offset angle will be wrong for whatever P2S distance that enables the arc to be tracked.

John

Hi Bill, thank you for your recommendation. So if I'm correct the specified P2S distance is not a "holy grail" that cannot be touched by mere mortals?
More important is the correct geometry at the null points (that is that the cartridge is parallel to the grid on the protractor)?

Chris

Hi Bill, thank you for your recommendation. It appears to me that the manufacturer specified S2P distance is not some kind of "holy grail" that cannot be touched by mere mortals.
According to an geometriy calculator as supplied by Vinyl Engine you can calculate the distortion if the geometry is changed.
If I change the S2P distance to 297 mm (instead of 295 mm) and the overhang can be kept at 12 mm, I would end up with reasonable distortion. But you will deal with other null points!

Chris

Chris,
If you change the mounting distance to 297mm it will be fine if this gives a 12mm overhang. If you are using an SPU the headshell offset is fixed, so the only variable is P2S.

If the arm was designed for a 50mm as opposed to 52mm pickup, then the real effective offset will be slightly less as the effective length increases, but the basic arm geometry remains the same, it just needs more P2S distance. You can use the original protractor to check, as the nulls will be the same.

John