Question on FR 66s

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

Hi John,

Yes, I've come to accept that the offset angle will be a little off using my arc protractors, both of which are designed for Baerwald, with the SME and SPUs. I just don't see any way around it. It seems to me that SPUs are inherently compromised in this regard--and inconsistent. For example, if I set the P2S to enable my Royal GMII to trace the Baewald arc and then swap on my Mono GMII, the Mono is slightly off, which I read as overhang inconsistency.

With the UNI-Pro, if I set the tonearm to SME's prescribed P2S and check alignment with the Baerwald template, which Daniel says is the one to use with the SME, it's way off, as one might expect. So, I can slide the SME along its base, changing P2S until the stylus drops in the template's hole. But then offset angle is wrong because the orientation of the template itself is no longer proper, seeing as how has been situated in relation to the prescribed P2S. And because the UNI-Pro is orienting the stylus using a single point rather than the multiple points available using the arc protractors, I trust it less--unless its controlling parameters like P2S remain fixed.

I suppose I could reorient the template and further change P2S until the stylus drops in the hole and the cantilever looks aligned (which I would take to mean offset angle is correct). However, I assume that the setting on the Uni-Pro's micrometer has been determined according to the manufacturer-specified P2S. Is this not so? Then changing the P2S scrambles this variable, too, doesn't it? With these two primary coordinates thrown to the wind, this alignment procedure seems rather random and chaotic.

But I'm certainly no expert in these matters, just a guy wrestling with SPUs and alignment tools in hopes of good sound, so please correct me if I'm not seeing things correctly.

Hi Chris,

Yes, that's what I've come to accept by shifting the P2S: slightly different null points and slightly incorrect offset angle in exchange for minimal overall distortion. I just don't see another way with SPUs.

Bill