Question on FR 66s

John

Thanks for the explanation. I stand corrected on the relation between P2S and alignment using the UNI-Pro, and on how the micrometer settings are determined. I had concluded (erroneously, it turns out) that they were related because the UNI-Pro requires centering over the pivot, which, in turn, has typically been located according to the manufacturer's prescribed P2S. I use the UNI-P2S, which is extremely handy.

As I recall, the variation between the SPU Royal GMII and Mono GMII is tiny--easily less than 1 mm off the arc. But that's too much.

I hadn't thought of it as a VTF and compliance issue. Could be. The differences between the two SPUs are a non-negligible half a gram in tracking weight (3g for the Royal v. 3.5g for the Mono) and 4 µm/mN in compliance (8 v. 12). I imagine that the extra weight and higher compliance of the Mono could cause it to splay, pushing the stylus beyond the arc, which is the direction of the error. That's a very good point.

Still, I've felt more comfortable--and gotten better sonic results--using the arc protractors on this arm with SPUs. After this discussion, though, I'll experiment further with the UNI-Pro.

Apologies to the OP if I've pulled the thread off topic.

Bill

John, Is it not the case that for your statement that the P2S is irrelevant for the Dennesen or the UNI to be true, the cartridge must be aligned (ideally) perfectly with the long axis of the headshell. If the cartridge is "twisted" inward or outward with respect to the headshell, then the P2S dictated by the alignment grid on the template surface would put the pivot at some position different from ideal. I guess this would still "work", in the gross sense of the word, but if you want highest precision for a particular specific classic geometry, what I say must be true. Yes?

Of course, for an SPU cartridge, the point I raise is moot.

Halcro,
Yes you are correct: the only way to adjust cartridges like the SPU on arms like the FR64 and 66 is at the base, as the headshell offset is fixed. Which is why on a specific arm you have to use the existing specific arm alignment which uses that headshell offset. In other words, the given nulls. Only by doing this will the cartridge line up correctly.

John appears to be offering no solution at all......and if he thinks the UNI-protractor uses Baerwald alignment.....I believe he is mistaken?

The solution I gave for SPUs is straightforward: adjust the base to fit the original nulls. Don't use an arc.

If you have two SPUs or FR7s or whatever, and they don't have the same dimensions, then the base has to move or you accept a compromise.

For removable headshells with slots, you can use whatever alignment you like, but as I have told you in a past post, if you want to swap with an SPU you have to set it up first, and only then use a slotted headshell to mount and adjust another cartridge, without moving the P2S.

and if he thinks the UNI-protractor uses Baerwald alignment.....I believe he is mistaken?
Dertonarm has developed his own alignment called UNI-Din which places the null points differently to the others.

You are right to question your belief ;-)
As it is the Uni (for universal) protractor, DT has incorporated an adjustment to allow different alignments, one of which is LofgrenA/Baerwald IEC, and another is his version of LofgrenA which is derived from inner and outer recorded radii parameters which differ from IEC and DIN, thus giving different nulls. In the case of the SPU, unless the arm has an offset which corresponds to that alignment, then the alignment can never be achieved, and the cartridge will only ever align at one null. That is the way it is. All else is compromise. Whether the compromise is to accept a misalignment of a couple of degrees is up to you.

In the case of the FR66 the original P2S and overhang seem fine to me. A change to 296 makes little difference. Nulls around 63 and 115 would work ok. The thing is to get the offset as accurate as possible, as this is the parameter most prone to error.

John

Hi Lewm
The Dennesen principle allows any arm to align to LofgrenA/Baerwald.

If the arm is designed to that alignment to begin with and is mounted at the correct P2S the cartridge will align with the grid when square in the headshell.

If the arm is mounted with P2S incorrect or is designed with a different alignment (say, Stevenson) it can still be aligned if it has the facility ( ie has slots) to move the cartridge to the correct offset and effective length to align with the grid.

In the case of Dertonarm's variant, various alignments are possible. The physical structure of the arm's headshell doesn't matter if it allows twisting by a few degrees and some fore and aft movement.

The way it works is that the protractor alters the angle of the grid relative to the arm pivot automatically as the sliding arm is moved back and forth, because their relationship is fixed for whatever effective length. So if the arm pivot is in a fixed position, the only adjustment available is to twist and slide the cart to obtain the correct offset and effective length for the fixed P2S whatever it is.

John

Halcro,
As an aside, if Dertonarm's recommending setting the P2S at 231.5mm for the FR64 with FR7, that presumably means that when using the FR7 pickup, the effective length is changed as is the cartridge offset, and he reckons that is the best way to adjust for that, using the reduced offset and increased overhang to allow alignment at different nulls of around 63 and 115. Simply changing the P2S if the effective length remains the same is not a good idea with an integrated cartridge.

A friend of mine has pointed out that these discussions as to P2S distance for integrated headshell pickups would be moot had the FR arms been designed with sliding bases like the SME. Perhaps someone will put one on the market, like the aftermarket adjustable VTA base which I believe was available, and put FR owners out of their misery...?
John