Question on FR 66s

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

Bill, John, Lew and others, I would like to have your opinion about the following calculations I made regarding the geometry of the FR-66S with a SPU (52 mm length). Please correct me if I'm wrong.
The effective length of the FR-66S with needle to arm distance of 50 mm is 307 mm.
Draw a imaginary line (line 1) between the needle and the pivot, which is the same as the effective length.
Draw another line (= line 2) from the needle, parallel to the head shell.
Between line 1 and line 2 there is an angle, which is the offset angle (16.8 degrees for the FR-66S).
So the projection of line 1 on line 2 is cosine of 16.8 degrees * 307 mm = 293.9 mm (right triangle).
293.9 mm + 2 mm (because the length of the SPU is 52 mm) = 295.9 mm.
The new effective length will be: 1/cosine 16.8 degrees *295.9 mm = 309 mm.
If we want to have an overhang of 12 mm, the P2S distance would be: 309 mm - 12 mm = 297 mm.

Chris

Hello John_Gordon,

I do like the way that your mind works through these well thoughout solutions...scientific, without a hint of marketing, thank you!

Cheers

Thanks, John. As it happens, I own both an original metal Dennesen and a UNI. I tend to use the Dennesen only to set P2S. (I mark the desired distance on a piece of white paper and then set up the Dennesen so its pointy end is on one mark and the center of the spindle hole is over the other mark. Then set the adjusted Dennesen on the spindle and "Bob's your uncle".) I don't like using it for cartridge alignment because I have a very difficult time seeing its grid and sensing the position of the little dimple that marks the target for the stylus tip. (Also, many of the vintage Japanese tonearms are not meant for Lofgren A.) The UNI is a little better for that purpose. As I mentioned privately, I have a bias against twisting the cartridge in the headshell, based on one bad experience doing that. So I tend to favor using the geometry for which a particular tonearm was designed.