Two Protractors - Only One Aligns

@lewm One thing that is unique about this tonearm is that I can change the P2S over a wide range without moving the mount, as the arm that leads to the pivot swings. This can be seen on the link in my first post. Thus I can change the P2S from 230-270mm.  I can also change the overhang by +/- 4mm with the slot on the headshell, and I can change the offset angle significantly since the headshell is only attached with one bolt.  With all this adjustability, I may be able to line up the proper offset angle and null points with the mint protractor made for my unique tonearm combo.  Does this make any sense?

peter s,

Sorry, I found your OP a bit confusing, I thought your Mint was made for 250 mm. Never-the-less, the Mint arc is specific to the P2S. A Mint arc for 290 P2S will have a flatter arc than one for 250. The null points have the same distance from center in each case but the arc between them cover a different number of degrees. Think of an arm 10 feet long. The alignment grid lines at the outer null point would be nearly parallel to the alignment grid lines on the inner null point. On a ten inch arm they are not. The null points are not in line with the spindle but on an arc that overhangs the spindle. With proper magnification - at least 16x or so for the Mint- the problem will become obvious on your smaller scale.

I agree with wlutke.  Nearly all other tonearms have a fixed angle headshell offset, unlike yours.  The P2S (pivot to spindle) distance, effective length, and the headshell offset angle are intimately related and chosen for whatever geometry the designer of the tonearm had in mind (usually Baerwald, Lofgren, or rarely Stevenson).  This is why the MINT protractors are unique for each tonearm.  If by chance two different tonearms work with the same P2S and headshell offset and effective length, then a protractor made for one will work with the other.  By adjusting the headshell offset angle on your particular tonearm, you may be able to cheat a little on this rule of thumb, but I think the tracking angle error in between the two null points might suffer. (I would have to sit down with a pencil and paper to figure that out, and I don't want to.)

While I've never seen a Mint protractor, I believe it is an arc type, which is to say the stylus must ride on an arc drawn on its surface.  If you can get your stylus to do that, with the Mint placed over your spindle of course, that might be valid.  The TTB assumes a priori that you have mounted your tonearm with exact correct P2S and that you can then align the spindle-mounted TTB with the pivot by eye, which I think is fraught with error, and I do own a TTB which I never use. Slight errors in aiming the TTB at the pivot can result in large errors of alignment.  I also noted that the TTB is a few mm thick, and the mirrored side is on the bottom, whereas the marker lines are on the upper surface of the glass.  That also is a potential source of error due to parallax.

I'll have to go back and click on the URL to see how your tonearm is constructed.  Sounds like it might be a copy of a Dynavector DV series.

Bottom line, now that I've re-read your OP and gone to the website.  First of all, don't expect the Mint you own to work with this tonearm unless the specs are identical to those of an SME 312S.  Seems your new tonearm is 10-inch effective length and the 312S is 12.  Not good. By fiddling with offset angle, you might get it to "work".

@wlutke I get what you are saying, and agree that the arc should be flatter for a larger P2S (or a longer effective length).  However, if the null points are the same for both configurations, perhaps this doesn't matter.  By definition, the null points are the two points where the cantilever is tangent to the record groove, and this is not effected by the "curvedness" (or flatness) of the arc.  I think the difference is that a flatter arc will create less error at locations away from the null points. That is why a 12" tonearm is preferred over a 10" tonearm.

I found a good turntable geometry calculator on this website. This calculator shows that for any given P2S, the zero radii are the same.  What changes is the effective length and offset angle required to meet the null point requirements.  For example, calculating the zero radii for a 250mm and 290mm P2S, I get the same zero radii (63.32, 119.63) with the following differences:

P2S                        250              290
Eff. Length              264.79         302.84
Off. Angle                20.25           17.61

I guess the cool thing about my tonearm is that the offset angle is adjustable, because the headshell is attached at only one point and can be rotated.  Thus, if I am able to find a P2S that works for the effective length of the tonearm, I can adjust the offset angle accordingly. Early indications suggest that I can accomplish that with the MINTLP - but I haven't confirmed that yet.

Where I get confused is applying the protractors.  My MINT and TTB protractors both have the same zero radii (67 & 121 mm) which differ slightly from those calculated above (don't know why, maybe different calculation method).  But they are clearly offset from one another in a different manner.  So if I could lay them on top of one another, the two null points would not line up, even though they use the same zero radii. That probably reflects the difference in P2S, and the TTB must have some assumed value of P2S.  The problem is that both protractor manuals state that you should fix the protractor - so that the differing offset makes it either possible or impossible to match the two null points on the protractor. If the only requirement is that at each zero radius the cantilever is tangent with the grooves, couldn't I simply rotate the protractor and use the lines on the mirror to make sure that the angle is correct?