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Two Protractors - Only One Aligns

So I've got a Technics SP10ii with an SME 312S tonearm and an Airtight PC-1 cartridge.  I had MINTLP make a custom protractor for the 312S, and it aligns just fine.  Recently I acquired a second tonearm (Musical Life Conductor SE 10" Cocobolo) - a very uncommon tonearm with essentially no documentation available.  Since the MINTLP states that it is fitted to only my SME312S tonearm, I defaulted to using a generic turntablebasics.com cartridge alignment tool. I was told that the pivot-to-spindle (PTS) length for the Conductor SE should be 235mm, but again, there is no written documentation. With the TTB tool, I could not get alignment at any PTS length, and I varied it from 225 to 270mm. Frustrated and confused, I pulled out the MINTLP protractor, and was able to align the cartridge at a PTS length of 250mm (FYI, the effective length is roughly 265mm, though hard to measure with the tonearm in place).  Then, in the spirit of scientific inquiry, I checked the alignment of the 312S using my TTB protractor and NO DICE - it was telling me that the cartridge was out of alignment (contradicting the MINTLP).  What the heck is going on here?????

I know there are more than one different alignments people use (Loëfgren, Baerwald, Stevenson, UNI-DIN, etc.) - but between the two null points on the TTB protractor the stylus was off by around 10mm.  That is way too much to be explained by variable alignment methods, right? And since I had success with the MINTLP protractor, I would like to call it good, except for the warning on the MINTLP ("BEST Tractor is tailor-made for a particular tuntable tonearm setup. Using it on any other setup will result in error and do harm to your cartridge").  I don't get that either.  Aren't the null points the null points, regardless of which tonearm you are using on a particular turntable?

Those with a high degree of vinyl experience - please chime in if you can.  Thanks, Peter
peter_s

Showing 4 responses by wlutke

wlutke
294 posts
 
peter s,
The arc on your Mint has a radius that works only at 250 mm P2S.   Both null points cannot be aligned simultaneously with ANY error in the P2S.  I chased a sibilance problem for a year before realizing this (original VPI JMW 10 arm - spec is 250 - yes, I was a newb).  P2S was 2 mm off.  If you are aligning both null points precisely, your P2S is not 265 mm, but 250.  If the null points cannot line up perfectly/simultaneously with the lines, the P2S needs adjustment or a single null point type protractor can be used.
wlutke
294 posts
 
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.
wlutke
294 posts
 
peter s,
You said; "But shouldn't the alignment lines be parallel to the grooves if they occur at null points? This makes the cantilever parallel to the grooves."
Yes.  It's the grooves that are not parallel to each other at the two null points.  As the stylus tracks past the outer null point, the stylus tracing angle turns too far inward.  As the stylus approaches the inner null point the groove angle turns inward even faster (nearer record center) and catches up with the stylus tracing angle at the null.  Overhang staggers the null points (a line cannot be drawn from the Spindle through both null points) to make this possible.   As the effective length of the arm increases, the arc gets shallower and shallower and the overhang is decreased, bringing the arc closer to ninety degrees to a line drawn between Pivot and Spindle - until you have no arc, no overhang and a linear tracking arm.  
wlutke
294 posts
 
peter s,

You said;
"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."
As I stated, the null point distance from the record center does not change. As the stylus traces the flatter arc, the stylus tracing angle changes less in relation to the grooves. The Mint inner and outer null alignment lines will be more near parallel to each other to compensate. Overhang will be different in order to compensate as well.
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