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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
peter_s OP
1,335 posts
 
PS - even though the tonearm is 10", if the effective length is 265mm (10.8"), perhaps it is not surprising that the PTS length that works is 250mm.  When I google PTS lengths for other 10" tonearms (not many hits) I see values ranging from around 228 to 232mm.  When I look at 12" tonearms, I see values on the order of 295mm. (this is also where my SME is set).  Using proportionality between 230 - 295, a 10.8" tonearm would have a value of 256mm.  So it seems I am in the ballpark.

I starting to not trust the TTB Protractor!!!  
wlutke
725 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.
lewm
13,019 posts
 
Peter,  believe the Mint and toss out the TTB.  I think your conundrum goes with it.  The TTB is not very accurate (because citing the pivot point by eye is not possible with +/-0.5mm error), even when you do plug in the correct data, which is another question as regards your current issue.
peter_s OP
1,335 posts
 
@wlutke The MINT was made for a 12-inch tonearm that has a P2S of 290mm.  Is it possible that a 12"/290mm combination has the exact same null points as a 10"/250mm combination?  I haven't yet achieved a perfect alignment with the MINT and the 10" tonearm, but preliminary results lead me to believe that I likely could.
peter_s OP
1,335 posts
 

@lewm  - I’m surprised at how inaccurate the TTB is. I tried multiple P2S values and I couldn’t get close for any of them. After lining up the outer null point, the inner null point was always at least a centimeter off. @wlutke - are you saying that there is no universal 2-point protractor - that all protractors must be made based on the P2S and the effective length of the arm? How is it that TTB seems to be selling a universal protractor with no mention of fitting to these parameters? Does it have to do with pointing the guide line to the pivot point? Can someone direct me to a reference that I can read to familiarize myself with the math and geometry of this situation?
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