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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
lewm
6,770 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
1,088 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
1,088 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?
peter_s OP
1,088 posts
 
@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?
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.
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