Tonearm Geometry and setup--help me with a brain lock

I've recently added a Jelco 750d to a new to me Thorens TD124 table. I had the armboard made by a very reputable maker and when I measured the pivot to spindle (P2S) distance using a newly acquired Feickert NG protractor, it was coming up as 215.5mm. I contacted the maker and was immediately offered a replacement. When it arrived, it was also about the same, maybe even 216mm (this is splitting hairs as there are no "half" indicators on the feickert.

I've also read where some feel the 750d has an incorrect spec of 214mm on the P2S...however I'm unclear on whether this means the TEMPLATE is incorrect or just that it should be closer to 216mm as I've read because the spec'd 214 sets the cartridge very far back in the headshell. (Calling @blakep here as I've seen him mention this before).

So my question is when I'm using Conrad Hoffman's Arc Template generator, do I just use the actual P2S of 215.5mm distance as it exists on the table? What are the compromises I'm making here? I was thinking of having Yip (MintLP) make one for me, do I go with the "as is" P2S distance? If so, doesn't that change the overhang as well? In an email to Yip, he noted this would also change the effective length to 231.5 while entering 215.5 P2S in the Conrad Hoffman notes an effective length of 232.294--but I'm just not getting how that could be.

I'm guessing this is a simple answer that I'm making far more complex than necessary as I'm just overthinking this and confusing myself...hence the brain lock.

 

Use the actual measured distance. A .5mm on that scale will throw things off too much. You are not making things far more complex than necessary, and your record collection will thank you for getting it right. You'll also be more confident in your set up.

 

If you have an actual P2S measurement this is what you must use, either for the Conrad Hoffman Arc Template or indeed if you ask YIP for a Mint protractor (I have a Rega table with P2S at 221.5mm not Rega 222mm so this is what he used when he made a protractor for me). These days I tend to use the Dr Feickert protractor on all the tables I own because once the pivot centre is located and locked, everything is accurate to that specific table irrespective of tiny manufacturing tolerances.

Overhang, offset angle etc will be automatically correct to your chosen alignment once you input the actual P2S.

 

Thanks to you both! The complexity I'm hung up on is what is the implication of not using the "stated" P2S per the manufacturer. The pivot is 2mm further away from the spindle, I assume the overhang changes (guessing it becomes less?). How does effective length change? I guess the effective length is just that--effective?

Interestingly, I tried using a Soundsmith headshell and I couldn't move it far enough forward. With the Jelco headshell it's very forward and angled towards the spindle ever so slightly. BTW, I'm using Lofgren A (Baerwald) with the DIN setting on the Hoffman generator.

 

Set those inner and outer targets as you like, but gotta get the overhang perfect within those parameters. My table is 219.5, so I have to make my own protractors, but I've learned. Pivot to spindle distance is the whole deal, and the more accurate that is, the better and more reliable/repeatable the results. Everything flows out from that measurement.

 

When you say "get the overhang perfect within those parameters", doesn't the arc itself do that? Calculated overhang is 16.794 in this case and so doesn't the arc line equal this? So once I've gotten that right, I have done as you suggested above correct?

Interestingly, I was not able to get the Feickert to work on two different decks (long story--confirmed with the distributer) but for the first time have a fairly accurate way of measuring this P2S. All three of my tonearms have clear marks for the pivot point. I've always used Hoffman protractors with the manufacturers stated P2S.

 

In the interest of politeness (as I was asked in the first post): Yes, as above two posts say, use the exact mounting distance as best you can establish it.

Once you change that, overhang and effective length and offset will change as well. Overhang will be slightly less, effective length slightly more if you are mounted further away than suggested but offset angle will be very slightly less.

I would use the IEC setting. It will give you the more "traditional" Baerwald nulls at 65,9 and 120.8

 

Yes. The overhang and null points will shift a bit based on the inner and outer dimensions used. I use a DIN for the inside because I have enough records that overshoot the max to justify, at least in my mind. In reality, my biggest issue is trimming the inside spindle hole with extreme accuracy. That's the weak spot for me, and that's how much spindle to pivot counts, at least in my estimation.

 

Thanks for chiming in Blake...I had tagged you because I seem to recall reading where you noted the P2S for the 750D was incorrect or inaccurate--forgive me I'm going off recollection so not trying to misstate what you said/meant.

If that's roughly true could you elaborate on that point? Is it the template jig that's off or just the stated number? I was curious also because I had two armboards made by a reputable builder be off by about the same amount.

 

I didn't really state that the 214 number is "inaccurate" but have commented in the past that mounting the arm at 214 will certainly give you problems if you want to align using the Jelco alignment, which is more similar to Stevenson. I think around 216 is probably a good mounting distance for the 750D and will allow you to mount the cartridge more in the middle of the slots with most alignments as opposed to running out of room at the front of the headshell or jamming up going back toward the pivot point.

I have the template, but have never really used it or really checked it for accuracy to be honest.

 

Thanks for the help and thoughts! I took to using a razor blade after trying a few other things not as successfully. I use a steel straight edge to get the line just right. I then laminate it and cut the lamination hole which makes it easier and cheaper since if I screw up I just print another one before getting it laminated.

 

I use my Exacto, and make the protractor with a fairly thin line (.5 pt). I get a local digital shop to print them out on a card stock, and they measure dead on, which is better than my PS3 laser printer.

 

try trimming the spindle hole with a push pin on a table with some posterboard backing. i just mads some protratctors by glue sticking the printed page to some posterboard and the push pin method worked perfectly.
take your time and make sure each pin prick is right on the line of the circle. after a few circles take the poster board off the table and punch through. fits extremely well.

 

I'll gladly put all you guys in charge of our next mission to Mars with your impressive understanding of esoteric geometry.

 

don't worry too much -- as long as you get the nulls fairly close, it's not gonna hurt records. Yeah you may have a bit more distortion if you don't them in the best locations, but obviously there are different alignment schemes so I would say any alignment that gets the two nulls on the record reasonably close to where they should be is going to sound decent and not going do any damages, that really requires gross misalignment.

well, worry as much as you wanna, the best sound will be achieved with as accurate a p2s measurement and alignment as possible of course...

 

Did you follow the Dr Feickert instruction to move the protractor in step 2 "Turn Protractor so that you are over area of step outer Null-point". It doesn't just say 'move the arm' in their printed instructions - although the translation from German seems ambiguous.

I have found that on my own turntables, step 1 sets the overhang but then if I move the arm over either null point, the stylus no longer falls into the centre of the circle and I have to turn the protractor slightly until it does & then after that adjust offset angle against the cantilever. Once set for either null, it remains so for the other so it seems to me that this is the correct way of doing things.

In each case, I am using 30x magnification and the stylus sits exactly in the centre of the crosshair circle. Everything sounds perfect so I guess this is correct?