Dear @mrearl :"" Suppose I have a tonearm that wants to be mounted 250mm from the spindle . ""
first that all is not that the " tonearm wants ", that could be the manufacturer spec ( normally the manufacturer gives the tonearm EL. ) and nothing more than that.
If you need to mount from the spindle at 240mm what you are doing is changing not null points but tonearm effective length, overhang and off-set angle.
So what you need is that the tonearm/cartridge new alignment be made it with a new overhang and offset angle.
A shorter effective lenght gives you a little higher overall distortion levels that maybe you can't even be aware of.
Here the parameters value you need for 250mm and you can choose between those 3 alignments:
https://www.vinylengine.com/tonearm_alignment_calculator_pro.php?arm1=Arm+1&l1=ps&a1lv=250&a...
here for 240mm:
https://www.vinylengine.com/tonearm_alignment_calculator_pro.php?arm1=Arm+1&l1=ps&a1lv=240&a...
If you use a good protractor then normally there will be no cartridge mistraking.
Btw, it does not matters the changes in tonearms EL the null points inside each type of alignment: Lögren A or B or Stevenson always are the same.
In 1938 Erik Löfgren stated over his calculation equations to find out the cartridge/tonearm overhang, off set and the alignment null points where the input parameters in his equations were and are: tonearm EL, innermost groove and outer most groove distances and the equations outputs: overhang, offset angle, null points and by difference P2S distance.
Regards and enjoy the MUSIC NOT DISTORTIONS,
R.
first that all is not that the " tonearm wants ", that could be the manufacturer spec ( normally the manufacturer gives the tonearm EL. ) and nothing more than that.
If you need to mount from the spindle at 240mm what you are doing is changing not null points but tonearm effective length, overhang and off-set angle.
So what you need is that the tonearm/cartridge new alignment be made it with a new overhang and offset angle.
A shorter effective lenght gives you a little higher overall distortion levels that maybe you can't even be aware of.
Here the parameters value you need for 250mm and you can choose between those 3 alignments:
https://www.vinylengine.com/tonearm_alignment_calculator_pro.php?arm1=Arm+1&l1=ps&a1lv=250&a...
here for 240mm:
https://www.vinylengine.com/tonearm_alignment_calculator_pro.php?arm1=Arm+1&l1=ps&a1lv=240&a...
If you use a good protractor then normally there will be no cartridge mistraking.
Btw, it does not matters the changes in tonearms EL the null points inside each type of alignment: Lögren A or B or Stevenson always are the same.
In 1938 Erik Löfgren stated over his calculation equations to find out the cartridge/tonearm overhang, off set and the alignment null points where the input parameters in his equations were and are: tonearm EL, innermost groove and outer most groove distances and the equations outputs: overhang, offset angle, null points and by difference P2S distance.
Regards and enjoy the MUSIC NOT DISTORTIONS,
R.