Technics SL-1200 Tonearm Auxiliary Weight and Effective Mass

The effective mass on a Technics SL-1200 / SL-1210 tonearm is 12 grams (7.5 g headshell, 4.5 g arm). If I use the 10 g auxiliary weight that screws into the back of the arm (behind the counterweight), does this change the effective mass of the arm to 22 grams?

 

No it will add about 0.8g to the eff mass.

 

But it also allows the main counterweight to be positioned closer to the arm pivot, which would reduce effective mass somewhat. Or is that already accounted for in your +0.8g figure?

 

Yes You are right, depending on the situation the total eff mass is not simply adding 0.8g, it will depend how the CW will be moved also.

 

So, how much of the effective mass is subtracted if the counterweight is all the way forward?

 

It will depend on which position the 12g is calculated, how long distance it is moved and the mass of the CW. It has to be calculated, but it will not vary much.
Maybe roughly less than 1g.

You can calculate it like; mass CW* (centre CW distance pivot)^2 = tonearm length^2* eff mass at needle. For each step you make.