effective mass of Jelco tonearms

Can anyone help with the effective mass of Jelco tonearms with detachable headshells [I believe SA 250 or 750}?I've found conflicting values, including 18.5 gms, and other values. Is this without or with headshells, and if with headshells, what is the weight of the headsshell?
The head shell on a SA750D with a ZYX R100 cartridge mounted with 2 screws weighs in at 17.97 gm.

The R100 is listed on ZYX site as weighing 5 gm, leaving a net of 12.97 gm for the head shell including the mounting screws.

Best of Luck

The SA-750 is said to have an eff mass of 20g. It's considered a good match with the DL103(d) or other low compliance carts.

I don't have one of these arms. The info was reported in VE a number of years ago by the UK importer. Because the 750 has fluid damping of the pivots, it's said to be compatible with a wide range of medium compliance carts. The S shaped 250 has no fluid damping, but looks similar in construction and the geometry is the same.
Save for the 750... it is much better than the 250.
vinylengine.com provides specs for most all tonearms.

For me, the SA-750D was very good with both a Lyra Clavis da Capo and an AT OC9/ml(I).

I agree that the 750 is better than the 250. The options and quality of the 750 are well worth the price of admission.
I have e-mails directly from Jelco stating that the effective mass of the 250 is 18 grams and the 750D is 20 grams, including headshell. They did not say anything about the effective mass including a cartridge. The e-mails are from 2009 and 2010.
The effective mass including the cartridge will vary with the weight of the cartridge.

Put a nude Denon 103r in a shaved-down lightweight wooden shell and that doesn't weigh much. OTOH, some of the larger boat anchor cartridges even need an extra heavy counterweight just to balance the tonearm.
My Denon DL-130R balances just fine with my 750D. And I think is a perfect match compliance wise. I plan on rebodying the 103R with a aluminum body some day. That's when the heavier counter weight will come into play...