effective mass of Jelco tonearms

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.

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.