@neonknight:
As @lewm just posted, adding moving mass to a tone arm is simple. Do it on the headshell for maximum efficiency. The lead weights for balancing wheels work splendidly, and if you need to add mass to your arm's counterweight to now get it to balance the weights will of course work there too It's decreasing the moving mass of a tonearm that is difficult.
You have heard correctly, the Deccas (old) and Londons (new) require a medium to high mass tonearm. The reason for that is that cartridge's very low compliance in one of the two planes of movement (either lateral or vertical, I don't recall which). To get a resonant frequency in the desired 10-15Hz range, a low mass arm just won't due. 15 to 20 grams of moving mass is correct for the Deccas and Londons.
The cartridge puts out a very healthy 5mV! Not 0.5mV, or 0.05mV, but 5mV. You need only 30-40dB of gain from your phono stage. Two manufactures have included accommodations specifically for the Deccas/Londons: Schitt with a 30dB gain setting on their phono amp, and Zesto with a 15k Ohms position on the MM impedance switch on their Andros pre-amp. I asked George Counnas about that, and he told me it was for his industry friends who use Deccas and Londons in their personal systems at home. Why 15k ohms? Deccas and Londons benefit from a lower-than-47k impedance setting, anywhere from 10k Ohms to 30k.
As for servicing, I just emailed London (the company) about sending them a Decca 4RC (from the 1960s) to be rebuilt, and got an immediate reply. Stewart told me the company is working on all Deccas and Londons, Deccas dating back to models from the 1940's. My newly-acquired 4RC will be on it's way to England tomorrow. When it returns it will join the London Super Gold and Reference pickups already tracing the grooves in my LPs.
A word of mild warning: Deccas and Londons put out a LOT of mechanical energy, and are known to cause rattling or chattering in some arm bearing assemblies. For the same I would advise choosing an arm with a very stiff arm tube. The stiffer the better (that's what she said).
