Interesting difference between 12" and 9" tonearms


So I just got my 1987 Decca Super Gold cartridge back from John Wright with a Decapod fitted and new Paratrace stylus. Modified Garrard 401 with 2 arms. (see my Virtual System). I installed it in my 9" Jelco TK-850 and it didn’t sound good. Lots of surface noise and missing inner detail. I adjusted VTA and VTF and improved it some but it hadn’t a patch on the other Decca (Garrott Bros Gold with new LC stylus and Decapod) on the 12". A little irked, I tried it in the 12" version of the same arm on the same table. Same cables and armboards. Ridiculous. Transformed. Minimal surface noise. Tons of detail. Crystal clear. Musical as all hell. In fact, I can’t stop playing album after album. Ridiculous! (Cross posted from a comment on @halcro post)
128x128noromance
@stringreen Set up exactly as the 12" except for overhang. They're on the same table. However, you got me thinking. The 9" has the standard Jelco base while the 12" has the Ammonite base. Could there be an azimuth issue? Would that cause surface noise? I run other Deccas without issue on the 9" but they all sound better on the 12". I wish the plinth supported 2 x 12". 
If you listen to Lederman and some others talk about jitter, and how much "surface noise" is really internal cartridge resonances that cause the stylus to bounce around microscopically, and then think about your surface noise and everything else you heard, you’ll have your answer. Its not the length of the arm. Its the interaction of the arm and the cartridge and resonances traveling back down the cantilever to the stylus that are causing the awful sound and noise. Call it jitter. Call it mis-tracking. Either way its a problem of vibration control.
Skip about 4 min in https://www.youtube.com/watch?v=WmwnN_T_wW8&t=7s
Or skip to 13:35 for a fascinating story about this very subject.
Jitter begins at about 21:30
Dear @noromance  : Your TK850 and Decca cartridge could be a not good match because the resonance frequency in between is 16hz and additional to that the Jelco comes with knife bearing that develops more resonances/distortions that gimbal bearing designs.

Regards and enjoy the MUSIC NOT DISTORTIONS,
R.
When i was reading about Decca cartridges i quickly learned that they are not for every tonearm, actually very limited choice of tonearms are suitable for this specific cartridge. I could not stand its design, so i never bought any Decca, the journey ended up with cantilever-less Ikeda 9 III.