The TP13 weight is a good indicator to the problems I have observed when arm swapping this table. The SME arm I'm using at the moment is nearly twice the weight of the TP13. Small wonder the third spring is nearly coil bound even when using the standard subchassis frame. That dealer who was using counterweights to even out the loading was on the right path imo.
Over at Vinyl Asylum a fellow inmate (Win) offered to take some poundage readings on two different types of Lp12 springs he has. Not having available a spring compression tester I suggested that a simple postage scale and ruler could be put to use by simply compressing the spring against the scale with one hand while observing deflection with a ruler and the weight reading on the scale. In the USA spring rates are measured in lbs per inch of deflection.
Win got 5.5 lbs for an early Lp12 spring. He also measured the free length at 40 mm. (1.57 in) He also had a set of later Cirkus springs that have an measured longer free length of 43-44mm (1.7 in) The Cirkus springs measured a whopping 7 lbs to compress 1 inch.
That was useful information as I was able to calculate that with the Cirkus springs I could achieve a natural frequency of just under 4 hz, and the earlier Lp12 springs would have been un-usable with the subchassis frame that I have. Btw I had measured 4.25 lb / in with the original TD150 springs using a postal scale at my location. Free length 1.7 in. Primitive measuring techniques but still useful.
why go back to springs when this sounds so good...! I haven't yet tried this with a standard subchassis frame so I can't comment on that result.
I haven't tried other isolator materials but if I did I would not look for more compliance than already exists in the current assembly. I think the idea is to provide resonance damping while maintaining a solid, unmoving base for the platter / arm to ride upon. To maintain a constant distance between motor pulley and the driven platter. Solid is better but isolation from motor vibes is needed. I should probably note that this TD150 motor I have runs very quietly. More so than either of the TD160s I have.
The current assembly of heavy grommets with a fiber reinforced length of automotive hose serving as a standoff may have some merit as a damped mounting. Background noise is quite low out of my speakers and the frame is completely quiet all over when probing with a stethoscope while running the platter. Isolation is very good in this assembly. I'd speculate that a solid piece of rubber in roughly the same shape configuration might actually be less effective as a damper. Of course parts like these aren't commonly available so I'd be looking at casting my own pieces if further experimentation is wanted. At the moment I am inclined to not try sorbothane (too compliant). Polyurethanes can be cast in different compounds of firmness. That seems like an option. Or various media filled isolator pad designs come to mind.
I used a bearing out of a spare TD160 subchassis frame that I have. That TD160 was less than perfect cosmetically but the bearing was in good condition. BTW the 160 bearing is the same as used in the TD150 and also the Mk II TD125. Dimensionally identical.
This bearing pressed out without harm but took considerable force and took some plating out of the subchassis bore with it. I used thick aluminum blocks with holes sized for the bearing to pass through and a vise as a press. This removed the bearing without damage but did distort the sheetmetal of the 160 pan. However the 160 pan was easily restraightened by pressing between panels and could now be reused. The bearing would need to be pressed back in, also with some force. It appears that Thorens used an adhesive in addition to an interference fit between bearing and pan.
I'm not suggesting that people start doing this to their old turntables. It's called canibalism...! Sacrificing some so that others of the same species might go on living. Horrible, just horrible...!
My optimum solution would have been to machine a new, and more massive, bearing housing for the new frame. Budget constraints modified that plan to using available parts.
Tonearm cables and their tension against a sprung subchassis:
This is a consideration when tuning the suspension. I don't know how the Linnies did it but with a Thorens the issue is fairly well taken care of with the standard tonearm. Change arms, however, and you now have this new influence of the tonearm cable working against the action of the suspended subchassis. It does seem that careful arrangement could serve to counter the influence of the elastic drive belt. A kind of equilibrium could be achieved. Very, very tweaky stuff. ....and all this trouble and effort because of springs..... Nice to be rid of 'em.
BTW, sound is very, very good. No turning back.