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VPI TNT Bladder replacement

Hoping for a little guidance on this because the bladders in my VPI TNT 3 legs are toast.  I've read that good substitutes are a squash ball, racquetball or paddle ball, the Gingko ball, or perhaps the Herbie's ball as well.  But, it's not at all clear how this replacement is actually installed - so hopefully someone out there can share their solution.  

Here's the background to my question:
  • yes, the bladders are easily removed
  • leg cavity holding the bladders is approx 2" diameter measured parallel to the plinth - and this diameter is reduced 1/4" or so by the two bolt heads (which intrude into the leg cavity) that connect the leg to the plinth 
  • leg cavity height (vertical) is about 2-1/4"
  • Squash ball diameter is 1.6" and seems too small
  • Herbie's ball diameter is 1-7/8" and might be a tad small and too hard to squeeze past the bolt heads
  • Racquetball diameter is 2-1/4" and might be too large to squeeze past the bolt heads.  And if it does squeeze past the bolts, it might not stick out far enough below the leg cavity to provide support
Sure, all this is easy enough to test out, but I'm hoping someone out there has solved this problem - Thanks for the help!
poonbean
dlcockrum's avatar
dlcockrum
2,105 posts
 
Good feedback poon. I suspected as much re: the tennis balls. I have found that vibration/resonance control has to do with two principles: 1) Dissipating the vibration coming to the component and 2) Drawing and dissipating the internal vibration/resonance away from the component.

Soft, squishy things can do 1) well but fail miserably at 2), actually trapping the internal vibrations inside the component with unpredictable results. Direct coupling the plinth to the sub-base as you have done will accomplish 2), but can amplify 1). Direct coupling the plinth to an effective constrained-layer base that draws the vibrations from the table while also isolating the table from the vibrations coming up through the stand addresses both. I like the Symposium platforms for this, but there may be other equally effective (cheaper) solutions.

Hopefully you have already isolated (separated) the motor from whatever platform you are using under the plinth.

Best to you poonbean,
Dave
poonbean OP
39 posts
 
Very helpful.  Hmm, based on that, I suspect my rack does a good job at isolating the gear from external vibrations.  So, I'll focus on dissipating internal vibrations.  Obviously a turntable will have those.  How much internal vibration can be generated by a power supply, preamp, or dac?  

Re isolating the motor from the plinth - good point - not clear how to do that beyond what I've got set up now, which btw, generates less rumble than original setup.  Currently, the plinth is resting on three stillpoint-esq feet which sit on 1-5/8" thick maple, which rests on the same maple shelf as the motor.  Thoughts?
dlcockrum's avatar
dlcockrum
2,105 posts
 
Hi poonbean,

Look at my Virtual System page. You can cut out the maple for the motor. Rest it on the cutout and then experiment with different shings underneath the cutout. Placing it on Jenga blocks to bypass the rubber feet helps IME, but don't remove them as I have read that the screws hold some of the motor's internals in place.

Thread drive is a big improvement over the belts.

Best to you,
Dave 
poonbean OP
39 posts
 
Wow - very nice system - congrats. The way I'm set up can't really place the motor on a separated shelf, but certainly could put something under it.  Thread drive...?  I'm game, any pointers on that?
Cheers!
dlcockrum's avatar
dlcockrum
2,105 posts
 
Try Jenga blocks under the motor, just so the rubber feet don’t touch the platform or the blocks.

re: thread drive, buy some unwaxed dental floss and use your belt as a guide for length. Practice tying knots that don’t slip (very important). Simply thread the dental floss around your platter (careful not to hook your stylus) and then work it around the flywheel pulley or motor shaft (whichever setup you have). Bypass the tri-pulley system if you have one. Tension is established by moving the motor or flywheel slightly backwards or forwards. Tension should be snug but not too tight.

It will help to give the platter a start by briefly turning the clamp knob by hand when starting up.

Best to you poonbean,
Dave
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