I took a staged approach developing the corner towers, evolving from sorbethane of the original TNT Jr., to springs, to a rigid mass-loaded approach using solid hardwood and later brass in feet, and finally to brass with a DIY Stillpoints-style core. There are a few others who have adapted Stillpoints or rollerballs to the TNT. With a chrome spring-steel hole plug from Lowes and 1/4" and 3/8" ceramic ball bearings from Boca Bearing, it is possible to emulate the Stillpoints idea inside the VPI stock elephant feet, which further benefit from being recored in brass. To solidify and better couple the now unsuspended plinth, I eliminated the threaded leveling mechanism from the suspension tower and substituted a leveling mechanism underneath the platform supporting the TT. With this last step the TNT and its platform perform as a system.
It requires a lathe to make a spindle bearing. There are several plastic materials harder and less compressible than delrin for thrust plates, and ceramic balls that sound better than steel for various reasons. AFAIK, I am unique in encasing a floating ball in a donut between dual thrust plates. I considered updating to the inverted bearing, but rejected this for the reasons stated above. Having owned and discarded the TNT 3-pulley system, I was curious to find a better way to center the spindle bearing that would also support thread drive. Theoretically the centering mechanism should lessen somewhat the need for closer clearance tolerances between the spindle and its cylinder. In fact, once the drive system finishes torquing through start-up, there may be reduced friction/noise with a bit more clearance. IMO the unfocused sluggish sound of soft rubber belts is the Achilles heel of all VPI designs. The idler wheel that I added centers the platter and provides better traction for the thread. Thread drive is the best thing that can be done to realize the potential of these TTs.
Mark Kelly's AC-1 sychronous drive controller kit has been out of production for several years. I believe he does similar work for OEMs and perhaps custom jobs. I've never compared it directly to an SDS or Walker controller, but the AC-1 killed the PLC in my system. The fact that the AC-1 is a two-phase controller has advantages. The SDS is a single-phase controller that works with a phasing capacitor.
The TNT offers simplicity and lots of space to work through DIY mods, which is the main reason I've stayed with it since the mid 90's.
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Yes it is a bit of a crap shoot but you really would not have much invested except time unless you use some super exotic bearing material such as rulon and even then you don't need much of it for a TT bearing.
I think making a turntable bearing for a one off is a bit easier than manufacturing them. On a one off you only need to get the clearance right, the absolute dimensions don't matter that much.
You would need a decent lathe with a tool post grinder and someway to ream or hone the bearing to fit.
I have not seen anyone attempting this task. As I understand it Dgarretson only made a weight to attach to his bearing, not the complete bearing.
I'm not sure what sort of clearance is involved but I have two VPI bearing / spindle sets and they are both pretty tight. |
Thanks Ketchup. You have confirmed my suspicion that it sounds easier than it is
plus you are taking a gamble that it will work and sound good. If I had machining skills and access to equipment, it would be a fun project to attempt with the only real cost being materials. I see a lot of threads on DIY plinths, outboard tone arm mounts, motor drives etc, but very few are attempting their own bearing and platter assembly (except for Dgarretson) |
Brf,
It's probably very hard to say. You would first need to know the bearing clearances that VPI is shooting for (it will be a range). For instance, if the bearing clearances are 0.0010", is 0.0015" acceptable or does VPI try to keep it to within 0.0003 instead? Maybe they're going for 0.0002"? The price would change as the tolerances change.
Then there's the surface finishes of the bearing components. Better finishes will cost more money.
The cost of for a bearing with loose tolerances and poor surface finishes will be different than one with tight tolerances and better surface finishes.
If you ask a machinist for advice on what tolerances or surface finishes to shoot for, he may recommend something not even close to the quality that VPI offers as he probably doesn't know how these parameters affect the sound of the turntable. I think it's safe to say that he would not know unless he was a turntable manufacturer. |
I am not a machinist, but I can surely appreciate the precision required to make a bearing that must run true along with providing a 90 degree platform for the platter to rest. How much would a machinist charge for a stainless steel bearing similar to that of the TNT Mark V? |
Very interesting and thought provoking responses.
I am not worried about aluminum threads stripping and suspect you could shear of the top collar of the bearing housing if you apply enough torque (consider the amount of material engaged in the threads compared to the amount of material in the flange).
Dgarretson has done some very nice mods. As I read more about turntables I had begun to wonder if more mass on the bearing housing would be beneficial and I think he is saying it is.
The feet are pretty slick also. Do I understand that the flex provided by the fingers of the cup which holds the ceramic balls is enough to give a bit of vertical compliance as the four balls on the bottom move in and out allowing the top ball to move up and down? Some more details on the foot construction would be nice.
Your AC supply for the turntable bears some research as I have been looking for something that would do that.
As to machinist, while I am not a pro I do have pretty good skills and a very well equipped shop in my garage so I can fabricate a lot of this sort of stuff.
Lots of food for thought... |
On my custom HW-19 acrylic Plinth, the entire MK-IV Bearing Well (with the 3 screw mounting arrangement) is a tight slip fit.
Ny Spindle-Pivot distance on the Plinth is at +- .003".'
Probably the condition of the Bearing, and well itself is more important than what version bearing well.
I could've had a local machinist I know fabricate me a bearing that would've looked like it came of a military 2-1/2 ton truck. With his knowledge, and inspection of my stock MK-IV bearing, and the Platter, his words were "this is fine".
Yes, the 1/2" bearing shaft looks sort of "spindly" IMO.
Reckon the issues at hand are, how much load is on the bearing? There is no heat to speak of. Tolerances, ans smoothness are probably some of the most important factors.
A local machinist who is worth his salt shpuld be able to do anything you want. Tney'll usually charge you a flat rate charge for labor, and then materials acquired-used. |
Whatever VPI has done with subsequent iterations of the platter bearing to improve coupling between the bearing housing and the plinth, my point is that massive brass coupling of the metal bearing housing to the platform below will surpass the acrylic plinth in dissipating parasitic vibration from the system. This is based on several modifications to TNT spindle bearing and suspension I've performed that substitute brass for plastic. If you have access to a machine shop, try it you'll see. The construct can be extended to a brass pedestal that extends from the tonearm pedestal through the plinth to the platform. I've done it and it works. After enough of this the acrylic plinth becomes little more than a means of locating the spindle and tonearm in the horizontal plane. |
I am not sure if your could get the appropriate torque without stripping the soft aluminum threads Does VPI give a torque spec for any of their bearings? If you were to strip aluminum threads tightening any component on a turntable you are applying way to much force. |
Please allow to offer some clarification. The MkV mechanical coupling is vastly superior to that of the 3 bolt coupling. VPI also changed to this type/method of coupling the newer JMW Memorial tone arm bearing; it is basically a mini MKV setup.
My only concern with modifying an older TNT bearing to utilize a single bolt coupling method as deployed in the MkV is that the older bearings are aluminum and I am not sure if your could get the appropriate torque without stripping the soft aluminum threads. If you are interested in my back up MkV bearing, send me a pm. |
Thanks for all the responses. My thought was not in finding a MK 5 bearing but in modifying the mount of the bearing I have. There are a couple of options that come to mind but the housing appears to be thick enough to thread and attach like the MK 5.
Interesting to learn of the material change for the housing. If I understand correctly from Brf the method of mounting makes no difference so the gain would appear to come from the stainless housing and or different rulon inserts.
It is interesting to note that my TNT mounts the bearing with three screws tapped into the acrylic but I see there are another set of holes with what appears to be a broken tap or screw in one. Perhaps that is why they went to through fasteners on the later models. |
That bearing, and its companion MKV platter is what I have on my TNT. According to Mike at VPI: "they are as good as anything we have ever made; including the Super Platter". Totally agree 100%. I used both the Super platter and the MkV platter. Kept the MkV platter and returned the Super Plater. |
That bearing, and its companion MKV platter is what I have on my TNT. According to Mike at VPI: "they are as good as anything we have ever made; including the Super Platter". |
Yes, the TNT non-inverted Mk5 bearing is a significant upgrade over the previous generations. The older TNT bearings were made out of aluminum and were attached using 3 small bolts. The Mk5 bearing is made out of stainless steel with MUCH greater mass. The bearing is mechanically coupled to the plinth using a very fine threaded 3 inch stainless steel collar which secures the bearing much better than the previous versions. In addition, the Mk5 bearing has much thicker Rulon (sp?) bearing collar sleeves which are made to a higher tolerance.
Some Mk5 bearings do not come with the 3 inch threaded mounting collar but come with a traditional nut and bolt combination, although not as attractive, performance wise they are the same. I own three examples of the TNT bearing. TNT Mk3.5, TNT Mk5 with nut and bolt, and a Mk5 with the 3 inch collar. The end result with the Mk5 bearing is a quieter bearing with improved micro detail and dynamics
VPI no longer stocks the MK5 bearing, therefore finding a bearing will be your challenge. The Mk5 bearing is a drop in replacement for older TNT bearing. The TNT Mk5 inverted, Mk6 and HRX inverted bearing require a larger bearing hole, therefore, older TNT tables will require modification. I am not sure if the new inverted Classic bearing will fit a TNT. Hope this helps |
You could also set the platter bearing into Panzerholz and just replace the entire vpi plinth with it and mounting your tone arm and cartridge would benefit substantially also.
Panzerholz is extremely effective at reducing parasitic vibrations that cause distortions. |
I did something similar to a TNT with a non-inverted bearing by extending the threaded thrust rod outside the bearing well. A massive brass donut threads onto the rod, with an extensible footer that drains vibration into the platform below. One of many modifications to TNT. I'm not a fan of inverted bearings. The non-inverted bearing stays better lubricated and puts bearing noise down below, where the brass coupler can absorb and drain it into the platform. I also replaced the steel ball bearing with a ceramic ball that floats between dual plastic thrust plates. Two points of contact spreads the load for reduced friction and stress at the thrust plate-- allowing for a heavier platter.
Adding mass and coupling to the platform should help an inverted bearing too. |
