The turntable started life as a B-stock unit (2015) that had a TNT-2 plinth, but where the 2-pieces of plexiglass were painted - that's why it has that unusual black-gray finish. VPI then added more modern pieces to finish out the table with the TNT arm-board being ebony-wood. I added the AL-plate and the 2nd tonearm as follows:
1.1 Added 0.25-inch thick ALCOA cast
aluminum MIC-6 plate for visual esthetics and acoustic performance. The 0.25-inch thickness does not extend the
plate below the lower edge of the corner posts.
The plate is not in any load bearing path; is not attached to the arm
board or platter bearing shaft (see photo of turntable underneath with plate installed). The platter/bearing is grounded to the plate
and the plate is then grounded to the 120VAC source (0.25-inch wide tin-plated
copper braid.). The plate is bolted to
the plinth; no adhesive was used. The
MIC-6 aluminum material was used because of the dimensional-milled precision along
with being softer than 6061 to allow for easier hand work and being a cast
material will have acoustic benefits.
1.2. The plate is bolted to the plinth with
four opposed 5/16-18 stainless steel fasteners.
The plinth two (2) existing 1.25-inch holes were repurposed and two (2)
new 0.75-inch top stepped to 0.5-inch holes added. The fasteners for the 1.25-inch holes consist
of the Kartek
Off-Road 1-1/2" Black Anodized Billet Aluminum Stepped Body Washer (https://www.kartek.com/parts/kartek-off-road-1-12-black-anodized-billet-aluminum-stepped-body-washer-uses-516-bolt.html);
and a bronze thrust washer (1.25-inch OD/0.75-inch ID), rubber quad ring, with
5/16-18 stainless steel bolt (length 1.25-inch), 1.25-inch-OD nylon washer,
nylon spacer and stainless steel coupling nut.
The arrangement centered/stabilized the assembly in the existing
1.25-inch holes sufficient to allow torqueing with no damage to the plinth. Sorbothane and Blu-Tack was used for the
1.25-inch holes to fill the area around the bolt to the bottom of the coupling
nut. For the new 0.75-inch stepped to
0.5-inch holes, the 1.25-inch OD bronze thrust washer, rubber quad ring, and
1.25-inch-OD nylon washer were not required.Four
(4) greased (to avoid galling) 1-inch length stainless steel 5/16-18 button
head fasteners thread from underneath via a nylon & stainless-steel washer (extra
thick) arrangement and were torqued to about 10 ft-lbs. When bolting, the top fasteners where held
fixed to prevent any damage to the plinth.
1.3. The MIC-6 plate is delivered with PVC
protector on both sides. The PVC has
some elastic properties so the side to the existing plinth (painted acrylic)
was left installed to act as a viscoelastic layer. The PVC areas that were damaged from the machining
were cut away (please note all machining was done with hand tools – the only
power tool was an electric drill; the motor square cutout was accomplished with
a coping saw so the precision has limits.
The location of one of the arm-board bolts (inboard center) required some
1.5 Vol 1, DAMPING CAPACITY OF MATERIALSby Robert E. Maringer,
Battelle Memorial Institute, 3 January 1966 (pdf report available on-line),
suggests that cast aluminum (older material Hiduminium) has better damping
capacity than extruded materials such as 6061 and under stress may increase
damping capacity by as much as 3X. This
supported the decision to bolt and torque the plate; and 5/16-18 bolts at 8-10
ft-lbs have a clamping force of about 1800-2200 lbs and the bolt locations are
on the platter perimeter.
1.6 CONSTRAINED VISCOELASTIC LAYER DAMPING OF THICK
ALUMINUM PLATES: DESIGN, ANALYSIS, AND TESTING, Bateman, Michael John, Monterey,
California. Naval Postgraduate School, 1990-03 (pdf report available on-line),
details successful use of a single thin (0.25-inch) Al plate with thin (0.030,
0.015 and 0.005-inch) viscoelastic layer to damp a large (48-inch x 15-inch) 0.5-inch
thick aluminum plate. The data only
covers 50 - 1050Hz; however,
frequencies above are much easier to dampen because of lower energy
content. An important feature of constraining layers is that to work effectively they need to be less stiff than the layer they are damping. The 1/4" AL-plate is about 1/10 the stiffness of the 2" plexiglass plinth.
1.7 Found an Avenger Delrin (pre-drilled)
arm board that was being sold E-bay for $25.00 (not to be seen again).This arm-board is shown on some early version of the Avenger and is 1/2”
thick using set-screws to secure to the post.The inside diameter (ID) of the hole was about 2-7/8”.Using files and sand-paper increased the ID
to about 3” to fit over the TNT 3.0” corner post. The arm-board sits flush to the plinth. In this arrangement it can handle the VPI 10.5/250-mm
Pivot to Spindle (PtS) or 10.5i/10-3D 259 PtS tonearm.
I added the two holes closest to the motor. They along with the original pulley holes (closest to the tonearm) are used to through bolt the plate to the plinth with 5/16-18 bolts (the hex heads have decorative caps) torqued to about 10 ft-lbs. There are opposed 5/16-18 stainless button head bolts underneath with stainless couplers in the center of plinth. There is old data that suggests that cast aluminum unlike extruded (such as 6061) when under compression the dampening factor can double or triple, and at 10 ft-lbs each bolting (total of four) applies almost 2000 lbs clamping force each. Also, the plate is in no load bearing path, providing the maximum benefit.
I did the plinth modification with a little change as possible and to allow bolting just in-case I did not like the sound. But the improvement in sound was good - The TNT was known for throwing a large soundstage, with the plate, it just got larger in all dimensions. After the installation, tapping on the plinth in-play eliminated any response through the speakers, and I liked the aesthetics, the black-silver motif is now carried throughout the table.
I will post some construction pictures tomorrow so you can better see the AL-plate.