Mounting a Grace G-707 on a Thorens TD-145mkII

[busoni]

Hello everyone!

I have recently bought a second hand Grace G707 to mount it on my Thorens deck. However, mounting distance for the Grace is 222mm against to 215.6mm of the TP-16mkII, the original Thorens tonearm. Due to the shape of the sub-platter around the arm-mount, I have to rotate the arm in order to mount it 222mm away from the spindle. Which means that when the arm stands, it won't be tangential to the platter. Is this ok?

If not, the only solution I think of is building a new plinth.

[busoni]

A sketch of what I was previously describing... In the drawing on the right, the arm base is moved by a small arc, somewhat 5-10 degrees clockwise, with reference to the arm position on the left.

[stoolpigeon]

The 222mm figure is all that matters so your plan is fine.

I remember fitting a Rega RB300 (which has the same mounting distance) to a TD160 and having to file away part of the subchassis to get the correct position.

sp

[busoni]

Another thought would be to mount the G707 on the original position of the TP-16(215.6mm) and then increase the overhang to reach 222mm. Would this be a correct solution?

P.S Stoolpigeon, thanks for your feedback!

[stoolpigeon]

I'm not sure what you mean but the arm to spindle distance is designed to be 222mm. This distance combined with the angle of the headshell will give you the best tracking performance. Assuming the cartridge is properly alligned that is.

If I remember the overhang for this setup is 18mm meaning the stylus tip will be 18mm further past the record spindle. This gives you what is called the effective length of the arm as 222 + 18 = 240.

My memory is fading so someone correct me if I'm wrong.

Bottom line is mount it at 222mm.

sp

[busoni]

What I mean is mounting the arm at 215.6, instead of 222mm. Would this be ok?

Then I would increase the overhang as much as possible to approach the effective length.

[stoolpigeon]

No, it has to be at 222mm. I just looked at a photo of a TD145 and it is essentially the same as a TD160. I have a TD160 I can check but not until the weekend.

I am also thinking I may have been mistaken when saying I mounted a Rega RB300 to a TD160 because I think it was a TD321 which is a different beast.

In any case I am happy to check my TD160 but not till the weekend.

sp

[sreten]

Originally posted by busoni
What I mean is mounting the arm at 215.6, instead of 222mm. Would this be ok?
Then I would increase the overhang as much as possible to approach the effective length.

Hi,

No.
If you reduce arm mounting length the actual arm becomes too long.
The problem becomes can you move the cartridge far back enough.
(The overhang 15mm (237 minus 222) does not change much)

If you understand arm geometry its not that intractable. Only when
you want the cartridge parallel to the headshell in the middle of the
headshell for the arms designed geomtery does the arm mounting
distance exactly matter.

Reducing arm mounting distance reduces the required effective
length and changes the optimum angle slightly (gets bigger).

What matters is getting a decent 2 point alignment within the
range of the headshell slots - cartridge angle does not matter.

Also note you can always set a null point near the record end,
what then is of interest is whether you get a null point at the
start of the record, as you move up and down the slots and
always setting the first point, the second one moves.

http://www.vinylengine.com/phpBB2/viewtopic.php?t=7141

Note that 15mm overhang is not part of the say Baerwald
alignment - its larger moving the cartridge forward which
would allow a shorter arm mounting distance (2mm or so).

# For Baerwald null points are 66.0 and 120.9 mm
# For Loefgren null points are 70.3 and 116.6 mm

Null points for the G-707 : 66.8 / 103.0 mm - not the best IMO.

Also I cannot see why the arm would not sit straight. You move it
backwards for the extra length and only sideways if you need to.
why twist it ?

/sreten.

[Ciscokid]

Hi,
Just now going through similar experience. I had a plate made to mount my Audio Technica AT1009 arm to my Thorens TD146. The arm length is 225mm. The person that made the plate stated that the length of 225 was all that was important. I dropped the plate off with my TT guy yesterday and hopefully will have unit next week. Both the guy who made my plate and my TT guy said would probably have to trim the sub-chassis a little below the arm mount to make fit. (The reason for me not to DIY)

Anyway, my TT guy knows what he is doing and I feel my Thorens is in good hands Can't wait. I have been listening to my spare Marantz but really miss the Thorens.

ck

[planet10]

Originally posted by stoolpigeon
The 222mm figure is all that matters so your plan is fine.

I remember fitting a Rega RB300 (which has the same mounting distance) to a TD160 and having to file away part of the subchassis to get the correct position.

sp

I put quite a few 707s on Thorens TD 160, 165 & 145. The subchassis needed to be drilled/filed out a bit IIRC (it was 25+ years ago) to get the 222, but the grace came with 2 big washers that made mounting it up easy.

dave