Tobes, I think you miss the point of the SME's brilliant design when you say "as you no doubt realise, the SME V cannot be adjusted for effective length because of it's unslotted headshell mounts."
First of all, with an SME there is no need to adjust the effective arm length. With standard mount cartridges, the stylus/pivot distance will always be 233mm and the overhang will always be 17.8mm; which is achieved by sliding the base back and forth. When the overhang is at 17.8mm, the pivot to spindle distance is automatically correct (no need to drill a precisely located hole in the TT armboard) and the cartridge, when adjusted true and parallel to the headshell, will be tangent to the grooves at two points; and if the cartridge is standard, one need only add the difference to the 233mm and look up the adjusted overhang (DB provides a chart) and you're in business.
My only gripe is SME's roundabout and imprecise way of checking the overhang -- typically British! However, the design objective of the SME V tonearm was to fashion a headshell/armtube/counterweight-carriage out of a SINGLE casting, in order to provide the most rigid platform ever conceived for mounting a cartridge; along with their use of outrageously expensive ABEC 7 ball race bearings that nobody else uses. So you see, the single hole mounting arrangement is not an arbitrary SME affectation, but works to locate the cartridge in exact relation to the other tonearm dimensions. Setup is really a snap with a standard cartridge andnot much harder with a non-standard one.
Granted, one gives up certain niceties to enjoy the unique attributes of an SME V tonearm. For instance you can't adjust stylus azimuth; but on the other hand, any cartridge worthy of living in an SME V should come with perfectly aligned stylus/cantilever; and if it doesn't? well at those prices, it goes back for an exchange!!
Adjusting the SRA (some call it VTA) can also be frustrating if you're expecting to just 'dial it in' like a Graham or a Tri-Planar. And if your (most likely modern MC) cartridge is too short, you'll need a headshell spacer to keep the back of the armtube from hitting the edge of the record ;-)
My advice to Pureretro: You have one of the finest, most foolproof, durable and easy to set up tonearms in the world. I've owned mine since 1990, and in 2004 sent it to England for its 200,000 mile checkup (they couldn't find anything to adjust!) and to update the internal wiring. That was it! So keep the DB and buy a nice thin metal mm scale so you can check the overhang accurately. (You'll figure it out.) And if your eyesight is shot, just get a great big magnifying glass ;-)
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