I just purchased a glass platter to replace the MDF one on my Goldring GR1.2 turntable. I bought it used and it supposed to be from a Rega p3. As you may know the GR1.2 is built by Rega and essentially is a lower tolerance clone of the Rega P2. The glass platter is 12mm thick. Though the MDF platter is considerably thicker it is milled on the bottom so that it is only about 10mm higher than the subplatter. I believe with the new platter my VTA has changed. And now the stylus with the tonearm fully raised clears the platter by only 1mm when a record is on, using my Hebie mat. I'm considering either using cork or some type of acrylic plate between the tonearm base and the plinth to raise it to account for the change. What should be the clearance. How do I measure the correct VTA to decide how thick of a base to use? Any recommendations for the material to use between the tonearm base and the plinth?
Roy Gandy of Rega will tell you VTA doesn't matter - which is essentially true if you're in the ballpark to begin with. Now that you've added a non-Rega style cartridge to a non-stock platter with an aftermarket platter mat on top ...
Did'ja ever see one of those original style VW Bugs with a blown Big Block Chevy installed in the front of the car and only room for one modified seat because the 2 1/2" headers ran through the passenger side of the interior and the 5:88 rear end took up all of the backseat which had to be removed to fit the slicks on 10" rims where the motor used to be?
Ahem ...
You determine VTA by experimentation.
Just what you wanted to hear, eh?!
Unfortunately, it's the truth once you've started to modify parts. The guy with the VW didn't know what size rims to use until he'd flipped the car over a few times and figured he'd need some traction bars to keep the front end down and some honkin' tires to get a grip so you're in good company here.
You begin with the arm parallel to the record surface. Which record? The record that is about average for all of the records you play. If you play lots of vintage albums, it will be a fairly thin slip of vinyl (stinkin' DynaGroove!). If you play lots of 180-200 gram pressings, it will be thicker. So pick whichever you play the most because the vinyl (along with about a half dozen other things) will determine what "parallel" actually means since "parallel" doesn't really mean anything and it will change somewhat with each disc you play. If you play all sorts of vinyl and some's thick and some's thin, forget all of this and go listen to your CD player.
Have you ever heard that records shouldn't actually work? Well, here's proof.
The industry standard is (supposed to be - but isn't always) 20degrees SRA (stylus rake angle) which is way more important than VTA because not all styli are mounted to provide that SRA when the cartridge/tonearm are parallel to the disc surface. But it's easier to change VTA than it is to change SRA 'cause SRA means you gotta look really, really close at the stylus so everyone bothers with VTA and thinks they're doing this right.
Not all LP's are cut with an actual 20degree head so there's no guarantee that the correct SRA will get you the correct tracking of the groove which makes all of this a bit of a crapshoot when you figure vinyl thickness and VTF also changes the SRA.
Did'j a know that? Yep, dial in more or less VTF and you change the SRA but not the VTA. This gets to be fun after awhile 'cause it's kinda like counting angels dancin' on a pinhead. Once you've determined VTA/SRA you should try different VTF's which will send you scurrying down the same rabbit hole all over again.
But, you'll need a teensy-weensie little protactor and a really expensive microscope to check where 20degrees SRA happens to be with your particular set up. You'll also need to be younger than about 10 to be able to actually see that tiny little protractor anyway so this doesn't really matter all that much.
Just blow it off and go for parallel to the record knowing full well that nothing about this is ever right in the first place.
But you get to spend more time playing with your record player than you get to spend actually playing your records - so there's the benefit there.
So, now you want to determine how much you need to shim your tonearm to achieve either parallel or 20degrees SRA - whichever you choose because once you choose one you've probably messed up the other. I think we said we're going after parallel so let's stick with that for now 'cause it's all going to change here in a minute.
Try a few playing cards or business cards placed under the arm base to get some idea how much height you'll have to add in the final shim to get to parallel. Then remove a few cards and then add a few cards then remove one card and add one card and listen to about a dozen discs with each addition or subtraction. This should give you an idea of how you want the VTA/SRA to settle in for your "average" record.
Stop and refill your drink.
You should hear a marked difference when the back of the arm is higher or lower than parallel.
Now, you're wondering what kind of "marked difference" you should be hearing, right?
Ya'know, everybody asks me that when I give these instructions.
Well, I don't want to tell you.
Not that I don't like you or I can't give you generalities but it's better if you just listen for yourself and figure out what you're hearing. It's your system and not mine so you have to take some basic responsibility for this, you know?
Once you've determined which "marked difference" suits your preference for the broadest bunch of "average" records, measure the height of the shim you'll need. If you don't hear a preference or find your preference shifts with each disc, you'll have to decide just how much work you want to go through to get results from what is not a very exact science on an arm without adjustable VTA.
Of course, you could add a VTA adjuster to the arm but that becomes way more hassle than it's usually worth on most mid-line tables. But you do get to spend some more money and dick around a whole lot more so there's the benefit to that idea.
Let's assume you've decided on the height from parallel (or parallel itself if your one of those weirdos I keep heearing about) and you're ready to cut bait on a few shims ...
... and then I'll tell you most people would think a standard Rega shim would be the answer here. They come in various thicknesses and you stack together what you need to get any height you want. And Roy Gandy says VTA doesn't matter anyway.
Now, I'll tell you anything you place between the arm and the plinth will affect the sound so you might want to try a few things before you make your decision to buy the Rega shims. Of course, the Rega shims will give you the opportunity to spend more money and dick around with this way more than you should be - so there's the benefit to that!
OTH, you can try three teeensy-weensie ball bearings under the arm (they sell them where they sell the teensy-weensie protractor). That's kinda what Rega is doing in their top line arms so that might work well as long as you don't dig the bearings into the top of the plinth. Or maybe you want to try that too!
You can try the cork, it will probably sound sort of smooth and OK for a lot of less than great recordings. You can try the acrylic and it will probably sound kind of fast with a hard attack and be brighter than the cork. You might like that at first but you also might get tired of it after a while. There's always that problem of changing your mind easier than you can change SRA.
You can try anything you want but most everything you try will give a slightly different sound. Now you get to decide which sound you like overall for the average record (remember them?) that you play.
You can also try the arm snugged up nicely to the underside of the plinth and you can try the arm just secure enough that it doesn't move and change the cartridge alignment when you cue a record. You can try it both ways with all of these shims. Both of those will give you different sounds too - and once you got the various shims it's all free after than so it's not as much fun as spending more money but there is a benefit to be had in more dickin' around.
Then you might want to try a different mat now that you've got a different platter and you've tweaked the VTA/SRA/VTF. By the time you get that worked out it should be time to spend money on a new cartridge which can begin the whole process over again - so there's the benefit of never settling in on any one final setting.
And, if you do one of these things and then change the isolation of the table ... WHOA! then you're in for some fun!
Wonderful response Jan, as usual. Not an exact science I get. A practice of experimentation, as well as sweat and blood, I get as well.
I've got some work to do but honestly at the moment I'm looking forward to it - lot's of fun. I'm sure I'm being a little quick with my eagerness. It makes me think of Luke Skywalker when he said he wasn't afraid to Yoda. Yoda's response...in his Grover like voice:
"You will be.....you will be..."
Not that I'm calling you Yoda, Jan. But actually it's a complement as he was a Jedi Master.
I'll try with the tonearm parallel, as measured by a spirit level, once the table is level. I'll use the card method to experiment. Now to get my TT setup again, as it's been out of my system since I moved in with my parents...I'm missing the vinyl badly.
If this is a Rega arm, the arm tube is tapered and a spirit level will not indicate parallel when it shows level.
Try the alignment gauge that came with the arm or create your own gauge on your computer. The Rega gauge has a straight line on a piece of card stock that has a straight edge. Sight the arm for parallel with the line on the gauge when the gauge is sitting on the flat portion of the LP. Do not allow the gauge to touch the lip of the disc or the label area or else the gauge will not be sitting flat.
I haven't yet been able to follow through on your suggestions as I'm still looking for an appropriate place to setup my TT in my temporary digs. That said, I found an article which described the impact of VTA adjustment on sound. They described it generally as:
"knowing that adjusting the VTA so that the cartridge is nose down will increase the high frequencies and decrease the low frequencies, while adjusting the cartridge nose up will reduce the high frequencies and increase the low frequencies."
IMO that explanation is somewhat like saying, "salt will make the bread taste better". There's far more to this than that little blurb suggests. If you understand the implications of caster/camber setting/tire inflation and footprint along with ambient temperature on a car's driving and steering wheels, that will get you a ticket into the stadium parking lot for understanding VTA/SRA settings - you'll still have to find your seat and buy some popcorn on your own. On an acoustic guitar the analogy would be to string gauge/neck relief/action/intonation/saddle & nut settings/personal playing styles. On an electric guitar the variables increase with the addition of pickups.
All of these things are to some extent inter-related as will be VTA/SRA and the final setting will depend entirely on what you own, how you use the item and how sensitive you are to the feedback you receive from changes.
On another point, analogous to adding pick ups to the guitar or changing between front/rear wheel and all wheel drive with yaw control, keep in mind VTA/SRA is far less important with the stylus shape and compliance of the suspension. The footprint of the stylus in the groove (and the configuration of the groove which is ever changing side to side and up and down and subject to skating effects which also vary with stylus type) makes a very dramatic effect on how critical you should be about set up.
At the mid-level though it might be best to just concentrate on high frequencies and not bother with the rest at first, just as a beginning guitar student is more concerned about managing a barre chord and a city street only driver should be concerned about tire wear than either would be about the more subtle implications of further fine tuning. As you listen more and learn more about what you own and what you hear, then you can make further adjustments.
Now, to seemingly contradict myself, on the whole I would largely ignore the information about high frequencies - it might not even be true for your system - and concentrate on listening for what you hear with your own system and sources. What has been stated above might not hold true for your specific cartridge as anything more than a gross generality. You have your own particular priorities and they should be heeded first and foremost. Listen and you will learn.
If you subscribe, buy or have access to Stereophile, go back and read a few of MF's recent columns.
I read Stereophile off their website and when I'm in the book store but haven't yet got a subscription. I'm thinking about it because I love the depth of their articles. Sure beat Sound & Vision which is a good quick read but with no depth and not much in terms of audiophile gear reviews.
Once you've determined VTA/SRA you should try different VTF's which will send you scurrying down the same rabbit hole all over again. 
