How best to eliminate LP warps

That one went right over the top of my head lol. Was wondering what you were referring to. The lad made me think you were english.  Thanks for the explanation, it restores my view of humanity 🙂

"No it is actually not, a wrapped record is stretched but all is depending on how bad the warp is"

optimize-

yes, that's what I envision what's physically happening. I'm thinking a  simple flattening during playback thru a VPI type outer weight makes more sense to preserve the LP, rather than any heating and prseeing/flattening of the disc?

Perhaps once an LP is warped, any attempts at straightening are technically(maybe not audible) compromising integrity of the groove? 

Because I'm a period press fan, I've got more than a few great albums, but they're real zingers-like a mogul ski run for the cartridge.

@lewm 

I think the premise behind flattening the LP by heat and pressure is the hope that the LP WILL return to its original flat shape, that the stretching represented by a warp, if it is indeed stretching, will be eliminated by a corresponding shrinkage.

Yes we might think and hope that shrinkage will happen.

Why should it shrink when we pressure it between two surfaces? It is not rubber.🤔

If you increase one, two or all three of the parameters temperature, pressure or time. It will only result in a bigger and thinner disc an it will never shrink back to its previous shape. Why should it? 🤔

That is exactly what is happening. The disc will be flatt (that is not a problem to achieve) but it's diameter has also increased by a very little bit when we apply as little force and heat we can get away with to just make I flat but the diameter will not magical decrease.

As I said it is small and something probably not something we can easily detect with small warps but the mechanics is the same. But if you have severe warped records you will also be able to flatten them out that part is not a problem to fix.

But you have now instead a severe wobbly grove that is not perfectly circular shaped. Yes you can only convert from one problem (warp) into another (egg shaped). When there is no shrinkage going on..🥰

I have first hand experience with a very warped record that I flatten out carefully over several iterations when I applied pressure and temperature during the whole night for each night and iteration when I did not get it as flat as I expected the first iteration. At the end I got it acceptable flat! (The iterations and time indicated that I didn't use to much heat so that it should destroy the record.) Yes, my happiness over that i had got the record flatter were shortlived when I saw the canteliver and cartridge work like crazy side to side! Never seen something like that in any of my 700 records and I concidered as still unplayable it looked as the needle should be thrown out of the grove, very disturbing. (Remember it were a badly warped record. So less warped will most likely give a less egg deformation). All the efforts was in vain. 🙁

Do you have any experience with record shrinkage? (Smaller warps will give smaller side to side movement but can be still OK and in tolerance, but it is impossible to know if it is little bit worse or not then it were before the warp event.)

@drbond , I have never measure the distance off center. I am not sure what for. Just watch your tonearm with a reference nearby. You can see the tonearm tonearm drift back and forth at 33 times per minute. That record? I'd guess it was somewhere around 0.6 mm off. The standard is within 0.2 mm. 0.6 is huge. That means the arm will drift back and forth 1.2 mm. 

optimize, You make a good point and it shows good thinking, but I question whether what you say is true for all warped LPs, because in many cases the warp you observe is compensated for by several smaller less visible warps in the opposite direction.  Let's call it "upward" vs "downward" warps.  This is one reason why trying to get around the problem with a central clamp won't work for both sides of that LP. Thus the LP itself has not lost its basic dimensions; they are just distorted in vertical space.  That should be reparable when you heat and flatten.  I don't doubt your story regarding the one example. 

Yes, you are right that there is "different" types of warps and severity of them.

The point is to get a better understanding of the material and how it works/act and flattening is not always the end goal and the assumption that we are done and all is going to be back to how it were only if we get it flat.. when there is other things happening and getting worse as a by-product. That we don't concider and take into account that is good that we know of and take it into account. (Oh, this record is to much warped or have a type of warp that makes that individual record "unrepairable" so it can be played again.)

Yeh the server warped disc experience I described before were a great educational thing for me that made me think about what really happened and how it works. When you have bigger warps then it is easier to see the actual effects of the flattening process. When the effects also will be greater.

I went into the project with the mindset that only I straighten it up to flat then all will be good and restored back to its original shape/state as a goal. And did not concidered any other factors that were a mistake from my side that I learned from.

And I probably think that many others also focus on to get it flat and think that it will be 100% restored back to its former shape. (But for smaller warps no one will notice any change or degradation even if they are there.)

@lewm , the solution is simple. Get a turntable with vacuum clamping :-)))))

And, a record flattener. 

@lewm , if i understand your situation, you have a very  small percentage of your LP collection with warps that concern you, BUT , when played, you dont notice a mistracking or audible problem....may i ask why you why wish to attempt to remedy this?  If its not broken, dont fix it.. I have a few similar, as well, BUT also dont note mistracking or pitch problems....it has bothered me, but I just shrug my shoulders....

but they’re only for sale in Japan, and only at 100V

@drbond  They've been available in the UK for ages...

Dear jw, Your assumptions are not quite accurate. I normally do not fuss over warped LPs, as I observe them very infrequently in LPs in my collection of 2500 or more LPs, but I would not dare to guess what fraction of that total is warped. However, in the last 2-3 weeks, I observed on separate occasions that two of the LPs I selected for play on those particular days (to be clearer, one LP per listening session) were warped "severely". Especially affecting the outermost one or two tracks on each. Since I only noticed the warps when the LPs were already spinning, I was also able to observe that both LPs sounded great, were tracked well by two different high compliance cartridges (B&O MMC20CL and Ortofon MC2000 mounted in Dynavector DV505 and Triplanar, respectively), had very quiet surfaces, no ticks or pops, and great music. Because of the latter good qualities, I am loathe to discard the LPs. Normally if an LP is as warped as these two, I discard them; they usually have some other defect that justifies trashing. So I started wondering what are the latest and most cost effective methods for flattening; I would not spend the $3000 cost of the Furutech, which I already knew about, because the problem is so unusual in my personal experience. I would not make the general statement that I don’t have a problem playing warped LPs, only that these two LPs play OK.

@optimize @jw944ts 

Mistracking and wow are only part of the problem. If you want to get the most from your vinyl, you have to set all TT parameters pretty accurately. That includes stylus azimuth.

A dished LP will significantly mess up your azimuth adjustment consistently for the whole record. A warped LP will severely mess up your azimuth every 1.8 seconds.

My DIY air bearing tonearm / Koetsu setup is sensitive to small azimuth changes - 6 minutes of arc from perpendicular is clearly audible. The effect manifests itself as a loss of beauty tending towards grating coarseness. It's not better when it comes and goes.

A 1 mm dish over the 150mm from edge to spindle is 1/150 of a radian, or 23 minutes of arc. Warp similar. 

My reflex clamp reduces most imperfections to nil, which works for me. YMMV

By 'most imperfections' I meant dishes and warps.

Terry, you wrote, "My DIY air bearing tonearm / Koetsu setup is sensitive to small azimuth changes - 6 minutes of arc from perpendicular is clearly audible".  That's remarkable because it infers that your Koetsu is perfectly constructed internally.  If perfect 90 degrees of azimuth yields audibly superior results even in comparison to a few minutes of arc off 90 degrees, then that would be the case.  There is a good case to be made for setting azimuth at 90 degrees notwithstanding electrical measurements of the results, but it usually means a slight compromise in crosstalk perfection, because most cartridges are not perfectly constructed. But your general point that warps throw off azimuth as well as speed is well taken. 

@lewm , you hit on it. It depends on what is playing at the time. You can hear the warps on prolonged steady notes with quieter backgrounds. I think solo piano is the worst. The violin note on The Lark's Ascending would do it.  

All the vicissitudes that plague the LP are part of the reason I'm just not as fanatical about the format as I used to be. It doesn't hurt that Hi-Def (and even CD quality) streaming can sound really, truly good through my system as it is currently constituted.

@lewm 

Sorry, I didn't mean to imply that the Koetsu was perfect. Although, it may be for all I know - all I do know is, that it's close.

I just meant that after optimizing azimuth by ear, a 6 minute change is clearly audible. Since my arm wand is artisanal and a one-off, it's almost certainly out more than the cartridge, and all that uncertainty led me to construct for adjustability.

I don't like to adjust for electrical measurements because, according to Peter Lederman of SoundSmith, many of the better cartridges have excellent separation on one channel but superb separation on the other. Equalizing crosstalk can lead to a stylus at an obscene angle, and that was indeed the case with the K.

Which is why I now adjust by ear. YMMV.

Back when I had an extensive vinyl collection I used 2 pieces of plate glass.  Put the record between them, set the sandwich in the sun early in the morning and retrieve it at night.  Here in the desert southwest it worked like a charm.

@piaudiol The use of an environment using the available Ambient Heat and not using heat applied as a point source or forced heat, is as said earlier, a low risk way to accelerate the reforming of the Vinyl Disc.

Sunlight Directly behind glass can reach 60 Celsius, so a Thermometer used to select a position that is creating a Temperature between 20-40 Celsius will be a lower risk method. 

Your success with creating the wanted reforming of the Vinyl Disc within a short timescale is good news and should serve as a guideline for methods that will create an ambient heat to assist with the reshaping.  

The best way is the AFI Flat record flattener. However it is also expensive.

@edcyn, you nailed it. Bowie was heavily into cocaine at the time and he was loosing it at times with severe paranoia. A lad insane.

@laoman , the AFI unit is a similar situation to the Furutech. My problem with the AFI is the silly stupid marketing on their web site. Temper PVC? You temper hardened metals to remove some of the brittleness. You can not harden and temper a plastic this way, utter BS. I personally won't tolerate that even if it was the best flattener out there. Furutech, as far as I have seen, does not feel the need to resort to this. To me this means their unit works so well they do not need to resort to misleading marketing. 

@terry9 , Peter Ledermann is absolutely right on that one but you can not adjust azimuth by ear. You adjust it by site which is easy to do. The best way to do it is with a mirror and a USB microscope but, I did it by naked eye for decades. The stylus has to sit symmetrically in the groove or you can accelerate both stylus and record wear. The message here is do not waste your money on a Fozgometer save and buy a USB microscope. For $300 you can get a really nice one.

@edcyn , I also share your enthusiasm for digital sources but for the majority of my life I have endeavored to improve LP playback and until I get to a point where little room for improvement is left I will continue to face the challenge. Digital is just digital, there is no challenge.  There is also no amazement. That a trench dragged along under a rock can sound as good as it does is amazing and as a collectable albums beat files by a long shot. IMHO the absolute best sound comes from properly mastered high res digital files. They are a 45 rpm record without the noise.

Who knew warps were so popular?

@mijostyn

"you can not adjust azimuth by ear. You adjust it by site which is easy to do."

Don’t agree. The Koetsu sits so low that even an approximate setting by sight is very difficult to impossible. In any case, sight is approximate to perhaps one degree, unless you have a microscope on a stand with crosshairs, which is registered precisely to the plane of the platter. Otherwise you are at the mercy of too many other variables, such as the construction of the tonearm wand - which is an absolute dealbreaker for me, as my wands are artisanal.

Speaking of USB microscopes, which ones do you find best? I looked a few years ago, and couldn’t find anything useful at less than thousands. A USB microscope would be useful to get me into the ballpark, and keep me there. A real timesaver.

Last, it is quite possible to adjust by ear. I use two tests: choral music in close harmony (Harmonia Mundi has lots of these; looking for ’sweetness’), and folk songs in dialect (looking for clarity of diction). But that’s harder to do without azimuth-on-the-fly.

If it's playable warp, don't eliminate it. If it's not playable warp, you can take risk of ironing it through cloth

Don’t buy LPs…get snap crackle and pop cereal instead. Lol

Cheersc

@terry9, Yes, the Koetsu is a PITA. You do it by placing the tonearm down on a mirror with good light from both sides and your eye or a USB camera directly in front. This technique quadruples your accuracy. The stylus and it's reflection form an hourglass. You simply adjust azimuth until the hourglass stands up perfectly straight. It hardly takes any practice once you get the set up down.

This insistence on using hearing to make accurate adjustments on turntables is bordering on the absurd. Human hearing is in no way shape or form calibrated. It is like trying to use a screwdriver to remove a bolt. Wrong tool for the job. You do yourself, your stylus and your records a gross disservice by doing it that way. I do not mean to be harsh but it is important for everyone to understand this. If you insist your hearing is sufficient for the job than your ego is taller than the rest of you. You will not really hear a difference until your stylus is way out of line. 

@terry9, I for got to answer your question on USB microscopes! Sorry about that.

There are many hobbyist scopes out the $50 to $100 price range that are good enough for azimuth and VTA adjustment but not stylus wear. The real problem is positioning the scope so that it's aim is right and it has to be absolutely still or you will get sick watching the image. It takes a bit of creativity to get the job done. For an all around brilliant scope that you can use for stylus wear and has a staging system specifically for turntables the WallyScope is the one and only scope for the job and at last count it was $1250.00 which makes it a luxury item for most people.  

For $100 and some velcro you can get the job done for azimuth and VTA. It is also a lot of fun and USB microscopes can be very handy for a variety of jobs.

Yeh adjusting azimuth ..is something that takes some time to realize how many factors there is..

For example you can dive into the rabbit hole and use for example a azimuth fozgometer or like I did with a digital oscilloscope.

There is many ways/methods to adjust one and each parameter..

Ok here is the kicker if you use one of the tools mentioned above with a calibration LP disc (just a good one of those are pretty pricey in my book) you adjust it perfectly to that individual disc.

But how is a calibration LP disc manufactured? Yes you guessed it. it is manufactured as a LP is usually done. Hopefully with some more care in the setup of the lathe that includes setup of azimuth with the sapphire cutting needles on the lathe that will cut into the lacquer.

So more precise you have just adjusted your azimuth to match the azimuth that the sapphire cutting needle HAD during the initial production of the mold for your specific calibration LP disc..

If you get different manufacturers/labels of calibration LP discs they will not be perfect if you measure those. 

Why.. yes you guessed it there is still some small variance in azimuth setup from sapphire cutting needles setups from lathe to lathe and time to another time.

My approach is when knowing that is to set it perfectly straight like mijostyn have explained.

The reason is to not adjust it after a/one specific lathe cutting occasion, only.

The second reason is that all of the different albums we own has its own lathe azimuth adjustment when they're cut. So the idea is that with a straight setup you will on average over the whole collection be OK. And not be better on some and then worse on some of the others..

In assumption that the cartridge manufacturer has mounted the diamond, canteliver and so on straight..

But the super best way is to get azimuth as good as possible is using one of the tools mentioned and getting different calibration LP discs from of different manufacturers and labels and take a average then maybe you are still little better on it on average.. and probably adjusted also for deviation from the cartridge manufacturer..

Anyway that is nothing that anyone is doing maybe one individual on 2 billion people may or can do that. 🤣

But adjusting VTA/SRA is a different story and has physical properties during cutting. And depending on your stylus shape and so on there is more or less ideal ways to make that adjustment.

@mijostyn

"You simply adjust azimuth until the hourglass stands up perfectly straight. It hardly takes any practice once you get the set up down."

It seems to me that the adjusting eye or lens needs to be perfectly normal to the cantilever, which means simultaneously adjusting two right angles. I doubt if anyone can do that without an adjustable, stable platform, repeated accurate measurements, and statistical analysis.

Whereas anyone can listen to a dozen challenging, flat, LP’s and set for the ’best’ sound and take measurements. Then throw away the two top and bottom measurements, and set to the mean or the median as indicated. At least, anyone with accurately calibrated azimuth-on-the-fly and a statistical background.

YMMV

Actually, this whole discussion leads me to the conclusion that we are chasing a ghost. As @optimize notes, there are irregularities in LP manufacturing, whereas we are treating the LP as identical precision instruments that just need to be set up via the cartridge.

Instead, we should be looking for a tonearm that can be easily and finely adjusted on the fly to suit each LP as it is played. Preferably, the tonearm should have a precise measuring system built into it’s adjustment mechanism, so that these numbers can be noted on the record sleeve, and the adjustment made in two or three seconds prior to each play.

And, of course, we should remove warps and dishes from the equation, coming back to Lewm's point.

All of this discussion about azimuth involving USB microscopes, test disks, oscilloscopes, Fozgometers, hour glasses, adjustment for each disk and such leaves me LOL. Better to put more faith in one’s hearing.

Assuming you have easy adjustment for azimuth, and if your stylus is symmetrically placed, adjust roughly for level by matching your cartridge parallel to its reflection on a disk. It doesn’t take much of an eye too get very close. Use a flashlight and/or thin mirror if that helps. If the stylus is a bit crooked, use a mirror and do the best you can to have the stylus as vertical as possible.

Then put on your disk with the best, widest, soundstage in your collection. Listen carefully as you adjust azimuth just a bit, first one way than the other. If the soundstage widens, continue slowly in that direction. If it doesn’t widen you’re done. If it does widen continue in that direction to the max soundstage.

it's scary to see your expensive cartridge and stylus on a badly warped record.

@terry9 , we already established that the two channels can have very different cross talks. (Peter Ledermann) So, by taking measurements you can assure that your stylus is digging into your records. If you want to do it this way then confirm it with a visual inspection, OK. I think it is a waste of time and money. Money that would be better spent on a USB microscope.

@melm , you are a little late with that idea but you are right on except with a good eye you are far more accurate visually than your hearing. If it make you happy.

I have made recordings with the arm set up correctly and with the stylus 2 degrees off and no one can reliably identify the right set up switching back and forth. 

@mijostyn

I think that you are talking about electronic measurements. I am not.

I doubt that you can set up correctly with your procedure. Therefore, I suggest that you may well be comparing 1 degree out clockwise to 1 degree out counterclockwise, which might be difficult on some records.

Further, what system are you using? In what state of repair is it? Perhaps the differences are swamped by TT noise. Or your tonearm may wobble. As per Lewm’s point, the records and their dishes and warps are also a factor.

In summary, your inability to find an azimuth effect is not evidence of lack of same, except to you on your system. IMO

@terry9 I am talking about any procedure other than direct visual adjustment of azimuth. The mission here is not to get the best sound, it is to get the stylus perfectly perpendicular to the record so that it does not dig into the vinyl. On a well made cartridge this will also give you the lowest crosstalk. If your cartridge is skewed it will not give you the lowest crosstalk but it will protect your records until you get a better cartridge. 

Perpendicularity, especially with the mirror method is easy to see and your eyes are 10 magnitudes more accurate than your ears. You do need to achieve the right set up particularly the lighting. A flashlight on either side will do it. A magnifier also helps. I use the one on the SmarTractor which is excellent.

Though late to conversation i think that azimuth is not the holly grail of alignment.

Too much effort with little return but fortunately once there you do not have to look back again. Set and forget.

Overhung, VTA/SRA, VTF, are definitely more important adjustments and should be accurate before any attempt for setting up azimuth. Bias included.

If a system is correctly designed and adjusted then the only source for error regarding azimuth would be any imprecision of the cartridge assembly. So we are trying to correct a faulty product for either stylus or generator, that would require two different settings. Impossible. I choose to check the stylus.

How many incorrectly, by a huge margin, assembled cartridges have you met, that would require so drastic adjustments? Twisting a headshell may help but they can become loose and have slack, so would put azimuth off plus they have to many connections, so finer details would be lost. My understanding is that an arm should not have so much freedom as it would introduce more errors than needed.

A visual set up with the aid of good lighting, magnifying glass or microscope ( much better than shooting macro photos), mirror and a test lp or one with good content of high frequencies would be enough to get you perfectly close (if needed).

Started with wraps and we talk about azimuth, audio is great.

Orb user here. Got it from ebay.  I've flattened thirty or forty records so far and they've all come out much better and although some still aren't perfect they play just fine. Saved me from the PITA of exchanging. Mine is rated 100-125 volts. As to audibility depending on the shape of the warp it's absolutely audible on my system. I have found the rate of change of the warp seems to be the most important factor...not sure if I'm wording that correctly.  If you only have a few warped records I can't see the value unless you're OCD or have money burning a hole in your pocket.

Hi Folks,

Here's a guy 😉 who made some modifications to be able to use a VPI Periphery Ring clamp on a platter it wasn't designed for. Together with a center clamp, best warp solution I am aware of - and reduces LP surface resonance reactions from lower compliance, more significant VTF carts.

Cheers, John

https://www.audiocircle.com/index.php?topic=124081.msg1307306#msg1307306

petgo, I guess the only clue an end user would have that a cartridge is internally misaligned would be if he attempts to set azimuth electrically (equal crosstalk in each channel) and this result requires tilting the cartridge in either direction from a 90 degree orientation of the stylus tip to the groove walls. By that criterion, most cartridges are not perfect, but they are not far off, either.

I have used the Vinyl Flat for several years.  Initially I would borrow from a friend that has one about 10+ years old.  I believe that pouch only had low and high.  I would use the high, regardless, for about 3 hours and worked well.  I eventually decided to get my own and the high setting for 3 hours melted the grooves.  It was pretty wild and thankfully nothing special.  I went to medium for about 2 hours and eventually found the sweet spot of medium for 3.5 hours.  

My experience is that this works well for new vinyl.  Any of my collection of older vinyl I have not had any success.  I don't have as much of a problem with older vinyl so there's that.  

Pressing plants basically suck for new vinyl as it can be so awful right out of the sleeve on new releases.  I used to return them to amazon but got tired of the fight as the replacement was often the same.  Now on new vinyl if it is warped at all, it goes right in the Vinyl Flat for the 3.5 hours and is all good.

I recently bought a second copy of the Cardas test LP. I’ve been using it for years to warm up cartridges that either need break in or have been out of use for long periods, which happens when you own 5 turntables and 2 systems. Anyway, the new copy was very badly warped right out of the sleeve. I of course immediately returned it to MICHAEL PERCY audio, and he replaced it immediately, no questions asked. Still, it is surprising to me that Cardas could let such a thing go out the door as a test LP, of all things.

I have thousands of 45 RPM records. I got 2 1/4 inch thick pieces of glass made with the center cut out for the label. I put the 45 between the glass, taped the glass together, and sat it on top of light bulb. (about 6" from bulb) I would give it 30 minutes and take it off and let it cool. It only failed a few times on the very thin reproduction 45 records. It should work on an LP.

In my audio system I have a Sota vacuum turntable and must say it works great. Basis make one that work great also.

Friend,

No! No! No1 The physics will not work for you. Once a round object is warped it is really not round w/an eccentricity of 1.0! Nothing you do will ever bring the eccentricity back to 1.0! In essence you will now have an ellipse and your lp is not round. Watch the tonearm swing and make your lp sound on the weird side. Benn there, done that! Also, too much pressure w/heat = NO MORE GROOVEs!!!!

Not so sure about that jazzguy.

Say:

1. a record is pressed as a perfect circle

2. then is warped, e.g. by heat or pressure, and assumes an elliptical shape with a warp

3. then heat and pressure could return it to a perfect circle without a warp.

Or am I missing something?

I see Jazz guys’ point, if you assume that a warp represents a local stretching of the vinyl, such that there is excess material in the horizontal plane and this forces the warp to occur. Like pavement expansion on hot days. If we assume that that is the genesis of a warp, then it must also be true that the LP will have thinned out in the region of the warp in order to produce the aberration. I have never observed such a thing, and I have never stored any LP ever under conditions that could conceivably cause vinyl to “melt” and warp in that fashion. So this is my reasoning why I also doubt jazz guy’s thesis. But I don’t doubt he saw what he saw. Perhaps the occasional LP comes from its factory in that condition, the warp having been caused by uneven heating in the press. But don’t we assume that most warps occur while the LP is in our own hands, in our own house?

Dear @jw944ts  : "  if i understand your situation, you have a very  small percentage of your LP collection with warps that concern you, BUT , when played, you dont notice a mistracking or audible problem....may i ask why you why wish to attempt to remedy this?  If its not broken, dont fix it.. "

You are totally rigth an accurated.

This thread goes around 100 posts in a thread that after the jw posts has no sense. It's the classic op style and he has the rigth to do it. So go ahed.

Regards and enjoy the MUSIC NOT DISTORTIONS,

R.

Thank you@rauliruegas. Sanity…

Interesting post Lew.  I believe I have a similar situation.  With a number of records in my collection (possibly a majority?), if I watch for arm movement while playing a record I can note slight side to side or up and down movement.  But I do not hear distortion with many of them.

Considering warps, I agree with you and optimize, there are different types of warps which may be worth identifying.  The two I see most often are "dishing" where the entire outer portion of the record is raised (when viewing one side).  With that, a weight or clamp may be effective, but only when the cupped side of the record is facing downward.  No help at all when the cup faces upward.

The second type warp I've experienced I call the "potato chip".  The outer portion of the LP has undulations.  I've never tried a record ring but assume it may work if the undulations are not excessive.

Regarding hearing the warps, again I expect there are different types of experiences.  Anyone who is sensitive to azimuth may notice the cupping type when tracking outer portions of the LP.  The potato chip warp(s) may change tones, particularly on sustained notes with piano or voice.  Of course if the warp is severe enough then mis-tracking or even the stylus leaving the groove will be heard.

Unfortunately I don't have any answers to offer from my own experience.  I did obtain two 12x12 panes of glass to compress a warped record in the oven.  But then I read too many failures with that to be brave enough to try.

@pryso , very true about warps and dishing. I send the dished records back. It is almost impossible to seal the convex side.

In order to hear the warp easily you have to have some steady tones like a cord held on a piano. Sine wave test records really demonstrate the effect of warps and off center spindle holes. When a turntable compensates for everything except the offset spindle hole which there is no compensating for, with vacuum clamping and rock solid speed control there is a sensation of solidity which greatly adds to the illusion. I can not tell you if this is psychological or not as I have not performed that experiment. It seems to me to be quite obvious but, out brains are powerful items and you should never write them off.