Two mechanical comments on warp removal.
1. Sometimes you just want the record flat for recording purposes. I have used small pieces of transparent tape with mostly good results. I tape over about half the lead-in-groove width and adhere the remaining tape to the vertical edge of my platter. This usually moves the warp a few inches away so that more tape is required. If more than three pieces of tape are required it’s usually not worth the effort as the warps will just continue to appear. Extreme care must be used in placing in the stylus into the first track to avoid catching the tape. I use the arm lifting mechanism to lower the cartridge.
2. Rather than using a soft foam washer under the record to reduce warps I use four small pads near the spindle under my rubber record mat. Their height is roughly 1/8 of an inch tall. I use a custom made screw on clamp that looks like a VPI but has a longer and lowered thread height. This is to accommodate an acrylic disc under my rubber mat. ( but, that is a different topic) Being careful to watch the amount of warp as it is tightened. Warps are usually reduced or can actually increase with excessive clamping pressure. Each record has its own requirement. The hold down pressure will vary a little with each album. I used to use a rigid spacer under the record over the spindle, but think pads under the mat are better.
I’ve experimented with record clamping for over two years to come to the conclusion in item number two. Item one is still a valid option but, item two is repeatable and less risky, (though maybe not as visually correct). Sometimes a mounted and clamped record will not be as flat as you think, but have a slight curve which can affect azimuth and antiskate. In my experience these undesirable variables are manageable and better than excessive warps. FYI, I have not tried a ring clamp for comparison.
