> is there any difference between the 12au7 and the 12au7A?
Back in 1959, the -A version was "better" in some way. Sometimes built by the best workers, or better materials, or extra testing. In a few cases, the -A is rated for more abuse than the non-A version. (A 6L6GC will take a LOT more abuse than an original 6L6G.) And sometimes the same as the no-A version, just priced higher.
Back then, if you NEEDED a best-possible tube, you bought the -A versions. A radio station with a dead tube didn't have time to test several tubes to find one with low noise or microphonics, so they just bought the Premium versions.
As far as function goes: all the same for this project. No abuse involved. Low noise and low microphonics would be nice, but "Premium" tubes may have aged poorly, and some non-Premium tubes were premium quality. In DIY you can try several tubes, unlike on-air broadcast where it HAS to work NOW.
Unless you know your eBay seller, I would get common tubes from http://thetubestore.com/ Good service and good tubes. Argh, the dull 12AU7 has become "hip" and there are too many "sexy" versions.
http://thetubestore.com/12au7types.html
If you are poor, get the $9 ElectroHarmonix. I hope it is a good copy of the classic 12AU7. EH does market decent tubes.
For $13-$20 you can get classic US old-production 12AU7s and their military equivalents. These are the tubes I based my sims and calculations on.
Don't buy generic Chinese "12AU7" except for experimentation. Small changes in grid-wire layout may work fine in guitar amps but have less (or more!) of the vari-gain bend we need.
Don't pay $50 for sexy-brand 12AU7, unless you really want to, and have proven basic operation with a $10-$20 tube. The Amperex is a fine tube, but so is the GE or Philips or Raytheon.