EI valves were made in Serbia, as far as I'm aware off the old Phillips tooling.
There were a load of EI 12AX7s floating round a few years ago at a very reasonable price, and so I bought a batch. Most were very microphonic (to the point of being unusable), although the acceptable ones did sound good. It's interesting that these are now being offered as NOS at a premium price!
I used to use EH-12AX7s which I thought sounded great, and probably only had a couple of rejects out of 300 valves. However these were all from an older production run. Talking to one of our suppliers apparently there have been problems with all the Russian spiral filament pre-amp valves, which include the EH and Tungsol 12AX7s (I expect they all use a common filament assembly), especially in amps with cathode followers where the cathode is often at greater than 100V, and I expect these problems are due to break down in insulation between the cathode and filament.
I tried some "specially selected" current production Chinese 12AX7s as wasn't that impressed (they weren't that cheap either), so I have now moved over to using the JJ-ECC803s, which is a long plate 12AX7. This is slightly more microphonic than the standard JJ-ECC83, but sounds very good, and I've not had any rejects yet after using installing about 70.
I've not had the best experiences with the standard JJ-ECC83. A couple of years ago I had a spate of amps coming in with JJ-ECC83s installed in them that had either failed or become very noisy. Of course these might have all come from a single bad batch.
Given the ubiquitous nature of the 12AX7 in guitar amps, it still surprises me that no-one can make a consistent 12AX7.
