Okay, here's an update. First off, I think that the Kern wins the competition hands down as to the easiest preamp to test tubes in. First off, the tubes can be accessed from the rear of the unit, without having to remove the top panel. This is true for the Read Purity, too, but the tubes themselves stick out a bit further on the Kern than on the Read, which makes them easier to change (but perhaps more susceptible to damage). At any rate, this worked out well for my purposes, and besides that, the Kern is a unit which lets you hear the differences between the tubes fairly well.
I plan on gigging with my Kern on Saturday, so I wanted to give it something better to work with than the Sovtek 12AX7WXT+'s that were in there (I find these to be full and punchy, but somewhat limited in frequency response both high and low, and they are not particularly clear sounding; the lows are a bit sloppy compared to the rest, too, but I could hear these working well if you needed a more full tone or more punch). I didn't run down every tube in my arsenal, but I did try the following NOS tubes:
Telefunken ECC83 (smooth plates)
JAN Philips 12AX7WA
JAN GE 12AX7WA (maybe the same as the Philips?)
JAN GE 5751
GE 5-Star 5751
JAN Philips 12AT7WC black plates
JAN Sylvania 12AT7WC (again, appears to be the same as the Philips)
Mullard CV4024
I was able to test pairs of most of these, with the exception of the JAN Philips/GE 12AX7WA's (which appear to be the same tube) and the JAN Philips/Sylvania 12AT7WC's (which also appear to be the same tube). The 12AU7 tube that was in the Kern was the JAN Philips 5814A, and it seems like a fine tube, so I haven't tried swapping out anything different (yet!).
In tests like this, the Telefunkens are usually my "gold standard" for NOS tubes (with the JJ's serving this role for new production bottles). As expected, the Tele's sounded great. They are well balanced; full and robust, yet articulate; and they have the most harmonic content of any of the tubes I tested.
I have found myself really liking the GE 5-Star 5751's in a variety of preamps, and the Kern was no exception. This tube was very impressive and had even tighter lows than the Tele's. It had good balance; good harmonic content (not quite the Tele's level); clear yet full tone; but wasn't quite as "exciting" or robust as the JAN Philips/GE 12AX7WA. A top contender, for sure.
The "regular" JAN GE 5751 was a very strong performer also, though not quite as impressive as the 5-Stars. It is a sold performer, and is very well balanced, tonally (perhaps the best in this regard) and was again full yet articulate.
The Mullard CV4024 is a tube that I have come to admire, and it can work miracles in some preamps (my Chandler Tube Drive loves it), but I don't think it was a good fit for the Kern. For whatever reason, it comes across as not very well balanced, with a definite favoring of the upper mids. It is a very clear sounding tube, though.
The JAN Philips/GE 12AT7WC black plates were very impressive, and sounded much like a cross between the GE 5-Star 5751's and the JAN GE 5751's. Very good tube.
But, I must say that the JAN Philips/GE 12AX7WA's stole the show. These tubes just sounded awesome. They are much like the Telefunkens in overall tonal presentation (with very, very close to the same complex harmonic content), yet were as tight and full in the lows as the GE 5-Stars, but yet more robust and articulate overall than either the Tele's or the GE's. Very full sound, too, and a natural presentation. At least in the Kern, these tubes rawk!
I probably would have been more than happy with most of the above tubes, and in the Kern, it ultimately came down to the JAN Philips/GE 12AX7WA's, the Telefunkens, or the GE 5-Stars, with the 12AX7WA's nudging ahead at the end. If I were looking for a bit less output from my preamp (which was not the case with the Kern), the JAN Philips 12AT7WC black plates would be a top contender (as would the Mullard normally be - in the right preamp).
At any rate, it's late (past 2:00AM), and I think I'm ready for bed...
Later, Tom.
