I have a Van Alstine 550 EXR and have rolled many tubes through it. While several 12AT7/6201 tubes do have that characteristic "rough" upper midrange, a few do not.
Telefunken 12AT7: most of the Telefunken tubes do not have a rough edge (I had one pair that were, but that was out of about a dozen pairs that I tried). In Frank's design the Telefunkens have great dynamics and bass but tend to be a bit "thin" in the upper registers (upper notes on piano, for example, don't have enough body and have too much of a "tinkly" sound). I have one pair out of that dozen, though, that trumps every tube I've tried--utterly stunning in every respect of sonic performance. Haven't been able to find another pair of Telefunkens like that set, and I use them only on special occasions.
The most consistently good sounding tubes in the 550 EXR are the Siemens-Halske 12AT7's (not newer versions from Siemens) from the early '60's. They are very smooth tubes (no grain), with good dynamics, bass definition and impact, clear mids, and highs. Very good soundstaging as well. They are available from Tubemonger on Audiogon but they are getting pricey. On the other hand, Frank runs the tubes so conservatively in his amps, that a pair ought to last many years. Those are the tubes I use now.
Next on my list are Valvos and RCA 12AT7 (blackplates for the RCA's). These, too, are well balanced tubes, but I notice some grain in the midrange that the Siemens-Halske do not have. If you can deal with a just a bit of grain, the Valvo and RCA have a fuller sound, especially in the upper midrange and treble, than do the Telefunkens, and can be found for a better price.
Amperex 6201: these aren't too bad (smooth sounding tubes) but lack dynamics and cover up too much detail. The bass is too soft and not sufficiently detailed as well. No midrange roughness.
The Sylvania Gold Brand 6201's are very smooth but thin throughout much of the sonic spectrum. That was surprising to me because I thought they'd be an excellent tube (I've heard them in other designs and thought they were great sounding), but they don't work well in the Van Alstine amp (and I tried a half dozen different pairs, gray and blackplate--sonically no difference).
RCA 6201 blackplates sound pretty grainy and unpleasant. But, as I said above, the 12AT7 blackplates are really good.
GE 6201 sound poor--lots of grain, not much soundstage, and pretty much unmusical.
GE 6201 5-star tubes have unbelievable dynamics and taught, dynamic bass and give a REALLY BIG forward sound. Too bad the upper midrange sounds so rough. When I installed them in the the 550 EXR, it sounded almost identical to the modified strapped pair of Carver Professional ZR1600 amps that I owned for a while. Those put out 1200 watts/ch, and I didn't think any amp could equal their bass response, but the AVA did. Other than that, I can't recommend those tubes in the amp.
Mullard CV4024 sounded terrible--thin, bright, and harsh. That was surprising, too, since I've heard them in other tubed amps and found them a wonderfully smooth tube. It just goes to show that the circuit design is critical when rolling tubes. What is good in one circuit can be awful in another.
Siemens 6201 of late 1970s and early 1980's production sound like crap in almost every respect in the EXR 550 and should be avoided.
The JJ Tesla tubes that Frank is now putting in as original equipment sound suprisingly good--better than many NOS tubes. There is a slight amound of midrange grain but they do so much else that is accurate, that I can forgive themthat for the price of the tubes. If you are on a budget, those tubes are hard to beat. For the bucks, the Siemens-Halske are better--not much of anything to fault with them. You might want to try a pair of Telefunkens--you can find decent ones on Ebay for much less than you'd pay for a pair of E88CC's/6922's of that variety.
