If you were a tube preamplifier manufacturer

I just stuck a '50's Chelmer (mullard) CV491 (12AU7 equiv.) in my AirTight amp, which tends towards neutral without being cool, and the expansiveness is luxurious, silences energized with presense...you know, Ralph, kinda like I recall your pre's sounding...(the rest in amp are: WE300Bs, RCA red letter 12BH7s, Natl Union 5u4)

My pre, a Joule LA200, comes stock with NOS tubes (a single Sylv green letter 5751 with a Sylv 6350 follower, GE 6EA7s and Russian OA2s in P/S), but I'm not sure that's unethical. Maybe sticking a CV491 in a stock unit would be that, but not a Sylv green letter - I think it depends on the circuit tube complement and the context, i.e putting the Sylv's in a circuit that needed five of em' might be disregarding the customer, but, on the other hand, paying for Kondo sure better get me primo NOS tubes with readily manuf-obtained back-ups! If not too many LA200/150s are manufactured, it outta work out over twenty years, a reasonable life for a tube unit, I would think.

Atmasphere is certainly right about the regulation, and others are right about the balance of tube characteristics when designing a unit. I just replaced the NOS GE 6EA7s in my P/S with some sweet-looking equiv's, Sylv 50s brown bases. Should be killer, right? Nope - collapsed into indistinct euphonics. Will a Sylv D-getter 6350 sound better than the the Sylv yellow letter I have now as stock? A green letter sq-getter Sylv 6350 that I plopped in yesterday had better dynamics but zero involvement, and the deepest field clapping went from flesh to distant crackles...I sub'd a world-beater Mullard U52 rectifier in for the 5U4 in my amp and everything went to compressed mush...

It all depends. NOS tube rolling IS a mine-field of gas leaks, unmatched sections, bad sellers, etc. but you do get better at it over time.

And, manufacturers are pretty good at finding a balance, if they have good ears to match their technical abilities, in current production tubes.

With that said, however, it has been my experience, that regardless of the pitfalls, an expeirenced hand can balance a sound further towards meaning/truth/beauty, and its perception, using NOS tubes. Which is the foundational reason that people go to them, I assume.

Hi Mr. Tennis, The 12AU7 does not have a lot of gain. If you are making a low output MC phono preamp, its not the best choice. I find the sound of 12AT7s to be better than that of the 12AX7 because you can get bandwidth, but we don't use negative feedback (phono EQ is passive) and that gets you a more relaxed presentation. With 12AX7s it is a lot more common to EQ the phono by using negative feedback. If running feedback on the 12AT7s I might be inclined to agree with you.

The bottom line is that while the tubes do have an influence, circuit design has an influence too and can often get around tube issues, although in the case of excess microphonics, there is not a lot you can do.

I agree that in general, the answer to this question is going to be tube specific. Take for instance the famed 12AT7. It is the general wisdom that the Telefunken NOS ECC801s is the best of the best. At $350+ per pair, they better be! But I have been blown away by the sound of a humble pair of Chinese (Penta Labs made by Shuguang) 12AT7s ($30/pr) and by Mullard 6201 gold pins ($150/pr). I am not crazy about the JJ Teslas (a bit bright and grainy). Of equal importance to the tube itself, is the selection stringency of the tubes. The 12AT7 is used as the first gain stage of my VTL Siegfrieds and triode matching, low noise and low microphonics are critical. I highly recommend Doug's tubes (http://www.dougstubes.com) for his superb personalized service in this regard.

As far as ouptut tubes, I have been around the block on this every way from Sunday. First, let me state unequivocally that I am not a fan of 6550s as I believe the KT88 is simply a far more musical tube. 6550s have an inherent grain structure in the upper midrange that is rather characteristic of the tube style regardless of manufacturer. Having long ago substituted KT88s in my ARC Ref 300 MkII amp for the stock 6550s, I strongly believe that an ARC owner who has not tried KT88s in his amp doesn't know the true potential of their amplifiers. Unfortunately, ARC does not support KT88s (unlike VTL) so you have to do it at your own risk. Fear not, the bias is identical and the switch should be seamless. Regarding KT88s I definitely not a fan of the famed SEDs from Russia. Once again, IMHO the Shuguang factory is making the best tube of this type on the planet. These are available in several iterations and bottle shapes. However my search has ended with the beautiful Shuguang 88-98 which is available from several sources. This is also identical to the Valve Art KT100 and the PentaLabs KT88-98, both of which are OEM'd by Shuguang for these labels. The Shuggies are spectacular tubes and puts the Russians KT88s to shame. The Valve Art KT100s have been running in my VTL Siegfrieds for some time now and are simply fabulous.