If you were a tube preamplifier manufacturer

hi ralph

designing a preamp using a nos 12 ax7, e.g. is ethically neutral. obviously, the preamp would "work" with any 12ax7, but, would presumably sound different.

it is possible that using tubes currently in production, could still create performance differences as a result of variation in manufacture. there is no guarantee that the performance of a preamp is a "constant", from day to day.

so long as the buyer is aware of all of the facts about tube sonics and tube availability, the buyer can make an intelligent purchasing decision.

Mrtennis, correct. I personally don't use the 12AX7 because you can't get much bandwidth (the 12AX7 trades off bandwidth for gain), which is essential to getting phase relationships right. So if I need high gain, I use the 12AT7; you can easily get bandwidth out to 200KHz with that tube.

To get day to day consistency, the key is regulation, not the tubes. Tubes generally degrade slowly- over months, not days (unless there is a gross manufacturing defect), but day-to-day issues are usually something else- often the AC power. Good regulation will take care of that.

hi ralph:

of the following 12 volt tubes--12ax7,12au7,12at7 and 12ay7,
my experience points to the 12at7 as having the least pleasant treble response. i have used 12at7 in a cd player, amp and preamp.

why not use a 12au7 or 12ay7 ?

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.