Some terminology: 'Hard' vs. 'Soft' vacuum: The lower the pressure inside the tube bottle, the 'harder' the vacuum is. Supposedly, a 'soft' vacuum produces a smoother onset of distortion, and a 'round', 'brown' sound, suitable for Blues. A harder vacuum is thought to cause a sharper transition, better suited to Metal. So, a tube from a factory that has a reputation for hard vacuum is favoured by Metal players, while Bluesmen flock to a maker of tubes with 'soft' vacuum. I get a chuckle out of the way people describe these sonic differences. It reminds me of the kind of crap that gets spewed out by most audiophile magazines, each of which has an in-house reviewer with 'Golden Ears'.
Case in point: 6L6 vs. 6L6GC vs. 6550 vs. 7027A: You will hear some really heated arguments about the relative merits, and supposed suitabilty/unsuitability of these tubes. The inside joke is ... they're all the same! The 6550 and the 7027A are constructed in a more robust fashion which allows higher voltages, which in turn permits more output power ([u]if[/u] the rest of the amp's circuits can support it), but in electrical terms, all of them have identical response characteristics. A metal player raves about how much more power and crunchier distortion he gets since he swapped his 6L6GC's for 6550's. A blues player swears he has much better 'vintage' tone since he got those NOS 6L6's. These guys do have better sound, but the solemn fact is, the real improvements they hear are mostly due to installing fresh tubes, [u]and getting the bias adjusted properly[/u].
The 'problem' with the quality of Marshall amps sold in the States about 25 years ago, was that the EL34 tubes installed in England before the amps were shipped stateside, were being damaged in transit, resulting in high waranty costs to the distributor. So the distributor replaced those 'fragile' EL34's with 'rugged' military-specified 6550's. 'Thing is, the bias was not adjusted to accomodate the slight differences between EL34's and 6550's. The result was 'bad' sound. Well, certainly not the same sound as the Marshalls in Europe and Canada. Properly biased, a Marshall with 6550's sounds very good. The transition from clean sound to hellacious doom-bringer distortion is more abrupt than with EL34's, but that's a good thing, to a metal player who wants to change his sound with a small adjustment at his guitar, without losing much loudness.
Some of the stuff you read is a legitimate attempt to quantify the sound quality of different types, and makers of, tubes. A lot of it is delusional crap.
Musicians are artistic people. Most of us don't know, or want to know, what goes on inside the gizmos we use. Sadly, there are many hucksters who prey upon that vulnerability.
[Edited by Lordathestrings on 06-11-2002 at 01:00 AM]