I have always been somewhat leery of amplifiers that claim to support a bunch of very different tube types without changing anything. It just seems not possible to be optimal for all tubes, and in some cases it may not be optimal for any tube.
Exactly.
Audiophiles routinely issue claims of substitutions which cannot possibly work—the most common being swapping a high µ tube for high Gm, or vice-versa, typically in front-end stages or splitters—with confirmation bias about the resulting "wonderful sound", actually distortion. From such specious claims one might conclude that it probably doesn't matter if the new tube properly functions, or if it lasts longer than an ice cream cone dropped on a sidewalk in July. Too many audiophools have more money than good sense, or even a rudimentary ability to hear distortion, and this warps perspective.
Its not just the bias points that are different. You get into different load conditions for a given voltage with different tube types, and different drive requirements as well. I just have to wonder how much compromise takes place under such conditions. Its perhaps a very vintage design philosophy, but I learned much of what I know from reading old books, and the old timers were not designing things with the notion of tube rolling in mind. They set a goal and selected components to meet that goal, and the amplifier was built with those parts to do that job.
Damn straight.
Output tubes have different characteristics and no single magickal circuit is going to be able to divine same and drive in any optimal fashion.
The driver circuit would need to measure rP, gain, transconductance, perveance, contact electrons, internal electron scattering, inter-electrode capacitance, maximum screen voltage, maximum grid voltage, etc. and then magickally determine the cathode resistor, screen voltage, grid leak, plate resistor, and output transformer impedance, and then rejigger those. (I'm ignoring issues of grid current limits, because grid current generally causes distortion and is not a good way to design a low-distortion amplifier.)
Beyond the magickal circuit, a specific circuit topology would be controlling for whatever tube was substituted: auto-bias (cathode bias), self-bias (grid-contact bias), or fixed-bias. Not all tubes do equally well, particularly as the device ages. Then we have issues about load lines and the optimal impedance of the output transformer's primary winding.
None of this is addressed by the magickal device, and I note that it's circuitry is labeled Top Sekret, KEEP OUT!
I equate "tube rolling" with random mucking about in the hope of a happy accident.
But that implies the accident could be successful; this is more or less impossible when tubes of different types—such as 12AX7 for 12AU7 where either voltage or current gain is required and the tube selected has the wrong parameters—are swapped, nearly at random.
Tube rolling, particularly amongst tubes with significantly different characteristics, is a hobby which, like dowsing or sceances, is driven by the most tenuous of correlations and confirmation bias and is not rooted in science, engineering, testing, or even objective reality. It should be debunked for the witchcraft it is.
Here's something I previously wrote on this subject, slightly edited since then:
HiFi enthusiasts often want to have a personal relationship with their gear and some bragging rights. Having little design or analytical skills, they substitute tubes, replace capacitors, tinker with component values (generally to the detriment of the circuit) and claim that their golden ears detected a miraculous improvement that only they can hear, now claiming that
while there are many amplifiers, this one is theirs. The phonier the words sound, and the more it sounds like an oenophile describing wine, the more likely it is that the claim is not grounded in reality.
I am reminded of Hume's commentary on the evidence required for extraordinary claims.
"The plain consequence is (and it is a general maxim worthy of our attention), That no testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavours to establish: And even in that case there is a mutual destruction of arguments, and the superior only gives us an assurance suitable to that degree of force, which remains, after deducting the inferior. When any one tells me, that he saw a dead man restored to life, I immediately consider with myself, whether it be more probable, that this person should either deceive or be deceived, or that the fact, which he relates, should really have happened. I weigh the one miracle against the other; and according to the superiority, which I discover, I pronounce my decision, and always reject the greater miracle. If the falsehood of his testimony would be more miraculous, than the event which he relates; then, and not till then, can he pretend to command my belief or opinion."
Hume, David: An Enquiry Concerning Human Understanding, "Part 1", Chapter 10.13 (1748)
In the next chapter Hume cautions, "
A wise man, therefore, proportions his belief to the evidence."
(Yes, I actually slogged through Hume and Kant while pursuing a philosophy double major in addition to an engineering degree. The epistemology occasionally proves useful in debunking pathological science and fanciful physics.)
Another observation I have posted is from M. Le Comte Laplace:
"De ce qui précède, nous devons généralement conclure que plus un fait est extraordinaire, plus il a besoin d’être appuyé de fortes preuves. Car ceux qui l’attestent, pouvant ou tromper, ou avoir été trompés, ces deux causes sont d’autant plus probables, que la réalité du fait l’est moins en elle-même. C’est ce que l’on verra particulièrement, lorsque nous parlerons de la probabilité des témoignages."
M. Le Comte Laplace, Pierre-Simon: Essai Philosophique Sur Les Probabilités (Sixième Édition) (1840)
In case your French is worse than mine, I roughly translate that (the Google translation is execrable, as it often is, and I am maintaining Le Place's archaic sentence structure, though it may be awkward to modern readers) as:
From the foregoing, we must generally conclude that the more extraordinary a fact is, the more it requires support by strong proofs. Because the attestor possibly is deceitful, or had been deceived, these two causes being all the more probable, since the reality of the fact will itself be lesser. This is what is particularly seen when one discusses the testimony's probability.
M. Le Comte Laplace, Pierre-Simon: Philosophical Essays on Probabilities (6th Edition) (1840)
The biggest improvements in HiFi come from mundane changes which reduce noise and stabilize the circuit, preventing oscillation or ringing in the kHz or MHz band which saps power and causes distortion. (Most designers never look for it.) This includes proper lead dress to avoid hum and noise, using better components (metal film instead of noisy carbon composite resistors with other undesirable properties, and polypropylene capacitors instead of ceramics or electrolytics in the signal path), good soldering techniques, and adding some components to prevent oscillation and ringing. All very doable to anyone able to learn simple skills.
None of these changes are conceptually difficult to understand or expensive to implement. I would sum them up as mostly straightforward, well-understood, and mundane. Which is the best type of improvement to make. But the audiophile seeks a reduction of all issues to a single and simple—one might say "simplistic" without hyperbole—cure for sonic bliss that lies within the means of the non-engineer, non-designer, and unskilled hobbyist, and that is "tube rolling".
We live in a universe governed by the laws of physics which, while not necessarily applicable across the entire fabric of spacetime, nonetheless remain uncontroverted in our tiny corner of the universe, at least for HiFi.
