Tubes warm up during the first hour or so of use, depending on the tubes role, during every session. Over accumulated time, they only get worse.
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A brand new tube will "burn in" over the first 10-30 hours of use and its sound will change over that time. Thereafter, Pawlowski6132 is exactly correct about the warm up time with each listening session. Whether the burn in will resolve your concerns with this particular tube is an unknown -- my guess is that the general characteristic you've noted will not change much.
The answer depends upon whether you are referring to the tubes themselves or an entire tube-based circuit.
First, lets clarify that the popular term burn-in really refers to forming dielectrics. This is where electrons find and groove a path of least resistance through the insulation materials surrounding the conductors. Dielectrics (teflon, polyethylene, glass epoxy boards, etc...) act as an impediments or road blocks to the free flow of a signal.
The actual vacuum tubes themselves require little or no burn-in because they have so little dielectric material to form in the first place. The conducting elements are surrounded primarily by a nearly perfect vacuum, which is a superior dielectric to all others including air.
Tube circuits like all circuits which contain capacitors, resistors, wire and circuit traces, require significant burn-in because all of those passive parts contain all kinds of plastic dielectrics which need to form to sound best.
Transistor amplifiers require and benefit from even more burn-in because unlike vacuum tubes, the transistors themselves contain significant dielectric material which also needs to be formed.
If there is any advantage for tubes over transistors in relation to the idea of burn-in, it is that the transistor contains lots of solid dielectric material tightly packed around the conducting elements, where a vacuum tube contains virtually nothing.
Davemitchell, thanks for a very cogent explanation of dielectric material forming process -- I agree that this is the way I normally think of "burn in." A different word is needed then, because whatever may be happening, in my experience the sound of a tube will change over the first 10-30 hours of playing time.
I've found that tubes don't really benefit much from burn-in. The first few hours might have a slight bump up in emission, but otherwise they don't seem to change much. And like the other guys said, it's a long road downhill from there. Fortunately, this can be very slow.
You'll hear much more effect by removing the tube and plugging it back in. In other words, cleaned contacts are significant.
I believe the question is excellent, but the listening comparison behind the question is flawed. You were comparing two different tubes from two different manufacturers. These will inherently sound different. The same tube from different manufacturers will sound different, and the fact that these were not the same tube types exacerbates the situation.
Tubes do a slow burn-out during their lifespan, so it stands to reason they "burn in", but the change occurs gradually over time. It will be impossible for most listeners to tell a difference from day to day, or week to week, but if one was able to compare the sound today with the sound several months from now a difference could very possible be heard.
I'm certain the goal for most premium tubes was to make any change imperceptible over the tube's lifespan.
My concern was not that after burn in the tubes would sound the same. I was just seeing if I liked the 12SX7's at all. They sounded different than the 12SN7's, as expected, but I did not expect them to have such poor clarity, especially noticable with jazz, acoustic, and vocals. But, as stated, they need to warm up ... really warm up. Maybe obvious to most people, but it really was not to me. My stock equipment sounds pretty much the same whether warming up for 10 minutes or 1 hour, so I was not used to allowing for more warm up time. After warming up the 12SX7's for a complete hour, they sounded way better than before. Now I like them, although they are not as clear as the 12SN7's, after proper warm up their clarity is acceptable, which is good because I like their other attributes: better imaging; a little tubier in the upper and lower freqs. Jimi Hendrix and Rory Gallagher never sounded so good. Thanks guys for all of your explanations and insight.
They definitely settle and smooth out after maybe 10 hours.. Just probably something with the metal getting heated and or some kinda substance burning off the plates and emiting a stronger more consistent current = sound.. Only thing I have to support that there is a DEFINATE physical change in a tube one way or the other although rather minor is, Many times when I plug a brand new tube in, even for the maybe first 5 to 10 power ups, the Puff, Peak, Pop, And fizzle a little thru the system, and even with no speakers hooked up you can hear this crackling right through the Glass on the tube itself, then all of a sudden on power up and useage they are smooth as butter and dead silent, so something is adjusting due to the applied heat I would assume.
Just probably something with the metal getting heated and or some kinda substance burning off the plates and emiting a stronger more consistent current = sound.
That is definately plausable Undertow. It could also be that there is some dielectric interaction especially near the base of the tube. I do also agree that whatever burn-in the tube itself might go through is sonically very minor.
Caig pro gold is an excellent conditioner to use if the metal is cleaned already on the pins, it gives positive solid / liquid contact... buy it in the Nail polish bottle like this and application is very clean and easy vs. Spraying aerosols all over your tube.. see link