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That's a cool experiment.  Tubes seem organic and so does that spectrum analysis.

Makes me wonder if you had two tubes from the same manufacturer, would you see differences also?

Or how about length of time it is on.  Like the same tube but the amp left on for a 1/2 hour?

And then how steady is the voltage going in and how does that affect it. 

Then I tracked the first take several times. I swapped tubes between each pass and ended up with a collection of similar tracks.

So does this mean you played each take into each different tube, or did you play back the recording into each tube? I didn't quite understand exactly what you were doing from your description.

In other words, could the differences also be attributed to different performances, no matter how careful you were in playing?

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man, i've been playing thru tube amps for close to 35 years, and have played for years at the time, doing 5 hours a night, 6 nights a week..

tubes do change.

and fresh ones do make a difference.

and different manufacturers do sound different (both preamp and power) 


and sometimes the same model tube, will sound different.

don't ask me why.


but that's my experience.

I played the same amp for 20 years (Vibrolux 1981) with the original tubes and I will say it begin to have a harder response, very non musical. Just flat tone. I purchased some GTs from Guitar Center and it made a difference the sound was softer edged and more musical. That is my only experience with changing tubes but I have wondered if all the raving about JJs and Mullards really makes a significant difference.

Bat... I will agree... when you run an amp 6 nights a week... and it's on and cranking for 5 hours or more a night... and running like I assume you would run it... pretty far up the gain/volume curve.... a tube will "wear out" as the cathode gets depleted of the stuff (dope) they put in it to emit the electrons. 

This can and will cause a change, decrease,  in the amount of current that flows and that will affect the sound quality and the levels. Installing a fresh tube  will be noticeable in a newer amp to some people's ears.  The difference will definately show up on high quality test equipment.

Like Middleman said... he played the same amp for 20 years... that's a long time on those tubes.... yeah I'd say they were worn out at that point, unless it was only turned on once a year.  regular gigging and jam sessions for 20 years...yeah, you should consider some new tubes.  The cathode current in those tubes had probably dropped of considerably, resulting in the sound becoming harsh, not full bodied and rich. New tubes were the solution. He could probably have put the el-cheapo radio shack tubes in that amp and noticed  the same (a similar) improvement in sound.

Had I been in his shoes.... as I have been several times over the years, I would have bought the better, more costly,  tubes as well. Give me a choice and I will not put the cheapest tubes in any amp except in a gotta have it now scenereo.

My Mesa Studio 22 became microphonic, squealie, and after a call to Mesa, I ordered new tubes.... they were not worn out, just noisy. The new tubes sounded just like the old ones.... and the problems were still evident... just not as bad.... some time later I bought some GT's as well.... the microphonics are still there... tap a tub and it is heard.....  I just figure that the amp has such high gain  levels that the tubes really amplify the physical noise. And I reason that the microphonics assist physically in attaining the controlled feedback that is so freaking sweet on that Boogie.

Dont bother running tests.

But where's the fun in that?

I did extensive testing of different tubes a few years back when I had a THD BiValve.  I used a direct recording of a guitar to reamp with the different tubes.  The results were recorded without moving or adjusting a single thing and the resulting tracks were easy to compare.
 
The verdict is...Yes, different tubes do sound different.  It is fairly easy to hear the difference when you are listening to a solo guitar track, but in a mix...not so much.  The sound of different guitars, different amps, and different speakers is WAY more noticeable.
 
That being said, the new production Tung-Sol 12AX7 are by far the best sounding of the currently produced tubes.  Big, full, rich sound.  For 6L6 and EL34 tubes, the SED Winged "C" tubes sounded the best.  For 6V6, I liked the Electro Harmonix.  Everyone says that JJ is the best EL84, but if memory serves me, the bottom end was a little flabby and the Electro Harmonix was better over all.  Of course I realize this is all personal opinion.
 
As far as "testing" tubes by looking at spetrum analyzers...I think that is pretty much pointless, no offense intended.  No one who listens to one of your songs will ever bust out an spectral analyzer to "see" what you tone sounds like.  It's totally irrelevant.  The only thing that matters about tone is how it sounds.  As EVH once said, "If it sounds good, it is good".  Nuff said.

Mgreene , thanks for the tip on VTV , good stuff, Now I'm going to have to pull all my tubes and see whats in there!

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Here's a tip for you'uns. Many people dont know that preamp tubes of old (like the 12AX family) were meant to have a service life of 12,000 hours or more, when run according to suggested specifications. Do you know how long that is in guitar amp years? Well, neither do I - but its a long time. :)
 
(Many guitar amps actually run tubes over spec - like the Silver Deluxe; that combined with heat, vibration and being physically bounced around are reasons why tubes dont necessarily last that long in a guitar amp.)   
 
Take a flyer on some older "used - but test good" preamp tubes from ebay. If you can win them for less than the price of  currently produced tubes you might be pleasantly suprised. Some might be noisy but some might sound like NOS and last for years. Also, hi-fi people are looking for matched sections in a dual triode like a 12AX?, for stereo operation. This doesnt make much difference for a guitar amp so a hi-fi guy's cast offs could be great for a guitarist.  
 
Mike