Darius,
according to Peter Müller, retired maintenance staff member at WDR Cologne, for example every 10.000 to 12.000 hours of operation the two F2a11 power tubes in V69a and V69b amps were replaced. Checking/adjusting bias for the sole reason of least distortion, using sig gen and distortion meters, was done on shorter periods, probably every 4000...5000 hours, he doesn´t remember exactly. The driver tubes usually were kept until the whole amp was put out of duty after a decade of duty, or something like this.
I wouldn´t call this tube rolling by any means, but just maintenance.
Tom
according to Peter Müller, retired maintenance staff member at WDR Cologne, for example every 10.000 to 12.000 hours of operation the two F2a11 power tubes in V69a and V69b amps were replaced. Checking/adjusting bias for the sole reason of least distortion, using sig gen and distortion meters, was done on shorter periods, probably every 4000...5000 hours, he doesn´t remember exactly. The driver tubes usually were kept until the whole amp was put out of duty after a decade of duty, or something like this.
I wouldn´t call this tube rolling by any means, but just maintenance.
Tom
OK, I'll nail my colours to the mast by saying that I agree with all that the threadstarter said. And then I'll offer some contradictions. 
Tektronix indulged in tube-rolling, as did EMI. Sometimes, circuits were designed that used valve properties that weren't specified on data sheets. That meant that you had to have valves selected for your purpose - your market just wouldn't have been big enough to justify a new type.
EL821 is a field scan valve; essentially audio frequencies. It's pretty poor as an audio valve, and one of the EL821 data sheets actually has a comment to that effect.
Oh, and kudos to Salas and Wavebourn for their amusing comments.
Tektronix indulged in tube-rolling, as did EMI. Sometimes, circuits were designed that used valve properties that weren't specified on data sheets. That meant that you had to have valves selected for your purpose - your market just wouldn't have been big enough to justify a new type.
EL821 is a field scan valve; essentially audio frequencies. It's pretty poor as an audio valve, and one of the EL821 data sheets actually has a comment to that effect.
Oh, and kudos to Salas and Wavebourn for their amusing comments.
Tubes4e4 said:
Maybe the question requires tighter tolerances. Those old distortion analysers were blunt instruments, one number burying any indication of harmonic structure. I learned on HPs and Potomacs too but wouldn't consider either much use today. So are we saying:
- Measured distortion spectra differences between different tube brands are inconsequential?
- They're not but typical textbook topology employed enough NFB to make the differences inconsequential?
EC8010 said:
There are SS devices today widely available created especially to keep tubes always happy. I can't swap some tubes in my hybrid amps only because of different socket bases and pinouts, but with appropriate adapters differences will be almost inaudible.
Edit: I don't mean tubes on the edges, like mic amp and power output, but even they may be happy if properly regulated by SS devices.
I always throught the idea of tube rolling wouldn't make a difference but everyone said I was wrong.In my tube amp one person said he spent $700 rolling tubes and would tell me what tubes to change.He said the chinese kt88 were teriable and another person told me the chinese kt88 were the best he ever heard! Since both can't be right,I just left the tubes in that came with the amplifier and saved my money
keithgreenhalgh said:I always throught the idea of tube rolling wouldn't make a difference but everyone said I was wrong.In my tube amp one person said he spent $700 rolling tubes and would tell me what tubes to change.He said the chinese kt88 were teriable and another person told me the chinese kt88 were the best he ever heard! Since both can't be right,I just left the tubes in that came with the amplifier and saved my money
Everything depends on design. Sometimes some gear is designed such a way so slight variations from specs give the wrong result. For example, manufacturers of guitar amplifiers forced the industry to make tubes that can survive abuse longer.
Wavebourn said:
But why do two different people have opposite ideas on the sound of the kt88 in the same amp? The difference is either not there or a personal preference. Even if its a personal preference, whats to say the personal preference is closer to the real sound eg. some like more bass,some like more treble.Replacing a tube may give a different sound but is it closer to the natural sound?
keithgreenhalgh said:
But why do two different people have opposite ideas on the sound of the kt88 in the same amp? The difference is either not there or a personal preference. Even if its a personal preference, whats to say the personal preference is closer to the real sound eg. some like more bass,some like more treble.Replacing a tube may give a different sound but is it closer to the natural sound?
Was it the same model belonging to two different people with two different systems and rooms? Or they just gave different evaluation on one amp in one system?
Two different people had the same amp as me.they Both were sure that changing the kt88 tubes in one case was a great improvement were the second person tried it and said the origional kt88 was the best.They can't both be right so it must either be that they think they hear a difference or perhaps one of them perfers a certain sound but whats to say the sound he hears(if he can hear a difference) is better?
Were they the same tubes? I don't mean just the same type or even the same brand but exactly the same tubes. The reason I ask is because there can be quite big variations between tubes of the same type, even from the same manufacturer.
Quality control can be lacking, leading to variability. If this were not the case, there would be no need to search for matched pairs to use in PP.
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