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Old 26th May 2008, 01:07 PM   #1
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Lightbulb Engineering vs tube rolling

Gentlemen,

stupid subject line I admit, but it draws attention, eh. Actually this is not even the real point of this article I am sure you will see.

Tubes have individual tolerances when new. For example, the special quality, high reliability, long life, tight tolerances type D3a pentode states for a new tube in the TFK spec sheet for given test conditions a deviation of (all numbers converted to percentage):

+/- 5% for anode current, spans 10%
+/- 10% for screen current, spans 20%
+/- 14% for mutual conductance, spans 28%

Tubes also are expendable parts, hence some end of life values are given in the same spec sheet:

-10% for anode current from starting value
-35% for mutual conductance from starting value

This data is from a "special quality" pentode spec sheet, hence "normal quality" pentodes can be expected to show even wider starting tolerances and acceptable life time spans.

Now it could be argued that triodes, being simpler in construction than pentodes, might show much closer tolerances. Lets have a look what the TFK spec sheet says for type 6189 / ECC802S, which is a special quality, high reliability, long life, tight tolerances variant of 12AU7 / ECC82. New tube tolerances are given as:

+/- 17% for anode current, spans 34%
+/- 18% for mutual conductance, spans 36%

End of life values from the same spec sheet:

-33% for anode current from starting value
-32% for mutual conductance from starting value

So, no tighter tolerances for the triodes. Likely, acceptable individual tolerances for just normal, plain new tubes, not considered to be of any special quality, might be even wider than that. End of life values probably could be considered to be similar to SQ types, if not even lower.

Known new tube procduction tolerance spans and end of life values give us valuable information how to lay out and dimension a certain circuit to work as intended (within layed down specs of the circuit/amp itself) right from the start and over the defined lifetime of tubes. We have to deal somehow with new tube tolerances, as well with degrading tube performance over its lifetime.

An engineering approach to this "problem" would be to develop circuitry that is not susceptible to new tube tolerances and tube performance degrading over tube life time within reason or just within the stated tube specs.

This can be done and has been done, examples include just plain standard mass production TV sets made and sold by millions - for many such TV sets you probably would be astonished about how tolerant their circuits are to new tube tolerances and tube performance degrading over time.

Another example, a bit more on topic, are many professionally designed recording/broadcasting studio amplifiers. For example, in a large but typical recording/broadcasting "Regiepult" of the old times at German federal TV/Radio stations dozens of the famous "V"-series cassettes/modules were working, each of them engaging several tubes. Very elaborated and tight specs for the performance of these modules were layed down by a consortium of the consumers (!) in the "IRT-Braunbuch", often including strict requirements on the number of several of thousands of hours of operation such a module must be guaranteed to work within the tight specs - often explicitelly stated to do without any maintenance / adjustment / tube replacement - to get accepted by the studio/consumer.

In other words, it would be not acceptable that such a pre, line or power amp module would change its (non-?) sound over thousands and thousands of hours of continuous operation, despite the tubes degrading in performance meanwhile. In the same regard, it also would be completely unacceptable that a module, freshly re-tubed / maintenanced for the tenth time would "sound" any different than a virgin one from the factory, or just one being short before planned maintenance after several thousends hours of operation.

I am sure similar specs and requirements were layed down by any professional recording/broadcasting organization or gear vendor all over the world, think BBC, RCA, you name it.

To sum up so far, there are scenarios where good engineering and circuit design is required to counteract or prevent any perceivable performance degradation with any new tube within its individual (accepted, stated, as per spec sheet) manufacturing tolerance range and while tubes degrade in performance for a certain ammount to their "end of life time" specs or even worse. Those scenarios range from plain mass consumer stuff like radios and TV-sets to then-"state of the art" recording/broadcasting gear.

* * *

Now, scanning this board, or any board or newsgroup or mailing list about any type of tubed amplifiers (doesnīt matter if guitar amps or high fielity stuff are the topic), you regularly will find people telling something like this:

- [generic tube type] is sounding horrible regardless of vendor! Use [completely different/other tube type] instead!
- [tube type x] from [vendor y] is always sounding best!
- No, but the same [tube type x] from [vendor y] is best sounding only at [position z] of [amp u] when listening to [music style v]!

And so on, you know such statements. Even more, there are lots of people asking questions like:

- Which is the best sounding [generic tube type] in general?
- Which is the best sounding [tube type x] in [amp y]?

And so on, you know such questions, too. Sometimes discussions evolve, sometimes they sadly also run out of line.

Be that as it may, obviously many people are deeply convinced that healthy tubes of the same type - hence completely within specs/tolerances - do sound different even in the same position of the same amp. Maybe even without any measurable differences (maybe we are measuring just the wrong things?) in resulting operation points, etc., so I am not talking about undeniable differences in microphony, hum, and so on. I admit I noticed such tube-dependend "sound effects" in certain of my own circuits I couldnīt explain by measuring and my electronics / acoustics / perception knowledge so far, too.

Even sort of a "sub-hobby" evolved around these effects - we call it "tube rolling".

* * *

As shown by well-engineered apparatus, influence of tube tolerances and tube wearing can be minimized to a degree acceptable even for highest demands, regardless of being mass consumer electronics (example TV-set, accepting almost any cheap "replacement type tube" of even questionable heritage and performance), or as another example, high-grade professional studio gear of the old times.

Now, in conclusion, it could be heretically argued that amps that show sonical differences by "tube rolling" (given same tube type, healthy and within tolerance range), are _not_ well-engineered circuitry because they canīt deal with absolutely normal and acceptable tube tolerances.

An alternative conclusion might be, such susceptible amps are _intentionally_ designed to exhibit sonical effects on "tube-rolling", but somehow I doubt this, especially considering that no amp manufacturer I know so far claims this as a desirable porperty of his certain amp product. Hey, this might even be a strong "pro" marketing argument for an amp aimed especially at the "tube roller scene", no?

My intention definitely is not to deprive the tube-rollers from their fun, but I have some nagging doubts about the engineering quality of amps (regardless of consumer-oriented industrial heritage of any price range or just DIY/homebrewed ones) that show susceptibility to tube tolerances and tube wearing "within reasons", means, within what is defined to be acceptable tolerance and wear as stated by the spec sheet.

After all, I feel a major quality point of system intended for good sonical reproduction should behave ... errr ... reproducible and long-time stable in any case, and especially when engaging parts like tubes that have (and are allowed to have) considerable tolerances right from the start and, tubes being expendable parts, degrade in performance over their life time.

Circuits that behave like this - not being tolerant on expendable parts ageing and replacement within reason - are just examples of bad engineering to me. I donīt want a "Diva"-like amp that only lives up to performance when a cryogenically treated extremely rare NOS tube assembled at full moon by virgins is plugged in. I donīt want an amp that sonically detinguishes between an EL34 that came from TFK or Mullard or from some shack in the middle of nowhere, as long as it confirms to specs and works reliably within them.

I just want to listen to good music, preferably with my homemade tubed amps, happily accepting that expendable parts have to be replaced from time to time and little general checkup/maintenance has to be done every few years or something, and thatīs all to it.

Your mileage may vary of course, but that is _your_ cup of tea or idea of what is fun with tubed amps, not mine. But that position is completely okay with me, of course. And I hope the tube rollers accept my position as well.

* * *

I think much more interesting than just exchanging mere positions and personal preferences would be to hear something on how to _deliberately_ design either a "tube tolerant" amp or a "tube roller" amp. As a matter of fact, a technical discussion about this issue could and should be of high interest to _both_ parties. Both parties should benefit from finding out how to get what they want by a systematical approach.

Comments please - what do you think?

Regards,

Tom Schlangen
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Old 26th May 2008, 02:50 PM   #2
Merlinb is offline Merlinb  United Kingdom
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Hi Tom, nice to see you again.
The literature I have has noted that datahseet values for ra, mu and gm are only accurate to +/-40%! So it does make me smile when I hear people complaining that their ECC83 only has a mu of 90, and must be "bad current production"!

I agree with you that it is benificial to have amps that are either immune to valve tollerance, or are not at all immune, for those people who like to tube roll. However, both types of amplifier already exist, of course

Negative feedback is the usual way to make a circuit more immune to component variations, which is normal in hifi amps. Obviously, the more feedback there is the more immune it becomes. I know from experience that a cathode follower usually gives identical performance with new or very worn-out valves.

The other side of the coin in zero-feedback amplifiers, which have their proponents. Obviously these will reveal the most deifference between individual valves. Additonally, the lower the amp's gain then the more of the transfer characteristic the valves will be forced to operate over, which will reveal the most individual detail. Guitar amps are the most extreme example of this.

Bandwidth will also count of course (the narrower it is, the less variation in performance will be found with different valves)- though we don't have much choice about that in hifi!

I absolutley agree with you, though, that some designer's circuits really are aweful- and that not enough attention is paid to longevity. Particularly in guitar amp circles, for example, I find people who insist on running 6V6s with 500V on plate and screen, then whinge that current production valve "can't take it", and they spend their days looking for the odd valve that does manage to last more than five minutes (and then claim that that particular manafacturer must be the only good one). No doubt a similar attitude exists among some hifi designers.

The funny thing is, that no matter how much those bad designers decide to run the valves beyond their published ratings, they still insist that a standby switch is essential and deplore any designs that don't have them, when they are quite unnecessary.
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Old 26th May 2008, 02:58 PM   #3
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I'm agree with you

Regards Andreas
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Old 26th May 2008, 04:18 PM   #4
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Default Engeneering and tube rolling...

Hi Tom,
and you think the manufacturers of "studio equipment" didn't make "tube rolling"?

Kind regards,
Darius
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Old 26th May 2008, 05:06 PM   #5
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Some comments in no particular order.

Audible Illusions preamps are notorious "tube eaters", due to operating conditions outside published SAO. The remarks about "bad" tubes are spot on.

I'm on record numerous times about the homogenizing effect of loop NFB. Reasonably reproducible results are a good thing in commercial products. Do what you like in a "1 off" that's for personal use.

I'm a transplanted New Yorker. IRT ist eine Eisenbahn. Why would somebody want a brown book about the subway?
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Old 26th May 2008, 05:30 PM   #6
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Default Re: Engeneering and tube rolling...

Hi Darius,

Quote:
Originally posted by oldeurope
Hi Tom,
and you think the manufacturers of "studio equipment" didn't make "tube rolling"?
Excuse me, but the idea of "V"-series vendors like TFK, Siemens, etc, meeting IRT Braunbuch specs by chance (aka "tube rolling") is plainly ridiculous. What they did instead to meet the specs is good engineering.

If it couldnīt be done to meet the specs with "usual" tubes at had, sometimes special tube types had to be produced/ordered to meet the requirements the manufacturers derived from the system design spec they were bound to by IRT themselves, but mostly regarding strictly engineering properties like long life, low hum/microphony, and so on.

If you think otherwise, please just point me to a paragraph in the IRT Braunbuch laying down a certain sonical imprint or "sound" specification. I for myself donīt know a single case whatsoever a certain tube was explicitely designed to meet a certain _sound_ imprint property - do you?

Please donīt tell about guitar amp manufacturers claiming that only their own (actually rebranded) tubes will sound best in their amps - everybody knows that this is only marketing stuff.

Regards,

Tom
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Old 26th May 2008, 06:08 PM   #7
rdf is offline rdf  Canada
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Comparing the circuit requirements of a tuned device like a television to audio amplifiers operating simultaneously over a three decade bandwidth seems questionable. Re: rolling, the differences in measured harmonic distortion spectra between different manufactures of the same tube model are surprisingly wide. Based on my measurements I wouldn't touch an EH EL84. Discounting those who unintentionally roll increases in pleasing distortions there's no question in my mind significant performance improvements can be achieved with careful brand selection (or avoidance.) Based on a quick look at the low-NFB Telefunken V76’s schematic for example, rolling in worthwhile measurable improvements would be no surprise.
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Old 26th May 2008, 06:23 PM   #8
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Hi Eli,

Quote:
I'm a transplanted New Yorker. IRT ist eine Eisenbahn. Why would somebody want a brown book about the subway?
but IRT means "Institut für Rundfunktechnik" in this case. Excerpts from the "Braunbuch" specifications book including lots of famous "V"-series schematics can be found/viewed here at the IRT site.

Regards,

Tom
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Old 26th May 2008, 07:18 PM   #9
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Default Re: Engineering vs tube rolling

Quote:
Originally posted by Tubes4e4
Comments please - what do you think?

Regards,

Tom Schlangen
I'm not so certain that there isn't a whole bunch of audiophoolery here. I put some new VTs in the amp, and I hear a difference because I expect to hear a difference.

Of course, there is legitimacy behind the idea of tube rolling. A spec for a type says nothing as to how the manufacturer is to go about meeting that spec. So you have different configurations across brands, and even models within a brand. That could make a sonic diference, especially since sonics aren't a part of very many specs. VTs, being low gain devices, have a bigger effect on circuit performance as opposed to higher gain devices such as BJTs and MOSFETs.

Here's another case where gNFB comes in handy: it tends to make circuit performance less dependent on both active and passive components. It's another part of why I use it. I can't be bothered sifting through a bazillion different VTs to find just that "magical" combination. The only tube rolling I did was replacing some very microphonic Sylvania 6BQ7As (you can recognize these by their series connected heaters) with quieter Motorolas and Sylvanias with parallel connected heaters. Other than getting rid of microphonic noise, I didn't notice they sounded any different from the series heater Sylvanias.
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Old 26th May 2008, 07:47 PM   #10
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally posted by rdf
Comparing the circuit requirements of a tuned device like a television to audio amplifiers operating simultaneously over a three decade bandwidth seems questionable.
I'm inclinded to agree with that. And let's not forget that TVs didn't use the same type of valves we're familiar with in audio. For example, when Mullard designed the EL84 it was purely for audio applications.At the same time, Valvo was working on their version, and, quite by chance, it was found to be far more consistant and closer tollerance. Philips therefore decided to rebrand the Valvo version as the EL821 and sold it as a TV line driver, which would imply that TV manafacturers could avoid the need to tube roll by using valve types that were naturally more consistent?
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