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|13th October 2003, 02:58 AM||#1|
Join Date: Sep 2003
Tuberolling - Is it for real?
Since I have come back to vacuum tube audio in the last several years, I am fascinated by the concept of tube rolling. I have heard both sides of this issue expressed. Maybe some of you guys can give my your further opinions on this subject. If you all have done this before, then by all means stop this before someone gets hurt.
When I was growing up, tuberolling was unheard of. I assume that this is because the rest of the components in the system (transformers, speakers, caps, etc.) were so poor that the differences between tubes could not be heard. Or is it maybe that tuberolling actually has no effect at all?
Let me offer some structure that makes sense to me. Please help me understand why tuberolling works.
A vacuum tube is defined by a very small number of physical characteristics. These physical characteristics, in turn, define the tube as a circuit element. I think it makes sense to divide the physical characteristics into two categories, first order and second order.
These are the first order characteristics that I am aware of:
1. The actual physical dimensions of the various elements (plate, grids, cathode).
2. The geometries and spacing of these elements.
3. The emissivity of the cathode (that is, how many mA/square-mm at what temperature).
Now a tube has only two circuit charactisitics that matter in audio: its interelectrode capacitances and its behavior as a voltage controlled resistor. So far as I know, these three first order characteristics almost entirely determine a tube's circuit behavior, its capacitances and resistances.
There are some second order characteristics too:
1. The actual materials used in construction.
2. Small structural features, such as ribs on plates.
3. Gas in the envelope (which becomes a first order effect if there is too much or if the tube is designed to contain gas).
4. The way the emitting material is layered onto the cathode surface.
5. Whether the cathode is directly or indirectly heated.
There may be others of these.
Now, it seems to me that two tubes of identical physical geometries with cathodes of approximately the same emissivity, must exhibit the same electrical characteristics. There is no way that I can think of that they can behave otherwise.
From the circuit's point of view, it doesn't matter what the tube looks like internally, all the circuit sees are the capacitances and resistances. So, two tubes of identical physical characteristics should sound exactly the same because they produce the same electrical behavior.
OK, so what? Well, let's take two tubes from different manufacturers. They may be made somewhat differently. But, if they behave identically as capacitors and resistors, then how can they sound different? That is, if they display identical plate curves and identical interelectrode capacitances, then how can they sound different since their circuit behavior will, must be identical?
OK, suppose they don't have "identical" curves? Well, if they are the same type of tube, the curves should be within some tolerance range. And if so, what exactly changes anyway?
If tubes were perfectly linear there would be no discussion, because there would be no distortion. But they are not. So, it must be the deviation from linearity that defines any particular tube (what else can it be? except for the capacitances and current handling capacity which I'll get to presently).
Two tubes of the same type, made differently, but with nearly identical plate curves must, therefore, exhibit nearly identical distortion characteristics. In fact, unless one of the tubes is just plain bad, my guess is that the differences in distortion would be audibly undetectable. Are you guys actually hearing these differences? You may be and I truly admit to being very, very far from an audio expert nor do I have wide experience with lots of tube amps. But I would like to know how you hear them.
Do the second order characteristics change anything? Well, if it's the same tube type how can they (unless the tube is bad or failing)? The only possible effect might be actual materials and small physical features. I don't see how the materials could have any effect if they are chosen properly for conductivity and durability. And, I don't see how small scale differences can matter. For example, plate ribs can't possibly change a tube's behavior. By the time electrons reach the plate they are moving so fast that a millimeter is irrelevant (or so it seems to me).
OK, so then the question is, do tubes of different types sound different? Well, doesn't it depend? If the circuit truly exhibits the nature of a tube's plate curves and capacitances, then the distortion characteristics will be different. Can you guys actually hear this in a properly designed amp?
And finally, for this post, the current handling capabilities between tubes is an obvious difference that I think everyone understands better than I do. This not only affects power delivery but speed, that is, the ability to charge the load capacitances fast enough for audio (say from 20Hz up to 300KHz). But, two tubes of the same type, if they measure the same, should do both of these things the same and so should "sound" the same.
OK folks, I am standing in front of the target with my arms wide . . . . . . . maybe I should go back to the CCS thread.
|13th October 2003, 05:31 AM||#2|
Join Date: Oct 2002
Blog Entries: 2
It gets much deeper than that, but to put in into perspective, you can have FETs with the same part number, same manufacturer and even the same batch show Idss and Vp spreads of 3 or 4 to 1. So we're not doing too badly with the real tolerances of tubes.
Some "rolled" tubes aren't even the same nominal part number. For example, there were a lot of 6DJ8s around that were actually relabelled 6ES8s.
There's a lot of myth out there, much as with other aspects of audio, but there are some real differences, too. A quick look at 3 or 4 different 12AX7s will make you appreciate this; I've got three examples here which don't even look vaguely alike inside. The fact that they all have roughly the same characteristics (to first order) is miraculous. The fact that they do function differently in real circuits is understandable.
"You tell me whar a man gits his corn pone, en I'll tell you what his 'pinions is."
|13th October 2003, 05:32 AM||#3|
Oops, that's a can of worms my friend...
Scientifically, the way a tube is used is its electrical characteristics. So any actual in-circuit behavior can be measured, as you observed (distortion, gain, frequency response, etc.). Any variations can be narrowed down to the specs you mentioned, plate curves (which cover well over the used range, allowing tight matching if you're really picky) and capacitance. Now really, if these are equal, and behavior depends on this, then there can't possibly be a difference. But apparently, there is! (Although I can't say I've heard of any tests, blind or not, involving precisely matched tubes of different contruction and brand.)
Overall I say it's all in your head. The human brain is suggestible, and if one believes that a particular modification (be it new equipment, tubes, wires, whatever) will improve it, it indeed will. And note also the "it was expensive, it damn well better be worth it!" effect on very expensive things *COUGH*cables* will have a similarly strong effect.
Personally, I can say that several amps of mine, speakers and other combinations can sound good for up to a week, but after the novelty wears off it sounds like crap again. Probably the mental effect of the effort of arranging, since I tend to do things as free as possible...
I can't say I've noticed any differences while rolling tubes, probably because I'm too sceptical, err scientifically-minded..
(I mean, except for 12AU7 vs. 'AT vs. 'AX in my preamp, but that's the differing gain and plate resistance affecting the tone stack.)
Oh, and not to mention rectifiers. Besides voltage drop and supply impedance, well you know where this is going.
|13th October 2003, 10:25 AM||#8|
Join Date: Nov 2002
Yes at first I was thinking of a new gimmick to improve the sound of tubes by first rolling them over the floor to run them in mechanically
But no joking, accuracy of mechanic dimensions is very important for the electrical behavior of tubes. Spacing between cathode, grid and anode directly defines Mu. How much are these elements // to each other? Also how even is the spacing between the grid wires? Uneven spacing of the control grid acts like many different tubes with different S in //. Further is the anode coated to reduce secondary emission? Idem for the material of the grids. And so on ...
It is clear to me that tubes from different brands differ slightly from each other.
|13th October 2003, 04:32 PM||#9|
Join Date: May 2003
I can't remember all that was mentioned in the first post, but, and not necessarily in order.
gas in tube
Materials makes a big difference in the sound. I just recently saw a tube ad that actually uses Nickel for the plate. Nickel will sure influence the sonics.
Different brands and years can sound different. Sometimes, tweeking the circuit is all that is needed to make the component sound the same, sometimes not.
|13th October 2003, 05:38 PM||#10|
Re: Tuberolling - Is it for real?
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