RCA Engineers Test Telefunken 12AX7 Tubes against Their 12AX7

Do you know the technical definition?
Flicker noise is also called 1/f noise as it has a 1/f power spectrum, that is the power increases from the noise floor at the rate of 1/f as the frequency drops below a certain point- i.e. if the crossover point (where the 1/f noise "emerges" from the constant thermal noise floor) is say at 100 Hz then it will be 3dB higher at 50 Hz, 6dB higher at 25 Hz. etc.
Many natural processes of all kinds exhibit this behavior, although the mechanisms are, in general, not well known. In semiconductors the processes are generally well understood and improvements can be had, but I have no idea what causes 1/f noise in tubes.
 
Flicker noise in triodes is discussed in this Melvin Blencowe book excerpt, under section 5.3.

I have no affiliations
 
Flicker noise in triodes is discussed in this Melvin Blencowe book excerpt, under section 5.3.

I have no affiliations
Isn't the work function of the cathode simply the amount of energy required to cause an electron to be emitted? If so it's saying that that flicker noise in tubes is due to variations in the rate of emission of electrons, which seems to be trivially obvious in my rather uninformed perspective. In semiconductors the mechanisms are fairly well understood and are modelled quite well. I believe that the sources of 1/f noise are generally identified as traps in the semiconductor- that is locations that restrict the uniform flow of holes and electrons- and can be improved by various processing "tricks". Is there a similar mechanism for tubes? For example, is the problem due to impurities in the metal used for the plates, or possibly dislocations/irregularities in the structure, which disturb the delocalization of the electrons?
 
I wonder if its not as simple as the shape of the anode or cathode, slightly different distances between the parts causing the issue? Most RCA 12ax7 tubes that I'm familiar with have 3 embossed ridges across the plates, but Telefunkens do exist in smooth plate fashion. It might help to know specifically which TFK tubes were being used here.

Could also be something about the grids causing an interruption in flow since there is a *thing* in the way.
 
No way to know my guess is smooth plate
Perhaps that's it, it sounds plausible to me. It's hard to believe that it's geometry related- it has after all constant energy/octave (like pink noise) below the cut off point and that suggests a quantum mechanical source.
Perhaps the rougher plate surface implies the existence of the kinds of dislocations and discontinuities that might cause irregularities in emissivity.
 
Flicker noise in a tube has nothing to do with the plate shape or geometry. Neither does the interposed grid, or its construction.

What matters here is simple: the cathode's electron emission process creates the noise because the cathode is a semiconductor.

Remember, the cathode's surface is an alkali metal which, not being pure, contains a bit of oxide plus orthosilicates (hold off on this for the moment, as it is later addressed). What do we know about oxides and metals? They act like semiconductors and form diodes. Copper, selenium, iron, lead, silver, etc. (Radio detectors can be made using lead oxide using a blob of solder or even a rusty razor blade. NB: I did not say "good radio detectors". Best used for foxhole radios and covert radios built under Nazi occupation. Keep that in mind for the Zombie Apocalypse.) A vacuum tube cathode is an N-type semiconductor.

In somewhat greater detail, the magic occurs at the junction of the metal cathode sleeve (base metal) and the alkali metal (which has some oxide) which forms a metal-oxide junction which, again, is a semiconductor. That's one source of the noise. Then the silicon used to both reduce the alkali carbonate—the carbonate form is used during manufacturing because pure alkali metals are pyrophoric, i.e. they burst into flame in air—during both activation and operation to replenish the surface, also forms a semiconductor on its own, adding the second source of noise. For the third source, the silicon, as a bonus, then reacts with the alkali metal to form orthosilicates aka "interface layer" which acts like an insulator on the surface. (This is why running a tube in cutoff for extended periods of time ruins the tube.) The increase in this layer's thickness and distribution over time explains why flicker noise increases over time. It also explains why the tube's bandwidth gradually reduces because of capacitive effects. Plus there's all sorts of impurities in the cathode, as it is a poorly controlled farraginous assemblage of elements and molecules.

So the electrons have a complex journey through a variety of materials before emission, and this creates noise.

Any cathode material ablated and deposited on other structures may, of course, act as a supplementary cathode and emit electrons and thus be an additional source of flicker noise. But that's a minor source since those surfaces are minor emitters.

As an aside, the granules in a carbon-composite resistor also form rectifiers, which is why the CC resistor has a Vcr. (The rectifier is all jumbled together so the resistor conducts in both directions, albeit with peculiar properties. My writings on the evils of the carbon-composite resistor are elsewhere detailed on AK in glorious detail.) Similarly, the corrosion of silver forms oxides and sulfides which are also semiconductors; this creates noise and other peculiar properties in switches and sockets.

Before posting this I checked Spangenberg (two volumes) and Reich (two volumes) off my bookshelf, and neither had any explanation better for this than what I above wrote. I vaguely remember Schottky's original paper addressed this, but he really didn't understand the semiconductor issue as it was the early 1920s. Took a while for science to advance.

The key thing in this discussion is to remember is that flicker noise is tiny and it really only mattered—past tense as such circuits are no longer constructed using tubes—for low-level DC amplifiers, or for scintillation counters or EEG/EKG sensors where low-noise greatly mattered. No audiophile is going to hear flicker noise, so we may all sleep at night knowing that truth, justice, and low-noise amplifiers will prevail.

Edit: corrected typo.
 
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Flicker noise in a tube has nothing to do with the plate shape or geometry. Neither does the interposed grid, or its construction.

What matters here is simple: the cathode's electron emission process creates the noise because the cathode is a semiconductor.

Remember, the cathode's surface is an alkali metal which, not being pure, contains a bit of oxide plus orthosilicates (hold off on this for the moment, as it is later addressed). What do we know about oxides and metals? They act like semiconductors and form diodes. Copper, selenium, iron, lead, silver, etc. (Radio detectors can be made using lead oxide using a blob of solder or even a rusty razor blade. NB: I did not say "good radio detectors". Best used for foxhole radios and covert radios built under Nazi occupation. Keep that in mind for the Zombie Apocalypse.) A vacuum tube cathode is an N-type semiconductor.

In somewhat greater detail, the magic occurs at the junction of the metal cathode sleeve (base metal) and the alkali metal (which has some oxide) which forms a metal-oxide junction which, again, is a semiconductor. That's one source of the noise. Then the silicon used to both reduce the alkali carbonate—the carbonate form is used during manufacturing because pure alkali metals are pyrophoric, i.e. they burst into flame in air—during both activation and operation to replenish the surface, also forms a semiconductor on its own, adding the second source of noise. For the third source, the silicon, as a bonus, then reacts with the alkali metal to form orthosilicates aka "interface layer" which acts like an insulator on the surface. (This is why running a tube in cutoff for extended periods of time ruins the tube.) The increase in this layer's thickness and distribution over time explains why flicker noise increases over time. It also explains why the tube's bandwidth gradually reduces because of capacitive effects. Plus there's all sorts of impurities in the cathode, as it is a poorly controlled farraginous assemblage of elements and molecules.

So the electrons have a complex journey through a variety of materials before emission, and this creates noise.

Any cathode material ablated and deposited on other structures may, of course, act as a supplementary cathode and emit electrons and thus be an additional source of flicker noise. But that's a minor source since those surfaces are minor emitters.

As an aside, the granules in a carbon-composite resistor also form rectifiers, which is why the CC resistor has a Vcr. (The rectifier is all jumbled together so the resistor conducts in both directions, albeit with peculiar properties. My writings on the evils of the carbon-composite resistor are elsewhere detailed on AK in glorious detail.) Similarly, the corrosion of silver forms oxides and sulfides which are also semiconductors; this creates noise and other peculiar properties in switches and sockets.

Before posting this I checked Spangerberg (two volumes) and Reich (two volumes) off my bookshelf, and neither had any explanation better for this than what I above wrote. I vaguely remember Schottky's original paper addressed this, but he really didn't understand the semiconductor issue as it was the early 1920s. Took a while for science to advance.

The key thing in this discussion is to remember is that flicker noise is tiny and it really only mattered—past tense as such circuits are no longer constructed using tubes—for low-level DC amplifiers, or for scintillation counters or EEG/EKG sensors where low-noise greatly mattered. No audiophile is going to hear flicker noise, so we may all sleep at night knowing that truth, justice, and low-noise amplifiers will prevail.
Thanks. That explains a great deal. What is the 1/f corner of a 12AX7 typically? What little I can find on the subject seems to suggest corners between a few kHz and a few 10s of Hz. As I said, tubes are not my area of expertise, so any guidance would be appreciated.
 
That does give some further insight; thank you for the detail and, as suspected, is not a major concern for our typical audio apps. Apparently, it is still in need of full understanding. Excerpt page 239 onwards here also discusses with early references to Scottky, Johnson, McFarlane, Spoull.

That 1/f corner is indicated similarly in these references.
 
Thanks. That explains a great deal. What is the 1/f corner of a 12AX7 typically? What little I can find on the subject seems to suggest corners between a few kHz and a few 10s of Hz. As I said, tubes are not my area of expertise, so any guidance would be appreciated.

It's been a while since I researched this, but as I recall the flicker noise in vacuum tubes was really only a problem around low DC, typically well under a fraction of one Hz where it starts to rise up like a tsunami off the sea floor. This is why building low-noise DC amplifiers using tubes is difficult. In the audio band the flicker noise is generally accompanied by Schottky shot noise, partition noise in tetrodes and pentodes, resistor noise, and all sorts of other noise. Any of the above noise types are likely miniscule compared to the noise of half-century old carbon-composite resistors, adding substantial waterfall noise and non-linear distortion.

As far as I know, a hard and fast rule exist for calculation of flicker noise won't exist because the noise is a function of cathode material, purity, tube age, and the amount of electrochemistry (involving the cathode, but also the heater, by way of migration of alumina and nickel into the heater, or migration of tungsten into the alumina and nickel, depending upon the heater-to-cathode potential difference) which occurred during activation and use. This is an experimental process. All of the numbers I've seen were experimentally determined, and vary by the tube and batch, and by its age. Yes, by age.

Flicker noise reportedly worsens with age, even when the tube is NOS, unused and sitting unmolested in a box. This suggests that the outgassing/offgassing of absorbed/adsorbed gas in the cathode and nickel sleeve slowly reacts with the alkali metals, silicon activator, and nickel substrate assembling a slew of metal oxides and other species.
 
Superb explanation as usual, Retro. My question is this: Would the electron cloud formed around the cathode not act as the great muffler to any noise (minute as it may be) produced by a cathode? The cathode is of course the source of the electrons to ultimately support electron flow within a vacuum tube. But it doesn't support the flow directly. Rather, the emitted electrons -- their production of which is certainly uneven over the total cathode surface, and as you've now explained, somewhat noisy -- gather to form an electron cloud around the cathode that both acts to protect the cathode, but also is the pool of electrons that actually supports current flow through the tube. Since the cloud then amounts to (for this discussion) a buffering element that provides a ready reserve and stable source of electrons to support the actual current flow demands through a tube, would it not also act to buffer any noise created by the cathode as well?

Dave
 
Thanks for the kind words, Dave. I'm never entirely sure anyone actually reads this stuff or cares.

The space cloud is typically—and notably inaccurately—described and depicted, using a water tank or capacitor analogy, as some sort of reservoir or buffer from which electrons are smoothly leaked across the tube at a rate dependent upon the grid's potential. The British name "valve" even suggests this behavior. But that's not exactly how it works. Here's what I worked out by reading through a lot of the ancient tomes and cogitating upon how it works. No guarantees, and a lot of it is handwavy since I'm not a quantum mechanic, only the regular kind that does the electronic equivalent of oil changes, brake pads, and tire rotation.

The space cloud is not a giant tank of noise-free electrons, stored to the point they are permitted to exit through gaps in the grid and transit across the tube, much like a water tank having a spigot at the bottom which releases water at a steady rate. (Even that analogy instantly breaks down, spigot would have issues of laminar vs. turbulent flow because of surface tension, plus other interactions of water molecules with each other, impurities, and the interior surface of the pipe.) If the space cloud were a tank, one might think the electron properties somehow ended up being averaged or muted, analogous to how a capacitor purportedly functions as a shock absorber, absorbing charge in fits and starts but releasing it at a constant rate. (That analogy, too, breaks down, as the capacitor has frequency-dependent behavior, particularly electrolytics, and voltage-dependent behavior, particularly ceramics, and electrons become trapped in the dielectric only to be released over time.) The pool analogy clearly ignores the noise issue. It may help to consider how electrons acquire noise and how it may be removed. Think of how a clean signal is modulated by a noise source, and once having been altered how the only way to make the excrement back into a cow is to extensively filter it and even that isn't perfect, as the filter may itself introduce noise in the pass-band because of ringing in the pass-band or stop-band.

The migrating electrons are modified, one might say "corrupted" since it is a negative outcome, by noise on their journey through the cathode's internal structure and then again as each escapes from the cathode's rough surface. Once emitted each electron—again, containing its own statistical component of the noise spectrum—would normally directly transit across the tube to the plate, except that the interposed grid (when in cutoff or semi-cutoff) forces all of them back into the space cloud, much like a holding pen. So the cloud is chock full of noisy electrons which are attracted to the plate (or screen) but which are not permitted to cross the tube. (The plate is essentially saying, "Give me your huddled noisy electrons yearning to be free, I lift my electro-positivity beside the golden grid", and the grid, decked out in a jet-black Hugo Boss uniform and mirror-polished boots, is saying, "Papers, please!".)

Thermal noise, Brownian noise, flicker noise, mains-frequency and power-line leakage noise from the heater; every possible type of noise is represented in the cloud's electrons. Once noisy the electrons can't become non-noisy without doing work to remove that noise, like using a filter. Even if some averaging could occur, the electrons in the aggregate are all picking up noise on their journey, so it's not like a noise-free source exists to average down the noise.

Shot noise (individual transit) and partition noise (electrons going to the screen instead of the plate, hence "partition"), are different types of noise, being statistical noise arising from how the electrons travel across the tube in randomized discrete units, so that's not the sort of cathode-induced noise we're herein discussing. Emission of electrons from a filament is random, so each electron is a statistically independent event, which is how noise diodes are made.

The cathode's space-charge doesn't have an infinite pool of electrons, which is why high spikes in current demand can damage the cathode as the emission moves from a steady replenishment of the cloud to lighting-bolt like emissions from the surface. The space charge essentially smooths out the electron emissions process, so the electron transit from cathode to plate isn't a random (statistical) process which adds more noise (cf. shot noise or partition noise) as electrons leap from the cathode to the plate, but the cloud does not and cannot smooth out the noise of individual electrons because no mechanism exists for that. A filter is needed. The cloud itself is full of noisy electrons which can't get rid of that noise because that would imply loss of information without some means to do that. It takes energy to move around information aka noise.

All of this becomes clearer, I think, when considering all of this from a different angle: electrons don't actually exist as particles jostling each other in that cloud. Instead, a wave function encodes the entirety of the electron's properties, including energy and noise, and the wave function collapses to actual particle behavior as needed. (Atoms, for that matter, also don't exist. They are just convenient abstractions and shorthand which explain the behavior of the software running on the computer we call the universe.) The aggregate of electrons has statistical noise properties but so do the individual electrons which retain their properties, just in a fragmented way since they're discrete entities. I don't know if this approach clarifies it or makes it worse.

I've rewritten this a number of times and simplfiied the arguments, but it still isn't as clear as I'd like it to be. Time to throw it out into the world.
 
Audio on the individual electron level. I had no idea that electrons could have their own unique noise value. I, like I'm sure so many others, just assumed that noise (or any other electrical signal) was the result of electrons flowing at the direction of other controlling factors (the conductors they flow through, source generator, etc.), and that the electrons themselves were just the munchkins "doing the work" so to speak. Can you imagine what a heyday the snake oil salesmen could have with this one!

Thanks for taking the time to share your knowledge on such a complex subject, and dumbing it down for us non-physicist types. Easy to dismiss it all as non-consequential to the bigger picture of our audio projects, and yet, the very foundation it all operates on. Amazing.

Dave
 
What we essentially have is a pink noise source (1/f) being superimposed upon, i.e. modulating, the electron steam that's transiting the tube from cathode-to-plate. So each electron must, of necessity, have some statistical contribution to that overall noise pattern being transmitted. That's the only way I can see it working.

As each electron migrates through the cathode and leaps into the space cloud to begin its transit it grabs a tiny noise packet for carry-on luggage. This makes it a statistical process. So we can't say what noise any particular electron has, but we can say that the overall stream now has additional information (noise) added beyond the signal that intended to be conveyed by modulating the cathode electron stream via the grid. Otherwise noise couldn't be added to the signal.

Think of the noise being added at the cathode as being equivalent to being added to the signal at the grid using a simple mixer. Basically the same result, even though that new signal is added at the cathode, and not the grid. So that's a very understandable process. This is the same mechanism whereby resistors in the signal path add undesired information (noise) to .the information (signal) we want to convey. (The amount of noise depends upon the resistor's type, for similar reasons to how different cathode formulations add different amounts of noise.)

So that's why I concluded this must be a statistical process. It's about undesired information (noise) being conveyed along with the desired information (signal).

I think this adequately explains how mains noise couples into the desired signal by leaking from heater-to-cathode as well. It's all about adding new information (noise) to the signal information. Sort of like using a magnetic or electrostatic field to modulate (impose a waveform) on a stream of electrons moving from cathode-to-plate in a CRT and lighting up phosphor in different places. The macro effect is being done by individual electrons all of which have some statistical contribution at the micro level to the observed effect.

I can't say that I properly understand the physics at any detail beyond this. Before I simplified it for you I simplified it for myself.
 
I still don't get why flicker noise has a 1/f power spectrum characteristic. From the descriptions above, it seems it would be more "white" noise (equal power spectrum across the entire band, rather than pink type noise with a 1/f characteristic). Comparing flicker noise to something I can relate to, 1 KHz pink noise, what is the resonant or base frequency of flicker noise? Does this frequency change based on the type of material from which the flicker noise is emanating?
 
They just bump each other like pool balls, not traveling through any distance. "..the speed of electric current (average electron drift velocity) is about 80 centimeters per hour or about 0.0002 meters per second."

Luminiferous aether was disproven by Michelson and Morley in 1887. Freshman physics.
 
I still don't get why flicker noise has a 1/f power spectrum characteristic. From the descriptions above, it seems it would be more "white" noise (equal power spectrum across the entire band, rather than pink type noise with a 1/f characteristic). Comparing flicker noise to something I can relate to, 1 KHz pink noise, what is the resonant or base frequency of flicker noise? Does this frequency change based on the type of material from which the flicker noise is emanating?

Flicker noise arises out of self-organized criticality such that the effects ripple through time. The sand piles of Bak, Tang, and Wiesenfeld, for example. It's about stability/criticality and thresholds for change.

Think of how the sandpile collapses or an earthquake both release a burst of energy, moving the system to a new stable state until more energy is added by dropping more grains of sand or compressing the tectonic plates.
 
After 40 posts, the "noise" hasn't been defined, described or shown how to be known or present, accounted for or dealt with in Tube Audio devices. I think there is a reason for that. That's why I ask. Ever found it in one of your projects?

Noise arises in many different ways. If you want definitions of noise I suggest google.

You have previously and routinely belittled my comments about the considerable Johnson-Nyquist noise, and distortion, arising from carbon-composite resistors. Your agenda is clear.

The fact that you personally disbelieve in noise adds no value to the discussion at hand, just as if you disbelieved in the spherical nature of the earth or our heliocentric solar system.
 
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