The Weak Link

The major problem with hollow state is that the vacuum tube is a high voltage, low current -- and therefore Hi-Z -- device. Electromechanical speakers are just the opposite: requiring high currents at relatively low voltages. The usual solution to this problem is the impedance matching transformer. Unfortunately, no transformer that has a ferromagnetic core is a linear device. Air core transformers that can operate at audio frequencies are not exactly efficient, and what you gain in core linearity you promptly lose in the enormous stray capacitance that the huge number of turns such a thing would require.

So the ideal situation would be to get rid of that transformer completely. There are a couple of factors that go against this: the Hi-Z nature of the device itself, and the fact that there is no such thing as a "P-Channel" VT. Given that, you are limited to some sort of "totem pole" output stage. Here, the problem is the lack of symmetry: one half will be acting as a grounded cathode amp with its characteristic voltage gain, while the other half will be, more or less, working as a cathode follower with its less than unity voltage gain. There have been various attempts to deal with this problem such as asymmetrical drive -- the SEPP output, or the Futterman variations that include feedback to either make the cathode follower half behave more like the grounded cathode half, or the inverted Futterman that applies NFB to the grounded cathode half to make it look more like a cathode follower.

There is another topology that at least takes care of the balance problem: the Wiggins Circlotron design, this being a bridged output stage. Even though originated for use with an OPT for the purpose of eliminating Class AB switching transients from the primary, it also makes for a very good OTL design as well.

The other problem remains: the low current nature of the VT itself. You really don't have much choice here, so far as possible types are concerned.

There have been VTs that were intended to give some serious output currents, these being the horizontal deflection finals. Even though the screen voltage specs limit the screen voltage to rather low levels, these can still pull some serious current at lowish V_PK's. Still, you are going to have to parallel up individual units in order to get the necessary output currents. The 6BQ6GTB can easily pull some 350mA of peak current when the plate and screen both operate at 150V_DC, if you go a bit into Class AB₂. For 30W of output into 8R, it will still require 16 6BQ6GTBs (8 per phase). That's a bit too much.

Other possible types would include the 6C33C (Series pass regulator used on the MiG fighter). There have been a lot of problems concerning this type, as its reliability is questionable, even with the special "burn-in" trick. Also, the pins are way too thin for the heater current requirement. MiG technicians replace not just the 6C33Cs, but also the socket as part of routine maintenance. I would avoid it.

The most likely type is the 6AS7 (and its relatives: 6080, industrial type, or the 6082, 26.5V heater version) -- another type originated as a series pass regulator tube. As such, it has the high current capability with reasonable V_PK's, and it is also a dual triode, so that you require half the number of bottles. Though its plate characteristics aren't spectacular, they aren't half bad either. It's not likely you'd want to use this in a conventional output circuit as there are better power triodes for that. Still, the 6AS7 can pull significant currents without taking the control grid positive. Eight 6AS7s (16 sections, 8 per phase) can get you 30W of output without serious spec busting. The type is also well known for having a high g_m and a very low r_p. The 6AS7 has been used to drive VT Class B audio output stages (AM plate modulators) for a very long time now.

Does getting rid of the OPT really represent an improvement in overall linearity? What you might gain in one area, you might lose back in another: operating the VTs into a very steep loadline (eight sections in parallel to drive 8 ohms is just 64 ohms per section). You might be just as well off in substituting MOSFETs for the hollow state output. There have been some complaints that paralleling finals leads to sonic degradation, since the individual VTs won't all be operating at precisely the same currents with the same exact characteristics. Whether or not this makes a difference is a whole 'nother story. At least the very low (for hollow state) impedance of the 6AS7 does help to mitigate this effect. This is another one of those areas where you just might have to try it to see and make up your own mind if it's worth it.

As for trying this, it is definitely something to take a shot at. It is especially helpful in that the 6AS7 is a current production VT, and at least in this case, the new production is superior to NOS offerings: better matching between sections, and a more robust design that will stand up to the slight spec-busting that can reduce the number of triode sections per phase. The 0.125A plate/cathode current rating is for DC regulator use. That peak current can be increased to 0.375A in audio final use. Music and voice programs won't be spending very much time at the maximum peak current. (It would be a different story for DC use or continuous max P_RMS applications. (If you were doing that, then consider the 0.125A spec to be a DC/RMS value.)

The only other requirements are DC offset detection/correction since you definitely want to keep DC out of speaker voice coils, fault detection in case one 6AS7 (or one section of a 6AS7) develops some fault that causes it to pull more current, and HV delay. Being that the intended purpose was series pass duty, the heater-to-cathode insulation is a good deal thicker to stand up to higher than normal voltages. This also slows down greatly the cathode warm-up, and the specs call for preheating to make sure the cathodes are at operating temp before hitting 'em with the HV.

The specs also strongly discourage the use of fixed bias. The problem of using the much more desirable fixed bias can be solved by mixed fixed/cathode bias, and over current detection. A 6AS7, OTL, Circlotron design is definitely "in the works".

The Numbers Game

In most of the electronics field, the characterization of
performance is pretty straight forward. If a video amplifier has
inadequate bandwidth that causes the loss of fine detail, and excessive phase nonlinearity that smears images across the screen, or puts out all the wrong colours, no one who isn't blind as a bat will disagree that it is a very poor design. If a servo doesn't track accurately, no one will argue about the finer points of that kind of distortion.

The audio field is something else entirely. Eyes don't lie, but ears deceive. Characterizing the performance of audio electronics is complicated by the very fickleness of the end user. What sounds just great to one will be judged inferior by another. Different people hear different things, and there may very well be those (the "golden ears") who hear things that the rest of us can not hear at all. Throw in musical preferences, and it gets even more complex. It's highly doubtful that the head-banger and the Classical aficionado will agree that the same amp is superior to other offerings.

The marketing department needs a selling point, and that has long been THD. The Williamson of 1948 made THD fashionable, and started the fallacy that gNFB was the cure for all ills. Between 1948 and 1956, the Williamson design was all but universal. The commercial "Big Box" manufacturers went for more power and ever greater feedback factors to drive those THD numbers ever lower. By 1960, Big Box products had excessive bandwidth (necessary for stability) and enormous feedback factors. UL EL34s in Class AB had become quite fashionable. The demand for more feedback led designers to abandon the lower distortion, lower gain types (6C5, 6J5, 6SN7, 6CG7) for high-u triodes and small signal pentodes. These high gain devices are less linear, adding more open loop distortion that was then corrected by high gNFB. Solid state was a natural progression since transistors have almost unlimited gain available, getting rid of the OPT removed an obstacle to driving up gNFB, and, of course, the gross nonlinearity of transistors demanded even more correction. This naturally led to a certain sloppiness in the open loop design. Why worry about poor open loop performance, add enough gNFB and you can sweep your mistakes under the carpet where no one will ever find them. Unfortunately, you also sweep away much of the musical detail.

The end result: sound-alike amps, with that lifeless "Big Box" sound.

The supreme irony of this is that there is almost no correlation between the THD figure and listener satisfaction. Unless something measures really badly can it be said that it will sound really bad. If THD had everything to do with it, we would have attained audio perfection a long time ago. Solid state amps have THD figures of 0.001% or even better, yet when was the last time you ever heard a manufacturer of VT equipment brag that their amps had that great transistor sound?

There are further complications that you just don't see in other areas of electronic design. An RF amp works on a very narrow band of frequencies into a well characterized load that's almost pure resistance: either a transmission line or antenna. If there are any reactive components, you can tune them out. Whatever device distortion is produced is of no consequence since one or more tuned LC circuits and/or bandpass filters will reject any harmonic distortion. This does not apply to audio. Even though we draw nice, straight loadlines representing a resistive load impedance, speakers are anything but. A spec that says a speaker is an "8 ohm speaker" refers to the DC resistance of the voice coil winding. In operation, the device is an AC synchronous motor. The only difference is that the voice coil moves back and forth instead of
going around. Any time you have coils moving in a magnetic field, you have a voltage induced in the coil. That induced "back EMF" adds to the incident voltage to produce an impedance that varies in both magnitude and phase. In practice, your loadline opens up into an ellipse (for one frequency only). This is bad news for your output devices, which like to see a constant load. Then there's the question of OPTs. A transformer designed to operate at a frequency of 20Hz is going to be very different from one that's designed to operate at 20KHz. And yet, OPTs are expected to do both, everything in between, all at once. By all rights, it shouldn't work at all, let alone as well as it does.

Distortion is not created equal, nor is distortion necessarily undesirable. If that were the case, then no one would ever use a tone control or graphic equalizer. Since these suppress or enhance various frequency components, what comes out is no longer the same as what went in. In other words, the output is distorted. The undesirable, but inevitable, distortions are not created equal either. You can see this with an oscilloscope. If you add 10% of second harmonic to a pure sine wave, the difference is hardly noticeable at all. Even if you know what to look for, you'll probably miss it. It's not until you get to about 20% h₂ that the disturbance becomes obvious (flattening of the peaks on one side only, all even harmonics produce asymmetric waveforms). Just 10% of the third harmonic will make for clearly distorted waveforms (nearly triangular waves). Smaller amounts of higher order harmonics make for even more severely distorted sine waves. This means that you can tolerate more lower order harmonics than high order. If the THD is several percent, but it is all h₂, you probably won't notice it at all. It is this type of distortion that low-u triodes tend to produce. On the other hand, even very small amounts of high order harmonics can be all but unlistenable. The cross over distortion produced by underbiased transistor amps -- or Class B VT amps -- is a good example of this, and it does sound quite nasty, even in small quantities. It is for this reason that Class B is relegated to RF amplification, or audio amplification where power and efficiency are more critical than fidelity.

As for what the various distortions sound like, well, that depends. Unless the distortion is extremely bad, it isn't really all that obvious. Some harmonic distortion doesn't sound any different than background static, or hissy "esses", or background crackles. Often, there really isn't any one thing one can identify, other than listening becomes fatiguing after awhile. The least harmful is h₂.
Since the second harmonic is musically correlated, being one octave higher, it tends to make for a "richer" sound. Excessive amounts tend towards a "darker" sound, as has been the complaint against the very nonlinear 12AV7. From a design point of view, this would not look like too much of a problem, since even order harmonics are nulled with balanced circuitry. As for h₃, this, too, is musically correlated, being nearly an octave-and-a-half. The effect of h₃ is to lend a sense of "detail". More h₃ lends "edge" or "brightness" to the sound. For some types of music, this can be desirable. As for higher order harmonics, these aren't musically correlated, and sound very dissonant and unpleasant. The higher order the harmonic, the less of it you can tolerate.

THD numbers really tell you very little. The spec sheet for the 6L6, for example, gives a THD figure of just 1.8%, before any NFB is ever applied. That is really quite outstanding. The lowest THD figure for the 6V6 -- another audio type -- is 3.5%. However, operating open loop without any NFB, 6L6s definitely sound worse, with lots of "pentode nastiness" that quickly wears the listener out. With some program material, the effect is as annoying as fingernails on a blackboard. O. Schade, the developer of the 6L6, recognized this problem back in 1936. O. Schade recommended that local NFB be applied to correct for the problem of higher order harmonics. Even though the 6V6 measures worse by a significant margin, it produces mainly h₃. This doesn't mean that you won't need NFB, however, running open loop, produces mainly an overly "aggressive", "edgy", or "bright" sound. The type doesn't need the added assistance of local NFB in order to sound right. Fifty or so years ago, Norman Crowhurst proposed a weighting system of distortion measurement that would account for this: undervaluing the less destructive low order harmonics, and emphasizing the higher order harmonics. Simple THD measurements don't do this. Of course, the industry wanted nothing to do with it.

There is another type of distortion that's much worse than harmonic distortion, and that's IMD. IMD arises from the same source as harmonic distortion: active device nonlinearity. IMD occurs whenever a lower frequency mixes and modulates a higher frequency. It's really the same as AM with a very small modulation index. All IMD is quite bad since its frequency components are rarely, if ever, musically correlated. This is the reason to stay away from nonlinear devices and nonlinear operating points. IMD is also the reason why poorly filtered DC sounds worse than AC for filament heating. With AC, you have just the one frequency to deal with. DC, unless well filtered, adds quite a few harmonics of the line frequency. Even if the level doesn't seem all that high, there is sufficient to mix with signal frequencies via intermodulation to degrade sonic performance.

The simplistic answer is to add as much NFB as you can manage while maintaining a reasonable level of stability. Unfortunately, it's not that easy. Over fifty years ago, Norman Crowhurst looked into the problem, and discovered that the main effect of NFB is the reduction of h₂ and h₃. It doesn't do much, if anything, for removing low levels of higher order harmonics. NFB can make the situation even worse by filling the noise floor with lots of low level, high order harmonics. This can, and does, ruin any pretense to sound stage. In much the same way as NFB, the unavoidable quantizing noise that accompanies analog to digital conversion also adds lots of uncorrelated harmonic noise to the noise floor. It has been proposed that this quantizing noise is the reason that some find the CD sound to be inferior to vinyl and tape. This is also a problem with certain audio file compressions, such as mp3. This is a "lossy" compression, in that some of the bits are thrown away. The first consideration in which bits those should be were the ones encoding near noise floor audio. These mp3 audio files just don't sound quite right because of that missing detail (And to think some people actually expect you to pay for mp3's!) What's going on at the nearly subaudible level seems to be a good deal more important than once suspected.

The main lesson is to design for the best open loop performance, and add just as much NFB as needed to improve the overall sonics. Of course, it is a trade-off: sound stage for better bass and less of the excessive harshness that often comes with open loop operation.

Common Amplifier Myths

 

You Should Always Use (Some Brand) of Tubes

There is seldom any reason to pay premium prices for "special" VTs. I'm certain that Western Electric NOS 300Bs are quite decent tubes. I'm also certain that they are not worth the $High Four Figure prices these command. You can see certain types going for some ridiculous prices these days. That's nothing but pure audiophoolery. Put an over priced tube in a mediocre amp, and you still have a mediocre amp. "NOS" is also not necessarily the magic word either. Back in "the day" good, bad, and indifferent tubes rolled off the assembly lines by the tens of thousands every day. Some war surplus is positively hideous, which is why it's surplus -- military rejects. Truth is: some NOS tubes are horrible, and there are current production tubes that are every bit as good as those made in "the good ol' days".

This is not to say that the seller is necessarily trying to put one over (although that does happen all too often). The advertised tubes may work especially well with whatever equipment he's using. It doesn't mean equivalent performance in random gear with random speakers.

"Tube rolling" does have a basis in fact. Unlike transistors, VTs are low gain devices. Being low gain devices, circuit performance is more dependent on the characteristics of the active device. Transistors have so much gain that circuit performance is totally dependent on the circuit itself. Unless you need some special property (such as an unusually low noise figure, or that can operate at unusually high voltages) it doesn't matter what transistors you use so long as the transistor in question can process the signal frequencies. Not so with tubes, especially in designs that don't incorporate any NFB. You may get a different sound from different tubes of different construction, name brands, model years. Whether it's worth it or not is something else.

It is not necessary to pay audiphool silly prices for good sound.

Point-to-point is Good, Circuit Boards are Bad

This is definitely true for RF circuits. For P-2-P wiring, the insulating medium is air. The resulting stray capacitance is much lower, and performance definitely enhanced. This is a good reason to opt for "dead bug" or P-2-P construction of RF circuits. At audio frequencies, there isn't enough of a difference in stray capacitance to make a difference. P-2-P has the advantage of allowing the free use of "parts on hand". Circuit boards have the advantage of easily replicated construction, and much better quality control for mass production. To be sure, a badly laid out circuit board will ruin the sonic performance of an otherwise excellent design. So too will a poorly laid out P-2-P job.

If You Use Circuit Boards, These Should be Teflon

Again, a good idea for UHF circuits, but the savings in stray capacitance will make no difference at audio frequencies.

You Should Always Use Teflon Insulated Silver Wire

Another excellent idea for UHF construction. The tiny savings in resistance and stray capacitance will make no difference at audio frequencies. The added expense isn't worth it for frequencies as low as the audio band.

You Should Always Use Carbon Composition Resistors

Another idea held over from RF practice. C-comp resistors have the advantage of having a very low inductance. C-comps hold their DC resistance well into the VHF band. Otherwise, C-comps are very noisy. Save them for your RF projects. Metal film resistors are a good deal quieter, and better suited for sensitive applications, such as low level stages. Those few nanohenries that are a big deal at 400MHz are of no consequence at 400Hz.

Negative Feedback is Bad

This bit of nonsense does have a basis in fact. In all too many cases, NFB is used to cover up a poor open loop design. Pour on enough NFB and you can sweep your mistakes under the carpet, along with a good deal of the vitality of the music. This, however, is not an indictment of NFB. Used properly -- to improve an already good open loop design -- NFB can make a good amp even better. This is the key: the open loop design must be a good one. If you find that you need excessive amounts of NFB, then it's better to go back to the drawing board and correct your open loop design. All too frequently, this is not the case.

Furthermore, there is no escaping NFB. Every device, vacuum tubes or transistors of every sort, have inherent NFB. This is the cathode/source/emitter resistance: r~= ¹/g_m. This resistance will cause degeneration just as surely as if it were an unbypassed resistor soldered into the circuit. Just because you don't see it doesn't mean it's not there. Furthermore, triodes have an additional feedback mechanism. Plate current is dependent on the V_PK. When a signal tries to pull the control grid positive, plate current rises, but the V_PK drops and tries to pull the plate current down. Since the plate and grid are pulling in opposite directions, this is negative feedback by definition. Yet triodes are preferred to every active device that doesn't have this property (pentodes and transistors).

If anyone claims to have a "no feedback" design, they're either ignorant or trying to put one over -- or what's worse: both.

Solid State is the Spawn of the Devil

Another partial truth here. The first solid state amps were truly awful: noisy, unreliable, and with hideous sonics. There has never been a technology with no down side, and there never will be. There are some things that SS does very well, and some things it does very poorly. The key is balancing the strengths with the weaknesses. The only reason to avoid all solid state is if you are replicating 1930s-era equipment for the nostalgia appeal. Otherwise, do you also say "No" to modern line, interstage, and output transformers and chokes that are made from exotic materials that didn't exist in the 1930s? All these devices can perform much better these days, and make those old circuits work much better now than they ever could then.

With zero impedance and perfect regulation, plate-circuit distortion does not exceed 2.0%... The driver stage should be capable of supplying the grids of the Class AB₂ stage with the specified peak grid voltage at low distortion. The effective resistance per grid should not exceed 500 ohms...

-- 1624 Spec Sheet

Back in "the day", this was asking the nearly impossible. Getting Z_o below 500 ohms is very difficult even for a cathode follower, unless it uses some of the big power triodes. These days, any power MOSFET source follower can come very close to the ideal of zero drive impedance, certainly below 10 ohms. Use solid state where it gets the job done right. The fact that it was a considerable challenge is testified to by the lack of Hi-Fi designs left over from "the day" that operate Class AB₂. Most of the designs that did are for AM plate modulators and PA systems where the premiums were lotsawatts and efficiency, not sonic performance. These days, there is no reason not to take advantage of going Class AB₂.

Another bit of mythology is the supposed noise that solid state diodes cause. Except for audiophile circles, I have never heard such a claim. Furthermore, I haven't seen it on the o'scope screen and I have never heard it from any receiver operated right next to any audio amp that used a solid state power supply. Gas and mercury vapor diodes, however, are notorious for producing all sorts of hash and incidental RF. Otherwise, solid state diodes offer much lower forward drops, don't get anywhere near as hot, have significantly higher I_surge ratings, and require neither heater power nor another hole in the chassis. They better lend themselves to voltage multiplier circuits, and bridged circuits. If you are that worried about high frequency noise from solid state diodes, the "fix" is a two or three pole LPF made from air coils and mica capacitors. Won't do any harm, but it sure doesn't look necessary either.

If there is any down side to solid state, it's that the high voltage comes up within a couple of seconds or so -- long before cathodes have a chance to warm up. This can overvolt direct coupled stages, possibly exceeding V_HK ratings. Since silicon diodes can source much higher currents, there is the temptation to use them with huge reservoir capacitors. The increased current demand can stress vintage power transformers that weren't designed to handle currents in excess of those expected from vacuum diodes.

A solid state power supply may or may not lead to a different sound. If there is a difference, it's due to the better voltage regulation of the solid state power supply. For some applications, good regulation isn't desirable. This would apply to over driven guitar amps where the voltage drop at max current demand lends to compression, and "sustain" as the voltage comes back up. The same effect can color the sound of a Hi-Fi system. If you're used to that sound, or actually prefer it, then, of course, solid state will be considered inferior.

It is questionable whether it makes a difference in a PP amp since the AC sums to zero at the primary center tap, making it a virtual AC ground. It might make more of a difference in a SE design, since the power supply is in the signal path. Even more reason to use solid state, as the AC impedance is considerably lower.

Tube Watts are Bigger than Transistor Watts

A watt is a watt: one newton * meter per second. Transistors clip hard and fast. This is what makes them superior digital devices, but horrible analog devices. Add NFB, and clip behavior becomes even worse. Tubes clip much more gently, first simply rounding off the peaks, as opposed to just flattening them. This gradual rounding generates mainly low order harmonics that roll off quickly with increasing frequency. The sharp discontinuities of peak flattening generates high order harmonics that roll off gradually with increasing frequency. Since low order harmonics are much less detrimental, one doesn't hear the occasional clip on fast transients. A solid state amp needs to stay out of clipping. Therefore, a tube amp will sound louder than an equivalent solid state amp, since you don't hear the occasional clip, you can turn up the volume higher before obvious distortion is heard.

Tube Amps Have Poorer Bass

This may have been true at one point. One of the advantages to NFB is that it reduces the output impedance. A low output impedance is needed to damp the woofers, otherwise they will tend to produce their own resonance note, as opposed to the notes the musician actually played. Back in "the day", the main problem here was inadequate open loop bandwidth. You can have a billiard table flat frequency response from 30Hz to 20KHz, however, if it's NFB that's causing it, the bass will not be as good as you'd expect. If the open loop response prematurely rolls off the low end, the NFB decreases, thus driving up the closed loop gain. Less NFB is less effective NFB, and won't dampen the woofer(s) properly. This problem was due to interstage coupling capacitors that were too small. At one time, your choice for capacitors was limited to either PiO, wax paper, or ceramic. PiO and wax tend to be leaky (not the oil or wax, but of current) and deteriorated with age. Ceramic wasn't leaky, but also didn't come in the sizes and voltage ratings you'd like to see. Consequently, there wasn't enough open loop gain at the low end.

These days, with new polymers, it is possible to get capacitors in the sizes and voltage ratings you need for good open loop performance at the low end. Vacuum tube amps can have good bass, with adequate speaker damping. (About the only exceptions are sub-woofers with ceramic or carbon fiber cones and heavy voice coils. These sub-woofers are difficult for even a solid state amp to keep under control.)

"Magic" Resistors

Yet another out growth of a real phenomenon. During the early 1970s, it was discovered that capacitors really did have an effect on sonics. Up till this point, no one really paid any attention to passive components. They knew about such things as ESR, DF, Loss Tangents -- however it was only VHF designers who needed to worry about this. Capacitors introduce distortions, due to D/E field nonlinearities, piezoelectric effects, and even self-excited resonances. So far as dielectrics go, the worst offenders tend to be metal oxides (ceramics, electrolytics) and other high permittivity materials. The more benign being the polypropylenes and teflons. Unfortunately, these are also materials with low permittivities, and so do not lend themselves to packing a lot of capacitance in small packages.

While dielectrics have a proved effect on sonic performance, this has also occasioned a lot of audiophoolery. From legitimate claims about the effects of various dielectrics with data to back the claims up, to utter nonsense regarding the "sound" of resistors. There are lots of claims with zero data about the superior attributes (available at superior $Prices) of these things. Don't be conned into paying dollars for some part that would normally cost pennies.

If you see extravagant claims with nothing more than vague generalities backing them up, better get a tighter grip on your wallet.

Component "Burn-In"

You see these claims about a variety of components: they require X number of hours of "burn-in" before they start to sound good. This is complete nonsense. Capacitors polarize within seconds of the voltage application. I've seen claims of 100 -- 500 hours for Teflon capacitors, and with AuriCaps. The first time I replaced an electrolytic coupling capacitor with an AuriCap, I noticed the improvement at once. Of course, electrolytics make lousy signal capacitors, and AuriCaps are pretty good. There was no "burn-in" required at all.

Resistors sometimes burn out, but they never "burn-in". As for cables, well, you figure that one out. The only components that require a certain amount of "burn-in" are vacuum tubes. It may take minutes to hours for the bias to settle down when new tubes are put in service. This is completely normal, but does not apply to any sort of passive component, nor does it apply to solid state devices at all. Such claims are largely disengenuous, in that the time it takes for the new component to "burn-in" is equal to the time it takes for you to get used to the new (and most likely) inferior sound of whatever over priced, whiz-bang, gizmo you just likely spent way $Too Much to acquire.

Next time you see this particular claim, ask yourself one question: why didn't they burn the thing in at the factory before sending it out the door?