Emission or transconductance?

There are two types of tube testers and a controversy about which is better: those of emission and those of transconductance, in fact both measure the activity of the tube under test, and this activity depends directly of the condition of the filament, although other types of wear and damage may be produced. A transconductance-type tester (like a TV-7) is better than a emission-type tester, but not too much better. The difference is that a emission tester is static (no AC voltage into control grid, just the bias) and a transconductance tester is dynamic (AC voltage into control grid). The dynamic test is a step further from the static test, but results are similar. There is also a static method to measure transconductance, which can be included in an emission-type tube tester.

Meter scales in Gm marking or with activity level numbering?

Another controversy related to transconductance tube testers is about the marking of the meter scales. Which ones are better? Those with transconductance values... or those with numbers indicating activity level values? The measure of transconductance is the ratio of the current change (max-min) at the output as compared to the voltage change (max-min) at the input, the meter gives the average result [BTW, it becomes evident that this value expressed in mA/V provides a more accurate translation of this event than expressing it in micromhos]. A transconductance reading in a tube tester tells the truth only when a tube is tested at design voltages in class A operation, but it is impossible to design a tube tester that have the voltage levels and specifications that many tubes models need, so the only thing that can be achieved is an equivalence effect at lower voltage levels and/or under simplified specifications, therefore micromho readings may cause misunderstandings. There is an easy test to check if the labeled micromho readings are true: Several NOS tubes should be tested recording micromho readings, and then verify in a tubes manual if these readings match the manufacturer's value for each tube. If a Gm reading on the meter does not clearly correspond to the Gm specification for that tube in the tubes manual (± x%), the Gm reading obtained is useless information as a transconductance value, it's simply an activity level value expressed with a sophisticated number; therefore it would be better if the meter had been marked with activity level values. IMHO, a numbered scale with activity level values is better because in practice it is very difficult to obtain full Gm correlation with what's stated in tube manuals, also, activity level scales are more suitable for making tube comparisons than transconductance scales; to do this, a TV-7 is a very good tool.

There is no magic inside a tube.

It should be remembered that a tube has NO sound, the "sound" of tubes is one of the greatest 'alternative-marketing' tricks ever. Within this environment, tube manufacturers are also often portrayed as designers of high-end stuff, thus "that brand" is better for jazz, "that other" for rock, etc. This is totally false and misleading, since tube manufacturers cross-ordered, and tubes weren't designer jewelry but an industrial product [GOLD pins were an effective design at VHF/UHF frequencies, USELESS at audio frequencies], there can be no such thing as "the top end detail of the German tubes and the rich bass of the Mullard and RCA tubes". If they read now these nonsenses would have a good laugh, but for competitive reasons they contributed in some way to create the misunderstanding. The sound can only change when technical characteristics involved in the frequency response change; all the "tube sound" fabrications are based on transforming any secondary aspect into an important one and sneakily associating it with the sound; so constructive aspects such as getter shape, plate dimension and finishing, number of supporting mica sheets, etc, "have sound" when in fact they were mfg adaptations that depended on the manufacturer, but none of these adaptations implied modification of the standardized characteristics established in tube manuals. It's surprising that an electronic component, not an electromechanical one such as a loudspeaker, can be analyzed that way. Even rectifier tubes "have sound"!!! ...what they must have is 'load-bearing', which must work in combination with which the power transformer can provide [if this combination fails, sound quality will fail]. That metaphysics of sound creates pompous phrases, such as that a tube can give "a three dimensional bold clean tone with a big bottom, warm mid range and silky top end" or it is "an airy sounding tube that has jaw-dropping imaging coupled with a ruler-flat response" One of the latest 'creativities' was based on theories of superconductivity, the so-called “multi-stage cryogenic process”, which claims that it “reduces material stress and tube working temperature, thus improving welds”; in short, promoting conspiracy theories for marketing purposes and disguising the imagination as technical analysis. I copy and paste here a text excerpt from the internet that includes IN ONE PACKAGE these 'philosophies': "...different brands of tubes sound different because their mechanical construction varied and their cathode coatings varied, the manufacture of a cathode is one of the blackest arts ever seen, oxide cathode is witchcraft..." A tube has a design that is not mechanical precisely, cathode coatings were totally standardized, nothing to do with black arts or witchcraft, and the sound is related to the impedances and frequency responses of the involved circuit, not with 'tube mechanics'. And it does not matter where the tubes are manufactured as long as it be in standardized conditions, so it does not matter Blackburn, Lancaster, or Eindhoven. The basis of this marketing trick is to make the anecdotal fundamental, and then argue at length using those "fundamentals".

A tube needs a dedicated environment for its design specifications to be met (a tube is not a plug-in circuit).

Tubes are not plug-ins to change characteristics, they are electronic components that make the circuit design features fulfilled. Therefore, the truth about any kind of "tube sound" is accurate output transformer design driven by a tube circuit adapted to it. There are tubes designed to 'process' audio, as well as capacitors that are better suited to audio frequencies, but the special sound of the "tube sound" is due to the response curve of the output transformer combined with the frequency response of the circuit (feedback control), not because of the tubes; the response of any tube to audio frequencies is much more than sufficient (in fact, the so-called "tube sound" should be called "high-impedance electronics sound", although that turns out to be less consumer-oriented). Advertising slogans like "the 'X' tube gives you outstanding highs and mids when the 'Y' tube brings deep lows" will be as true as the 'Z' output transformer allows it; and it can only be true when the circuit containing 'X' is set for treble and mid output, the circuit containing 'Y' for bass output, and 'Z' is wonderfully made to fully reproduce the high and low ends of both ranges [hard to get; regarding frequency response, the best transformer is the one that does not exist]. Playing with misunderstandings is a constant in the so-called "tube sound". The output transformer is what gives the differential 'personality' of tube amps (warm tone and sonic presence), because the transformer makes up a frequency selective pass filter (rich midrange effect); any other consideration may exist, but it is complementary, of a lower level. In a nutshell: transformer, not tubes; what "sounds" is: a) the output transformer [aided by the power transformer] + b) how well designed the circuit is + c) a quality loudspeaker suitable to be connected to the secondary of that output transformer. And regarding tubes, the final ones are chosen by output power, and the rest by sensitivity and amplification level, not by much else. On audio frequencies there isn't much problem regarding tubes, the circuit design is what makes the tubes electronically "sound" good or bad.

The pleasant natural sound of a quality tube amp is due to the frequency response curve of the human ear is being matched by the output response of the electronic device which, in this case, is determined by the limited passband of an output transformer (no 'extra sounds' bothering speakers), perhaps it could be an example of 'less is more', but that audio response is technically less perfect than a quality solid-state device can provide. The average frequency response curve of the human ear can be simulated in a solid-state amplifier by inserting a good equalizing filter at its input; and similarly, the influence of the transformer on the sound can be verified even in transistor devices, surprisingly some of them had 'tube circuit sound' as they had driver/output transformer in the audio stage [e.g. early 1960s Matsushita-National transistor radios]. When a tube is replaced what happens is that the circuit values may shift and the result may be contradictory depending on taste, hence comments like that such tube "softens up a harsh amp" as if that was an intrinsic characteristic of the installed tube. An used tube has different frequency response than a new one due aging, and another type of tube (e.g. replacing 12AX7 by 12AU7) has different load impedance due internal construction, so the effect of the change in sound will be drastic. Because of that, on AF applications an old/used tube may sound "better" if the listener likes mid tones and soft sound, but on RF applications a new tube always "sounds" better. Tubes of the same type, but from a different manufacturer, manufactured to specifications have few differences between them, but beware about some modern manufacturing, in fact the last true modern tube manufacturing is... the 80's manufacturing!

How a tube acts in a circuit depends upon the impedances which face it in the circuit; so getting the most out of tubes is a matter of getting the right impedances, not believing that the existing impedances will adapt to any tube. A tube will work at its design characteristics (Gm, µ, etc.) only when it is surrounded by the circuit that allows those characteristics to be operational. A tube is an electronic component, it has no life of its own. In short: a) It's the circuit that transfers its properties to the tube, not the tube to the circuit; b) A tube meets expectations only when the circuit allows it to meet them; c) Sound quality depends on the entire circuit.

Cliches.

A common misunderstanding on tube gear is to believe that tubes have relatively short lifespan, or they are the source of all issues, this is only true for tubes operating near or above their maximum design limits. Many times, tubes are blamed for external or circuit design errors, this is common in receiver oscillator tubes; e.g. it is impossible to avoid damping at the ends of a broadband coverage, the tube may stop oscillating, whereby the tube is blamed for a manufacturing defect when the defect rests with the circuit designer [the unfortunate 6C4, which is an excellent tube, is usually found guilty]. If tubes are well manufactured and operated, they are one of the most reliable and durable components in an electronic circuit. Filaments are strong, vibration resistant and durable if they are fed within voltage margins (±10% nominal), if the rest of electrodes operate within specifications a tube has a very long lifespan. The death of a tube is analog, not digital, usually tubes die slowly, not suddenly.

Facts.

Tube effectiveness over time and aging: Tubes lose effectiveness because the number of electrons emitted by the cathode/filament decrease over time affecting the frequency processed by the tube. This type of aging is more noticeable on VHF than on HF, for it to be noticeable at audio frequencies a tube must be quite aged. There is no type of tube tester or analyzer that verifies effectiveness at high frequencies nor under load, so used tubes rated "strong" on a tube tester may not be as "strong" operating in a real circuit.

Tube testers are not lab gear, in fact the lab gear for testing vacuum tubes is very scarce, this specialized equipment was only accessible to manufacturers. It has been said that the only thing that tube testers can do is a simulation, but despite that fact, tube testers are very useful for taking a "snapshot" picture of a tube (needle deflection on meter's face) and making comparisons among tubes of same type because the "test environmental background" is common to all.

A TV-7 is a very good tool for testing American "modern" tubes (40's, 50's, 60's production + European clones), except exclusive European types, high power, or special ones; but important deficiencies must be taken into account when testing some types of tubes. It's necessary to keep in mind the TV-7 is basically an I-177 successor (the portable tube tester used in the WWII) designed when the modern high transconductance tube didn't exist, these type of tubes were "adapted" (poorly) to the instrument later, that's why the basic TV-7 of 1952 evolved into model "D" in 1959. This latest model is the easiest one to align, since there are internal potentiometers for all of the required adjustments; earlier models have a less comfortable alignment, because they require replacing fixed resistors to make some of the adjustments.