The 12AX7 Vacuum Tube: The Miracle Postwar Vacuum Tube?

After almost single-handedly ushering in the post World War II hi-fi revolution in America and the rest of the world, can the 12AX7 preamplifier tube lay claim to be as the “miracle postwar vacuum tube”?

By: Ringo Bones 

Currently used in myriads of products intended to sound musically consonant to the musically-trained human ear, the 12AX7 small signal thermionic vacuum preamplifier tube seems able to resist and be dethroned by the tide of solid state progress of reliable silicon transistors and ultra-miniature integrated circuit operational amplifier revolution of the mid 1960s onwards. But does anyone still remember the origin of this humble and ubiquitous small signal preamplifier tube that almost single-handedly usher in the “miracle” of postwar hi-fi revolution? 

The 12AX7 small-signal vacuum tube – also known as the ECC83 in the European Union – is a miniature indirectly-heated dual triode vacuum tube notable for its high voltage gain. It was developed around 1946 by a team of RCA engineers in Harrison, New Jersey under the developmental number A – 4522. The tube was released for public sale under the 12AX7 identification code on September 15, 1947. The 12AX7 was originally intended as a replacement for the 6SL7 and 6SN7 family of small signal dual triode amplification tubes, but the 6SL7 and 6SN7 soldiered on because they are much better sounding when used in zero negative feedback single ended triode audio amplification applications. The 12AX7 tube is so popular with high fidelity vacuum tube based amplifier enthusiasts during the Golden Age of Stereo onwards in its ongoing use in both vintage and authentic replica vacuum tube based high fidelity audio equipment makes it one of the few small signal vacuum tubes in continuous production since it was introduced.  

In truth, the 12AX7 tube could be regarded as the most widely used modern low level, small signal vacuum tube known to mankind. A double triode with 12.6-volt / 6.3-volt series / parallel heater, B9A base and high amplification factor make it convenient to use in a wide variety of audio amplification applications. Gains of 60 to 70 in one of its stages can quite easily be achieved. This allows one section of the 12AX7’s triode (or ECC83’s triode) stage can quite easily be achieved. This allows one triode section of a 12AX7 tube to replace a pentode in a lot of circuits, leaving the other triode section free for some other use. 

The very high gain of this tube also makes it ideal for cathode coupled phase splitters, ensuring accurate balance. The Mullard 5-20 used one for exactly this reason. The 12AX7 tube is quite linear, but because of its high impedance, it doesn’t accept high input / output signal levels. A good 12AX7 / ECC83 has a warm, smooth sound, really tubey sounding – the Sovtek made ones, like the ones stamped with the Electro-Harmonix brand, really excel. 

Noise may be a problem in moving coil pre-amplification stages because of the 12AX7 tubes’ rather low transconductance or gm. Though it haven’t stopped many leading electric guitar amplifier manufacturers during the late 1980s and early 1990s for making 12AX7 tube based electric guitar preamplifiers that use 7 compliment of 12AX7 tube gain stages able to do gains of 90dB or 1,000,000 despite of the resulting noise. The 6DJ8 or ECC88 tube or the ultra reliable military spec Sovtek 6922 small signal preamplifier tube is way more suited for such applications. 

Even though the 12AX7 tube / ECC83 tube is primarily a preamplifier tube, an audio engineering genius by the name of Tim de Paravicini of Esoteric Audio Research managed to use the 12AX7 preamp tube as the output stage of the power amplifier he released back in 1998 – the Esoteric Audio Research EAR Yoshino V20 which uses 10 pairs of 12AX7 tubes in the output stage. The preamp tubes were connected in Enhanced Triode Mode, a circuit configuration originally applied to triode-connected pentode output power tubes, whereby the grid is maintained positive with respect to the cathode, so that the grid normally flows. Under these conditions, a typical thermionic vacuum tube now behaves effectively as a current-controlled device – rather like a solid-state transistor – than a more normal voltage-controlled mode of vacuum tubes. Though Tim de Paravicini is very much a genius when it comes to using unusual tubes in audio or using familiar audio tubes in unusual applications, such audio engineering iconoclasm allowed him to use the Enhanced Triode Mode principle to use the PL509 tube – a tube normally used as a saw tooth wave generator for the vertical deflection amplifiers of old TV sets - into a very good sounding PL509 tube based audio amplifier during the early 1990s. 

The 6SL7 And The 6SN7 Family of Tubes: Venerable Small Signal Tubes?

Even though they’ve been only introduced at the dawn of World War II and oft used to drive power tubes much older than them, are the 6SL7 and the 6SN7 family of small signal thermionic vacuum tubes the most venerable of their kind? 

By: Ringo Bones 

Even though they have became popular to the point of ubiquity during the thermionic vacuum tube amplifier revival in the hi-fi scene of the 1990s and were often asked to drive output power vacuum tubes that were developed between the 1920s and the 1930s like the PX25 and 300B output power tubes and the 211 and 845 transmitter power tubes pressed into audio use, though all are directly-heated triode types and the 300B is probably the only one of the tubes specifically designed for audio amplification use. Historically, the 6SL7 and the 6SN7 family of small-signal thermionic vacuum tubes were originally released in 1939 and were officially registered in 1941 as the glass-cased 6SN7GT. During World War II – a slightly improved 6SN7 was developed then a more rugged 6SN7W was also developed a bit later for military use. 

At the height of World War II, the 6SN7 was one of the most important components of the first programmable electronic digital computers, the ENIAC, which contained several thousand 6SN7 tubes. The SAGE computer systems used hundreds of 5692 – an ultra high reliable version of the 6SN7 famous for its bright red base and much often used for missile and space exploration applications at the height of the Cold War and the USA versus USSR Space Race – uses hundreds of 5692 vacuum tubes as flip-flops, also known as RAMs. 

Major Western consumer electronic manufacturers considered the 6SL7 / 6SN7 family of tubes virtually obsolete around 1964 and ceased its production because the more modern 12AX7 vacuum tube was starting to replace it in most consumer electronic applications. Though the US Department of Defense and the then NACA and then NASA were still using ruggedized military spec ultra reliable versions of the 6SL7 / 6SN7 family of tubes around the mid 1960s, they probably building up their stock of these types of tubes via existing – but slowly dwindling – NOS or new old stock supplies and sources. The renewed interest of single-ended triode zero negative feedback audio amplifiers in Japan during the start of the 1970s and the single-ended triode vacuum tube hi-fi amplifier revival of the 1990s eventually triggered a renewed interest and remanufacture of such family of tubes. 

Even though the ultra reliable ruggedized military spec American NOS 6SL7 and 6SN7 vacuum tubes famed for their unmatched sound quality probably only became widely available to post-Soviet Russian tube hi-fi amplifier enthusiasts way after the disestablishment of the US Strategic Air Command back in June 1992, such small-signal tubes are indispensible in the construction of zero negative feedback single-ended triode amplifiers. 

The 6SL7 tube is an Octal based high mu double triode with a 6.3 volt heater. Like the 6SN7 tube, the 6SL7 tube has been around for a long time – since 1939 in fact - and there are quite a few versions to choose from. The 6SL7 would be used in similar applications to the 12AX7 – or to those in the EU, the ECC83 – as phase splitters, low-level preamp stages, etc. Linearity is good but as with other high mu vacuum tubes, large voltage swings aren’t possible. As a driver, the 6SL7 will perform better than a 12AX7 tube but not by much because its anode impedance is too high. 

Although its amplification gain is slightly less than that possible from a 12AX7 tube, a stage gain of around 50 is available from the 6SL7. Generally, microphony and excess noise is low but as its gm is low at 1.6 milliampere per volt, moving coil input stages aren’t really practical. There are military versions of the 6SL7 which are ruggedized, ultra reliable and of course ultra expensive. The 5691 vacuum tubes are the missile silo / Cape Kennedy version of the 6SL7 tube and is regarded as the best example. Used in a lot of older American hi-fi audio equipment, the 6SL7 has a warm but very clear sound. The later remanufactured versions of the 6SL7 have a different presentation compared to older types as with the sister 6SN7 tube. Modern hi-fi amplifier manufacturers who use the 6SL7 tube include Cary and Audio Note in their single-ended triode zero negative feedback power amplifiers. 

The 6SN7 (the B65 tube is the British version) is an Octal based double triode with a 6.3-Volt only heater, the 6SN7 ranks along with the 12AX7 / ECC83 as one of the most popular tubes of all time. A quick flick through the Radio Designer’s Handbook show its pages littered with 6SN7 tube based designs. It is one of the most linear tubes of its class. A gain in the region of 15 for one stage is quite normal. 

Generally, the noise and microphony of the 6SN7 tube are low making this tube suitable in preamps as well as power amp driver stages and most competent audio designers would use it in place of a 12AU7 / ECC82 tube in almost every application. The US Department of Defense used 6SN7 tubes by the truckload and had special versions made. There is even an ultra high reliability version for telemetry kits to analyze missile trajectories and space exploration applications during the 1950s – like the 5692, famous for its bright red base, but don’t try to buy any unless you’ve got a bulging wallet! 

The sound quality of most 6SN7 tubes is excellent, but like wine, it tends to get mellower with age. The later 6SN7WGB or STC types have a lean, clean sound. Earlier manufacture versions of the 6SN7 have a slightly softer but all have a very open, natural sound quality. The best known application of 6SN7 tubes during the Golden Age of Stereo was the British Williamson amplifier which used GEC B65 tubes.  

Whatever Happened To The Vacuum Transistor Based Audio Amplifier?

With a working prototype demonstrated by KR Enterprises in the Netherlands back in March 1997, whatever happened to the Vacuum Transistor based audio amplifier? 

By: Ringo Bones 

Back in March 1997, Riccardo Kron of KR Enterprises accompanied by his wife, Eunice, visited the Netherlands to demonstrate a power amp said to use a totally new – in the audio world that is – output device called the “Vacuum Transistor” The amplifier had only just been finished and this was also the first time that Mr. Kron had taken it on a trip abroad. The demonstration took place in the listening room of a Dutch hi-fi magazine Audio & Technick with only four people present. One of them was Dutch hi-fi journalist Peter van Willenswaard and also includes the representative of Dutch distributor Analog Audio Productions who was also hearing the amp for the first time. 

The new Vacuum Transistor device is said to be a descendent of a device developed by the Russian military for use as a servo motor driver in Russian cruise missiles. It is well known that the Russian military continued using vacuum tube electronic devices in their defense equipment because, unlike solid-state devices, vacuum tubes can recover from the high-density electromagnetic pulses that follow nuclear explosions. Even though General Electric, RCA and United Electronics had been producing ruggedized military spec reinforced anode tubes for the US Air Force since 1947, one wonders if the RCA’s Nuvistor and General Electric’s Compactron also recover from high-density electromagnetic pulses – i.e. EMPs - that follow nuclear explosions. 

Back then, Mr. Kron refused to give clear answers to any technical questions about the device; this despite his assertion that the device had been patented and the attendees’ objection that a patent means protection. Nor was Mr. Kron prepared to show any measurements or give precise operating voltages or currents. Such an approach makes life sore for your typical journalist and for a while Mr. van Willenswaard thought he’d drop the subject. But when Willenswaard heard the amplifier, he thought that it was pretty thrilling. 

Then Mr. Kron told the attendees that the Vacuum Transistor was contained in a very thick glass tube, indicating that some tubes were 100 to 200 millimeters long – surrounded by a black metal body that serves as a heatsink. The Vacuum Transistor is capable of very high peak currents at 11 amperes. No other vacuum tube device of this size at the time comes anywhere close to this value, though solid-state power devices attain such a figure easily. This could be the reason why the name “Vacuum Transistor” was chosen. 

The amplification process takes place in the vacuum inside the glass tube and the attendees’ suggestion that it uses a heater was not met with direct denial. As for the bias and operating voltages, Mr. Kron disclosed only that the device’s input had to be driven with several hundreds of volts. Though this is probably a peak-to-peak figure, even so, it would imply a very low-mu device and a tough job for the driver stage. Maybe a military spec Sovtek 6922 preamp tube, with its higher voltage rating and higher anode dissipation in comparison to its “civilian” ECC88 and 6DJ8 preamp tubes have enough grunt to drive the KR Enterprises’ Vacuum Transistor device to full power. Optimum operating temperature is said to be 60 degrees Celsius, which Willenswaard later confirmed by touching a running Vacuum Transistor amplifier. The device can easily be replaced if necessary. 

Because of the enormous current capability, the amplifier is equipped with active protection circuitry that permanently monitors the current drawn from the power supply; the absence of such circuitry would mean that, in case of malfunction, the output device would fry the output transformer and / or power supply. The amplifier as a whole is without overall feedback and is capable of delivering several tens of watts into the load connected.  

There was about an hour’s time to audition the amp, which had been warming up during Mr. Kron’s introduction of the amp’s working principles. For comparison purposes there were a 6C33 based amp that Willenswaard was reasonably familiar with and a VV32B amp from KR Enterprises that Mr. Kron had brought along. Neither of these fully attained the level of sound quality of the better 300B amps of Willenswaard’s experience. 

But the Vacuum Transistor amp blew both away and by a wide margin. The sound of the Vacuum Transistor was big but not heavy, powerful and dynamic; with lots of low-level information; voices hung completely detached in space. Most important of all, the sound was incredibly real and to an extent Willenswaard never heard before. But he had only one criticism: The highs were a bit on the cool / analytical side. Caveats aside, today’s top vacuum tube manufacturers had never made a competing product aimed to challenge the KR Enterprises’ Vacuum Transistor audio power amplifier. 

Nuvistors: The Forgotten Thermionic Vacuum Tube Technology?

Despite being briefly reintroduced back in August 1998 in the Musical Fidelity Nu-Vista preamplifier, have the nuvistors become the forgotten thermionic vacuum tube technology? 

By: Ringo Bones 

Despite the value-for-money high fidelity gear manufacturer Musical Fidelity’s rather heroic attempt to reintroduce nuvistors to the unsuspecting but eager hi-fi world of the late 1990s with its Nu-Vista preamplifier back in August 1998 and the Nu-Vista 300 hybrid power amplifier in June 1999, it seems that the current top vacuum tube manufacturers never got interested enough to start making nuvistors again. Given that the thermionic vacuum tube renaissance of the 1990s had made Russian based vacuum tube manufacturers like Svetlana and Sovtek / Electro Harmonix to name a few have become profitable in their reintroduction of the most used tubes to the hi-fi world - i.e. the 12AX7 / ECC83, ECC88 / 6DJ8 / 6922 preamp and the EL34, EL84, 6L6 / K5881, 7591A output power tubes just to name a few – why is it that Russian and Mainland Chinese vacuum tube makers never mentioned it or even ventured into remanufacturing nuvistors? 

For the uninitiated, nuvistors are a type of miniature vacuum tubes introduced by RCA into the consumer electronic market back in 1959, though the famed British vacuum tube maker Mullard also made them in quantity with permission from RCA during much of the 1960s. The technological possibility behind nuvistors is most likely the result of the 8-year research made by Dr. Harvey C. Rentschler that led to the conclusion that atoms of gas – oxygen, hydrogen or nitrogen – actually dissolve in the crystalline structure of some metals just as salt dissolves in water. These gas particles then “loosen” the electrons in this structure, causing them to be emitted from the metal more readily as heat is applied. Dr. Rentschler’s research findings were later published in the July 1943 issue of the Scientific American magazine. And such then newly discovered phenomena that lead to the development of smaller and more efficient vacuum tubes that eventually paved the way to the development of the nuvistor. 

Most nuvistors are basically thimble-shaped but are 5 to 10 times smaller than an actual thimble and are much smaller than typical conventional preamplifier vacuum tubes of the day. Typically, they are 20-mm high and 11-mm in diameter. Nuvistor triodes and a few tetrodes types were made. The newfangled ultra-miniature tubes were made entirely of metal and ceramic. During its heyday, manufacturing nuvistors require special equipment since there is no intubation system to pump gases out its ultra-miniature metal envelope. Instead, the entire structure is assembled, inserted into its metal envelope, sealed and processed in a large vacuum chamber with simple robotic devices. 

Even though the “vacuum” inside a typical thermionic vacuum tube is 50,000 times less rarefied than the Horsehead Nebula, the vacuum chamber used to mass produce nuvistors back then must had been a technological manufacturing tour-de-force of its day to be able to maintain a “vacuum” of about 0.000001 Torr or millimeters of mercury – a level of vacuum required for a high-quality thermionic vacuum tube destined for high fidelity and other high quality consumer electronic use. By comparison, normal atmospheric pressure is 760 Torr or 760 millimeters of mercury. 

One of the most popular types of nuvistor during the electronic device’s heyday was the RCA 6DS4 Nuvistor Triode vacuum tube. It measures 20-mm high and 11-mm in diameter. During much of the 1960s, nuvistors were among the highest performing small signal receiving tubes. They feature excellent VHF and UHF performance plus lower noise figures in comparison to first generation germanium based point contact transistors. Nuvistors are widely throughout the 1960s in television sets beginning with RCA’s “New Vista” line of color TV sets in 1961 with the CTC-11 chassis and top of the line radio and high fidelity equipment’s RF sections. Nuvistors competed with the solid state revolution of the 1960s along with General Electric’s Compactron and probably held it at bay for a few years. RCA discontinued the use of nuvistors in their television tuners by late 1971. Given that RCA along with General Electric and United Electronics had been supplying ruggedized military spec anode reinforced vacuum tubes to the US Air Force since 1947, rumor has it that nuvistors are supposedly able to resist the powerful electro-magnetic pulses – i.e. an EMP - produced by nuclear explosions. 

One famed use of nuvistors during its heyday was the Ampex MR-70 open-reel tape recorder – a costly tape recorder for professional recording studio applications whose entire electronics section was based on nuvistors. Another commercial application of the very small nuvistor tube was in the studio-grade microphone of the early 1960s – the AKG / Norelco C12a, which employed the 7586 medium-mu triode transistor – the first ever nuvistor released on the consumer electronic market. It was also found out later that with minor circuit modifications, nuvistors could serve as a sufficient “ad-hoc” replacement of the then obsolete and no longer produced during the 1960s Telefunken VF14 tube used in the famed Neumann U47 studio microphone. 

Given that Musical Fidelity CEO and Classically-trained clarinetist Antony Michaelson managed to bulk-buy Mullard made 6CW4 high mu triode nuvistors (and the most commonly made nuvistor variant) before he became too busy playing / touring with the Michelangelo Chamber Orchestra in the Henry Wood Hall, London while being recorded by Tony Faulkner, the few lucky 300 or so hi-fi enthusiasts lucky enough to buy one or were in line of the only 300 or so of the Nu-Vista Preamplifiers and the Nu-Vista 300 Hybrid Power Amplifiers that were ever built between 1998 and 1999. 

Luckily, I was able to audition both the Nu-Vista Pre-Amplifier – which costs 2,500 US dollars back then - and the Nu-Vista 300 Hybrid Power Amps – which costs 5,500 US dollars back then. When the Nu-Vista pre-amplifier was paired with a competent solid-state transistor power amp, it managed to make that amp sound as if it was made of 6550 tubes in the output stage. Both Musical Fidelity nuvistor based products managed to extract fine detail and low-level information with the grace of a single-ended triode amplifier – only this time they are driving somewhat your everyday difficult to drive speakers by SET amp standards. The 5,500 US dollar Nu-Vista 300 has comparable sound to the JFET input Audio Research VTM 200 monoblock power amp, though the 6550 output tube equipped Audio Research costs around 14,000 US dollars per pair. Compared with old–fashioned SET tube amps, Musical Fidelity’s nuvistor based amps are slightly “cooler” in the midband. If only Russian based vacuum tube manufacturers like Sovtek / Electro Harmonix or Svetlana would start remanufacturing nuvisors for hi-fi and / or electric guitar use.   

Can the Audion Tube Be Used As A Hi-Fi Audio Amplifier?

Given that the iconic Lee De Forest’s first ever active electronic amplifying device initiated the 20^th Century electronics revolution, will it still work today when used as the output tube of a high fidelity audio amplifier?

By: Ringo Bones

Ever since the 1970s when dedicated Japanese audiophiles were experimenting with first generation vacuum tubes that were first designed and manufactured during the 1920s and 1930s into single-ended triode zero negative feedback audio amplifiers, audio amplifier design technologies long ago abandoned by the industrial West, there are probably dedicated audio hobbyist today that are starting to wonder if the Lee De Forest designed iconic first triode vacuum tube- the Audion – would be feasible when used as an output audio tube in a single-ended triode audio amplifier design. But will such an unseemly “old school” electronic audio design still viable and work well into the second decade of the 21^st Century? 

Based on established written history on the development of electronic engineering, it was the pioneering work of Nikola Tesla and Guglielmo Marconi on radio that initiated the development of the thermionic vacuum tube. Before the discovery of the germanium detector, early crystal detectors employed a piece of galena – a type of lead ore – and a catwhisker. Not all spots on the galena were sensitive and you had to hunt for a spot to touch with the catwhisker. A slight vibration on the workbench and you’ll lose the sensitive spot. Also, if the galena should become “dirty” via further exposure to atmospheric oxygen, you might never find a sensitive spot for your crystal radio set to work again. Obviously, the galena detector had serious drawbacks. 

Oddly enough, the first hint as how to improve the detector came in 1883, long before the crystal detector was first used as a radio receiver – or become a favorite of electronic hobbyists and elementary school level science projects. In that year, Thomas Alva Edison was experimenting with filaments for his new invention – the electric light bulb. He placed a filament in a glass bulb and then exhausted the air, creating a vacuum. By means of an electric current, he tested the filament until it glowed brightly and produced light. 

Edison soon observed an undesirable feature about his bulbs. After short time, a black substance was deposited on the inside of the glass, interfering with the light given out. In an attempt to eliminate this deposit on the glass, Edison inserted a metal plate. Now, this plate did not help much to solve the problem, but one day he connected a delicate electric meter between the plate and the positive end of the filament. To Edison’s amazement, the meter showed that a small electric current was flowing through the circuit. He did not know why this current should flow and he merely jotted down this strange fact in his notebook and forgot about it.

Today, we know why this current flows. When a filament is heated to incandescence (heated to when it becomes hot enough to give off light), it shoots off streams of electrons. This behavior is known as the “Edison Effect” or “Thermionic Effect”, of a filament heated to incandescence. These electrons given off by the hot filament collect on the cool plate and, if a path is furnished for them, they will flow along the path of the filament. The electric meter in that path can show that electrons are flowing. 

Since the discovery of the Edison Effect back in 1883, electron theory was still a relatively under-investigated phenomena. But in 1904, J. Ambrose Fleming, an Englishman, who understood the flow of current in terms of electrons, decided to experiment a bit. To depend upon the electrons piling up on the cool plate, thought Fleming, is too slow. Suppose we were to create an actual deficiency of electrons on the plate by placing a positive charge on it, wouldn’t that attract still more electrons from the filament? Fleming connected a battery in the circuit from the plate to the filament in such a way that the positive post of the battery was connected to the plate. He also connected another battery to the filament to heat it to incandescence. Note that this filament battery is not in the plate circuit. With such set-up, Fleming basically invented the first thermionic vacuum tube diode. 

Soon after Fleming’s thermionic vacuum tube diode appeared, in 1907, an American inventor, Lee De Forest, undertook to carry further some ideas suggested by one of Fleming’s experiments. De Forest knew that when Fleming placed a positive charge on the plate of his tube by means of a battery connected between the plate and filament, a much greater electric current flowed through the meter than when there was no such charge. Further, the greater the positive charge on the plate, the greater the flow through the meter. Actually, this did not go on forever, after the positive charge reached a certain value, placing a greater positive on the plate had no further effect. 

It was then that Lee De Forest had a stroke of genius. Since the flow of current in the plate circuit starts with the stream of electrons shot out by the heated filament, he began to experiment with that electron stream. After more experiments, De Forest eventually met the difficulty of practical amplification of weak RF signals received by the Fleming thermionic diode detector by making the new electrode in the form of a mesh of very fine wire – a grid. Since most of the grid consisted of open space, most of the electrons pulled over by a positive charge on the grid now shoot through these open spaces an d continued right on the plate. The grid was the solution to his problem. Since charges on the grid control the flow of electrons from the filament, we are able to control the large plate currents by means of a small charge on the grid – and this is what De Forest set out to do and eventually created the first ever triode thermionic vacuum tube which he called the Audion. When Dr. Lee de Forest placed a third element, the grid, between the cathode and the plate of the Fleming thermionic vacuum tube diode, he introduced the magical word – amplification.   

Given that it was from the first improved Edison light bulbs that the first thermionic vacuum tube triode were derived by Lee de Forest in 1905, every hi-fi hobbyist is probably now wondering whether those old Audion tubes could be made into single ended triode audio amplifiers. After all, during the 1990s, tubes developed and manufactured during the 1930s – as in the Western Electric 300B – or from the 1920s – like the old light bulb looking PX25 tube – were successfully used in the finest sounding hi-fi audio amplifiers during the latter half of the 1990s, would an even older triode tube – as in the turn of the century (early 1900s that is) era Audion even sound better?

Looking at a typical Audion tube – even recently constructed working replicas constructed with parts that then exist before 1910, one would wonder if this tube would be more” microphonic” in comparison to “newer” designs like the 300B or the PX25. And given that the Audion doesn’t have the much improved plate current ratings – therefore lower output impedance – of the output power tubes that were invented years after it – like the PX25 from the mid 1920s or the Western Electric 300B that cam much later in the 1930s, one would wonder if the Audion would produce a “good musical sound” if connected in parallel since most output transformers in the hi-fi DIY market today were more likely to have been designed to work with the more robust plate current capable PX25s and 300Bs. But like most DIY audiophiles, I would probably never pass the chance to hear first hand the sound of a working SET amplifier using authentic period-correct replica De Forest’s Audion tubes.