Sunday, July 7, 2013

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.   


Nadine said...

Given that Musical Fidelity's Antony Michaelson used the Mullard 6CW4 triode nuvistor when he designed the Nu Vista 300 hybrid power amplifier, would the inherent "Miller Effect" problem of triodes rear up its ugly neck here? After all, you cannot use - inherently - large(ish) amounts of negative feedback with triode type vacuum tubes like the 6CW4 nuvistor, ESPECIALLY when the Musical Fidelity Nu-Vista 300 uses bipolar output transistors which by nature requires appreciably large amounts of negative feedback compared with the Golden Age of Hi-Fi era pentode and tetrode output power tubes.

Ringo said...

When it comes to the limitations of a "plain-vanilla double triode phase splitters" in push-pull thermionic vacuum tube, as well as nuvisor triode based amp designs, Dr. A.R. Bailey was clever enough to utilize triode-pentode tubes for phase splitting in his Radford designs instead of the double triodes oft used in Mullard 5.20 type amps like the famed Marshall electric guitar amplifier. Even though the Nu Vista 300 hybrid power amp uses a solid state bipolar transistor output stage by the nature of the beast it is very reliant on large(ish) amounts of negative feedback for its operation - like all Golden Age of Stereo era push-pull tube hi-fi amps. The problem with the conventional triode phase splitter is its rather high input capacitance caused by the Miller Effect (the Miller Effect not only affects tubes and nuvistors but also solid state devices like bipolar transistors, FETs and MOSFETs). The Miller Effect causes undesirable high frequency loading on the first active device - whether vacuum tube, bipolar transistor or FETs - or in the case of the Nu Vista 300, the first nuvistor input triode - thus reducing useful bandwidth and making it very difficult to use appreciable amounts of negative feedback without the resulting instability due to the phase shifts incurred. A pentode tube - or a pentode nuvistor if you are lucky enough to still find one - has a very low input capacitance and high gain due to the shielding effect of the screen grid. This means that the loading on the input tube or nuvistor is greatly reduced, increasing bandwidth and decreasing troublesome phase shifts. I, too, wonder why Musical Fidelity's Antony Michaelson didn't use a pentode nuvistor type (probably because pentode nuvistors are now virtually extinct?) as a phase splitter for the Nu Vista 300 hybrid power amplifier.