Sunday, July 7, 2013

Can Nuvistors Be Used As An Electric Guitar Preamplifier?

Even though they are for all intents and purposes just ultra small vacuum tubes that were discontinued back in the late 1971, why do electric guitar preamplifier manufacturers never made one using nuvistors? 

By: Ringo Bones 

Even though nuvistors were briefly reintroduced back into the consumer electronic marketplace when Musical Fidelity CEO and Classically trained clarinetists Antony Michaelson released the Nu-Vista Pre-Amplifier back in August 1998 and the Nu-Vista 300 hybrid power amplifier before Michaelson got too busy playing with the Michelangelo Chamber Orchestra while being recorded by Tony Faulkner, these ultra-miniature vacuum tubes were sadly largely discontinued when the 1970s rolled around. It was largely Michaelson’s fortunate bulk-purchase of a stash of unused Mullard 6CW4 high-mu triode nuvistors that somewhat saved the largely forgotten electronic devices from total obscurity. 

Nuvistors are a type of miniature vacuum tube introduced by RCA into the world’s consumer electronic markets back in 1959 though Mullard also produced it under license from RCA during the 1960s. The technology behind nuvisors became possible after the 8-year long research of Dr. Harvey C. Rentschler allowed him to conclude that atoms of gas – like oxygen, hydrogen or nitrogen – actually dissolve in the crystalline structure of some metals just as salt dissolves in water. These gas particles “loosen” the electrons in this structure, causing them to be emitted from the metal more readily as heat is applied. Dr. Rentschler’s findings were later published in the July 1943 issue of the Scientific American magazine. And such phenomena lead to the development of smaller and more efficient vacuum tubes that eventually lead to the development of the nuvistor. 

Nuvistors are thimble shaped but are actually 5 to 10 times smaller than an actual thimble and much smaller than conventional preamplifier vacuum tubes that were manufactured after World War II. Nuvistor triodes and a few tetrode types were made. The newfangled ultra-miniature vacuum 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 of 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 must have 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 required for a high quality consumer electronics grade vacuum tube. By way of comparison, normal atmospheric pressure at sea level 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 tube. They feature excellent VHF and UHF performance plus lower noise figures than early point-contact germanium transistors. Nuvistors are widely used 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. 

One famed application of nuvistors during the electronic device’s heyday was in the Ampex MR-70 open reel tape recorder for studio use – it was a costly studio tape recorder whose entire electronics section was based on nuvistors. Another application of the very small nuvistor vacuum tube was in the studio-grade microphone of the 1960s – the famed AKG / Norelco C12a, which employed the 7586 medium-mu triode nuvistor – the first nuvistor released on the market. It was later found out that with minor circuit modifications, nuvistors could sufficiently serve as an “ad-hoc” replacement of the then obsolete and no longer produced in the 1960s Telefunken VF14 tube used in the famed Neumann U47 studio microphone.
Given that the “roadie / guitar tech” who first made those fuzz-boxes / stomp boxes for the famed 1960s guitar-god Jimi Hendrix used to be Jimi’s regular TV repairman, it is quite a mystery why no examples of do-it-yourself nuvistor based electric guitar preamplifiers or effects / fuzz boxes are in existence. The famed Dallas Arbiter Fuzz Pedal used 1960s era germanium small signal transistors – imagine if some enterprising TV repairman familiar with the working of nuvistors experimented replacing the early germanium trannies with one. 

Technically, nuvistors can operate up to 110 volts DC and as low as 30 volts DC so powering one with a 9-volt PP-3 could be an engineering challenge back in the 1960s, though today’s high-voltage lithium polymer batteries used in Airsoft gaming applications could easily supply the 30-volt DC anode voltage required for nuvistor fuzz pedal operation. A typical nuvistor has an amplification factor or mu of 64 – comparable to the 12AX7 preamp tube’s 100, but  nuvistors have a much stronger transconductance or gm compared to the 12AX7 preamp tube. So if today’s top Russian vacuum tube manufacturers like Svetlana or Sovtek / Electro Harmonix  or those in Mainland China decides to remanufacture nuvistors, it would probably end up as electric guitar preamplifiers and fuzz / distortion effects pedals and boxes. 

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