by Phil Taylor – December 2013
Author’s note: I’ve bitten off more than I can chew with this news post which was supposed to be 300 words long. It’s much longer than that and will become an indepth article on NOS tubes.
My investigations into improving guitar tone sometimes lead me to search Ebay for NOS (new old stock) tubes in the hope of finding bargain miniature B9A types such as the Telefunken E82CC or JAN Philips 12AX7s. The scarcity and escalating prices of these more widely adopted types make the thought of discovering unplundered stocks of unused vintage tubes an alluring prospect. At the time of writing this article Telefunken E82CCs (12AU7s) are exchanging hands for upwards of $200.00 each. The reason certain tubes command these astronomical price tags is in part because they’re a non-renewable resource, but more significantly because of their excellent audio characteristics.
It’s almost certain we’ll never see the likes of companies like Telefunken, Amperex or Mullard again – the huge pool of expertise, the physicists specialising in thermionics (emission of electrons from substances at high temperatures), the chemists who knew which alloys and coatings made the quietest and most stable cathodes and all the specialist engineers and technicians who made it possible to fabricate high-grade vacuum tubes are gone. All the machines and production documentation were scrapped over a half a century ago and the specialist chemical and metallurgy companies are gone too. It’s no longer possible to manufacture true intrumentation grade tubes anymore. It’s decades since these companies ceased manufacturing tubes and stocks of widely used NOS types are finally drying up. My search for them becomes ever more fruitless – there are no more bargains to be had. So I turn my search to undiscovered (by me) tubes that were developed for other applications that might be used as substitutes – substitutes that might even offer improved tonal performance or reliability.
Sound reproduction equipment that is radios, phonograms, hi-fi and guitar amplifiers, professional studio gear mixers, microphone preamps, equalisers and compressors were just one application for vacuum tubes. In their heyday they were also utilised in all kinds of other electronic equipment including televisions, projectors, lab instrumentation such as oscilloscopes, frequency counters, signal generators, volmeters and accurate D.C. amplifiers for physiological measurements. The Second World War effort pushed tube technology even further to develop radar, proximity fuses for guided missiles and logic circuits for digital computers such as UNIVAC and ‘Colossus’ (for German message decryption). A diverse range of thermionic devices were developed to fulfill all these applications – including vacuum diodes, triodes, tetrodes, pentodes, gas filled thyratrons and many other specialised and exotic tubes for light sensing and display purposes. Countless tubes were manufactured during the 20th century and the legacy is unimaginably vast.
Today there are still large stocks of the lesser known types around. Dealers and tube amp manufacturers have acquired millions of these devices and military warehouses scattered around the world still contain sealed boxes – time capsules containing tubes – untouched since the day they were manufactured and packed. They’ve not seen the light of day for decades, let alone been used – perfectly preserved. This is no ‘Area 51’ or ‘Roswell Incident’ urban myth. I’ve purchased a few boxes of NOS miniature and subminiature tubes in my time. They are typically packed in brown cardboard boxes containing a hundred tubes with each tube protected in it’s own little individual white carton. As an enthusiast, it comes close to a mystical experience to open one of these boxes to find the pristine, untouched tubes inside and I can all too easily become enveloped in a warm, cosy sense of nostalgia for the idea of a golden age of of that has long passed by.
So naturally my expectations for noise and microphonic performance of NOS tubes is high, perhaps unrealistically so. However, nostalgia aside, it is true that manufacturers went to unprecedented lengths to ensure rigorous quality control procedures were adhered to throughout the fabrication process of tubes. The raw materials such as nickel, tungsten, molybdenum and barium used in the heater filament, grid wire and getter would have been chemically analysed for purity. Dimension tolerances of the wire used in the grid and the thickness of glass tubing were checked using a micrometer or vernier calipers. Once a tube was manufactured a final burn-in test would have been undertaken to stabilise the cathode and the gain and microphonics tested to ensure they were in within acceptance limits. This was an impressive achievement, yet despite all this supreme effort in quality control to maintain consistency in the manufacturing process, the bottom line is that it’s easy hear variations in noise and microphony between tubes when testing a given batch. In practice this means some percentage of the tubes may be considered unacceptably noisy and/or microphonic and have to be discarded. The percentage of tubes that clould be rejected is dependendent on several factors.
Firstly what is acceptable is subjective. Some tolerate higher noise levels – typically shot noise hiss and lower frequency flucuation (flicker) noise. and as it is dependent on what type of circuit the tube is utilised and the listener’s expectations. For example a tube used in a phono preamp stage or ribbon mic preamp will need to hbecuase it these will be tubes that perfrom better than others. Different application require grading of tubes. However, not all tubes are a good choice for use as audio amplifiers.
Have they been graded? Well the manufacturer would certainly have a tested samples of each batch of tubes as part quality control scheme. Min aim is uniformity of product and minimising rejects which are costly to a manufactuerer. But how rigorous was this testing? Well that would depend on the application. During and after WWII the US forces became extremely obsessed with ‘ruggedisation’ of tubes which are pretty fragile by nature. They wanted their tube equipment to withstand being bounced around in the back of a jeep and survive combat situations. New specs and nomenclatures were created. For example, you’ll a JAN (joint army navy) 12AX7 tube with additional letters appended to its type like 12AX7A, 12AX7WA, 12AX7WB and 12AX7WA in quest to engineer more reliability into the tube by thickening or adding additional mica support washers and altering the geometry of the internal electrodes by shortening the plates. of theThe military however due to maufacturing tolerances there will be variation in hum, noise and microphonics within any given batch. Now this is a key point. bwHave the tubes been graded? Now depending upon the application for which the tubes were originally designed for will determine what the strictness of the testing deem acceptable as good tube tube. If you own an expensive tube rig then you are undoubtedly going to have high expectations. – the subjective audio performance, that’s hum, noise and microphonic sensitivity the tubes is going to have to be good. and if you’re using the tuges in sensitive high gain preamp stages then very good. This means you’l end up having to grade and select the quiestest tube from your batch. The bottom line is a percentage of any box of tubes will be unuseable becuase o. For example the E182CC was an extra special quality version of the E82CC which was itself a special quality version of the popular ECC82. The E182CC was designed with both an anti-microphonic construction and with two matched triodes within the envelope. These features made them especially suited for differential amplifiers in such instrumentation as pen recorders and bridge circuits. The Honeywell pen recorders used the E182CC for optimum performance.These are “premium” 12AU7’s which were marketed by European manufacturers. I don’t think that there was any difference in manufacturing technique, to create these, rather they are just specially tested 12AU7’s that exhibit very low noise/microphonics and matched sections.5963s are NOT good tubes for audio or RF, I’ve found. Their curve,
while good for switch and flip-flop applications, is curiously
non-linear in the middle.
Designed as AF voltage amplifiers, two independent matched valves exist in the envelope.They were also were supposed to have been selected for low noise/microphonics. Note that this type of tube may not match well between tubes, and/or tube sections. This is because this tube was designed for “on-off” digital operation, not linear operation to amplify analog signals. It will still work as a linear amplifier, it’s just that this was not its intended mode of operation so it may not have consistent linear-mode characteristics.
However my enthusiasm and nostalgia often clouds my objectivity carries me away However the thing is have to hand-select. My experience, just becuase a tube is NOS it doesn’t mean it’s good. It’s very easy yo get overtaken.
. This means there are plenty of NOS tubes out there however there are plenty that are not suitable for audio circuits. So the search for for audio tubes is not straightforward – there are many different types NOS tubes some being fit for audio applications and others that are not duitable for use in audio circuits
The first time I saw the 5963 in an RCA catalog was 1956, and they
were marketed for VT digital computers, mostly, such as the UNIVAC.
NORAD used these by the boxcar full in their early UNIVACs that were
used in the NORAD system in the ’50s through the ’70s. Many of the
used 5963s on the market now are probably part-outs from the old NORAD
machines, which would occupy large buildings and need 100s of tons of
refrigeration to cool.
The big difference here is that most “computer rated” tubes are not screened or spec’d for noise. The 5963 is notorious for having a large percentage of examples being noisy. then the beginning of production for the 5963 might be 1949 or 1950.Me too, I asked an old timer about it once at a auction and he told me that folks get excited about special purpose tubes, but they were just that, and if the purpose didn’t require them to be silent, you would likely get lots of noisy ones. The 5963 is a great tube though, if you do find a quiet one.These tubes are intended for digital circuit designs (it explicitly states this in the datasheet), hence the poor microphonic performance (if the tube is hard-on or hard-off then some sensitivity to vibration certainly isn’t that critical in a digtial switching application – unless it so terrible it is at the logic switching threshold). If you do happen to convince someone to purchase these 5963 tubes for hi-fi audio applications then they will be serverely disappointed and not a happy customer. “I don’t see anyting in the datasheet about higher microphonics.” – Of course not – the manufacturer is not going to bother to specify a parameter which is pretty much irrelevant for digital applications (switching, frequency division, counters, etc). That would be like specifying the gain of a CMOS ripple counter – it’s just not relevant to the way the device is used.
Probably if you audio test and hand select these tubes for low microphony (have you tested any?) or they’re used in a low gain application, then the microphonics won’t be an issue. But if you think JJ are tubes are poor for audio, then these computer tubes will be worse. They’re simply not appropriate for use as an audio tube let alone an audiophile tube.
We are fortunate to live in a time where, although past its zenith, the hi-fi audio industry still exists in tubes are still manufactured by companies such as JJ Tubes. there are valid reasons that audiophiles seek They’re. As audio electronics engineer I count myself fortunate to still be able to obtain these tube are fortunate to inherit this legacy. – the years between them from their date of manufacture are bringing them closer and closer to being valuable antiques rather than vintageIt seems to me we’re not too far away from a time where it may be considered foolhardy or even bad taste to actually use NOS tubes in amplifer – they will be coveted by an elite of wealthy collectors who use them only on special occasions or ultimately strive merely to preserve them in their original cardboard boxes as rare and fragile museum pieces of a bygone, golden age.
What makes this search so tantalising is knowing that there was a time before the transistor where tubes ruled supreme. They were the glowing heart of all domestic audio reproduction equipment such as radios, televisions, phonograms, hi-fi and guitar amplifiers and professional studio gear like mixers, mic preamps, equalisers, compressors and spring-line reverb units. The great audio product manufactureras of the time – RCA (Radio Corporation of America), Philips, Quad (Quality Unit Amplified Domestic) and H.J.Leak & Co researched, experimented, innovated, made genuine contributions to the art and set new standards for hi-fidelity audio electronics., but there is also a sense of nostalgia tinged with guilt whenever I unpack pristine old tubes to use them for a new circuit design. They’re a scarce, non-renewable resource and it seems to me we’re not too far away from a time where it may be considered foolhardy or irresponsible to actually use them in amplifer – they will be coveted by an elite of wealthy collectors who use them only on special occasions or ultimately strive merely to preserve them in their original cardboard boxes as rare and fragile museum pieces of a bygone, golden audio age.