NOS tubes explained: new old stock vs new production, and how to check them

In tube listings NOS stands for new old stock: tubes made years or decades ago that have never been used. Many of the tubes audiophiles seek carry the label, and it shares its three letters with a kind of DAC that has nothing to do with tubes. This guide explains what NOS means, how it differs from new production and used tubes, why vintage tubes are sought after, how to check what you buy, and how to store it.

What does NOS mean for tubes?

A NOS tube was manufactured in the past, often by a factory or on a line that no longer exists, and has been kept in storage since: unused, ideally in its original box. “New” describes its condition, “old” its date of manufacture. Several related terms appear alongside it:

Relabelling was a common, legitimate practice for decades: tube brands and equipment makers sold tubes from other factories under their own names. It becomes a problem only when a label is used to pass off a cheaper tube as a prized one. And NOS describes age and condition, not quality: a NOS tube may have been a lower grade or a reject in its day, and decades on a shelf do not improve it.

NOS tubes vs new production

• Construction. NOS tubes were made with the materials and methods of their time. Several types in our amplifiers, such as the 4E27, AL4 and EF14, survive today mainly as old stock.
• Supply. Old stock is finite; as it shrinks, matched pairs become harder to assemble. New production can be bought in matched sets and replaced years later.
• Consistency. Both vary. Some vintage production runs are known for consistency, some current factories for careful quality control, and both have weak batches.
• Sound. Owners often describe good vintage tubes as more natural and refined, while others find modern production more robust and dynamic. The circuit and its operating point usually matter more than the era of the tube.

Why audiophiles look for vintage tubes

Western vintage tubes carry the names of the companies that defined many classic types: Western Electric, which designed the 300B in 1938, RCA, Philips, Mullard and Telefunken. Audiophiles seek them for their construction and for a sound commonly described as natural and finely detailed. Their supply is small and shrinking.

Factories made tubes in very large numbers through the 1970s and 1980s for industrial, communications and broadcast equipment, many to strict acceptance testing. Large stocks of unused tubes survive, which is why vintage NOS tubes such as the GM-70, G-811, 6P3S, 6Zh4, 6D22S and 5Ts4M are still available. Owners value the better types for their consistency and their sound.

Demograf Audio amplifiers use both. Ether runs NOS GM-70 Triode / GK-71 Pentode output tubes and NOS AL4 input tubes; Helios MkII uses vintage NOS G-811, 6P3S and 6Zh4; most of our other amplifiers have vintage NOS 6Zh4 input tubes, and all eight of our single-ended amplifiers use 6D22S rectifiers as standard. Tantal uses vintage 3C24 triodes, Tantal Senior vintage 4E27 pentodes of 1930s–1940s production, and Endymion vintage AL4 input tubes. Every tube is selected before fitting.

300B tubes compared: vintage, NOS and new production

The 300B shows the whole picture in one type. Western Electric introduced it in 1938 and made it for decades, and original examples are now rare and keenly sought. Since then the 300B has been made by several factories in Europe, Asia and the United States, including revived Western Electric production. Versions differ in anode construction, filament, glass and base, and are commonly described as ranging from warm and rounded to lean and detailed. A ranking says little without your amplifier and loudspeakers, so three practical points matter more:

• Buy a matched pair from one batch.
• Compare in your own amplifier, at matched volume, after a few dozen hours of use.
• Check the ratings. Every 300B has a 5 V filament, but some variants differ in filament current or maximum dissipation; ask the maker of your amplifier before fitting one.

In Atlas the 300B output tubes are selected and matched before fitting.

How to check a NOS tube

Testing on a tube tester

A tube tester measures emission or transconductance and compares it with the figure for a new tube; a curve tracer goes further and plots the tube’s characteristics. Ask for the measured figures and the tester used, not just “tests good”, since testers use different scales. A thorough test also checks for shorts and gas. Noise and microphony are judged more reliably in an amplifier.

Matching

Matching means measuring tubes at the same operating point and pairing those whose current and gain agree closely. It matters above all for the output tubes of a stereo single-ended amplifier and for push-pull pairs, and tubes from the same batch are easier to match. In our amplifiers every pair is measured and matched before fitting; Tantal Senior’s 4E27 pairs are matched for emission and distortion.

Date codes and markings

Many Western and European tubes carry printed or etched codes for the factory and date of manufacture. Tubes of this kind usually carry a printed date, often month and year, and a quality-control stamp. Etched codes are harder to fake than print, which can be rubbed off and reprinted. A code should agree with the construction: a 1950s date on a tube built in a later style is a warning.

Getter and appearance

The getter is a deposit, usually silvery, on the inside of the glass that absorbs gas left in the tube. In an unused tube it should be bright and intact. A white or milky getter means air has entered and the tube is dead. A thin getter alone does not prove wear: getters differ between makers and types, and some transmitting tubes use other methods and show little or no flash. Look at the pins too: fine scratches or bright marks suggest the tube has been in a socket. Check the glass for cracks, the base for looseness and the print for retouching.

Risks: fakes, relabelled tubes and tired stock

• Fakes. Cheaper tubes are reprinted with prized names, and the more collectable a vintage brand, the more it is copied. Compare the construction (anode shape, getter type, mica spacers, base) with a known genuine example.
• Misrepresented relabels. A relabelled tube is not a fake, but a listing that names the wrong factory misleads. Construction details show who made it.
• Tired stock sold as NOS. Used tubes are sometimes put into new or original boxes. Test figures and pin marks give them away.
• Old but not good. Poor storage, a slow leak or a tube that was a reject in its day. Some technicians bring a long-stored tube up gently, heaters first, before applying full voltage.
• Price as a guide. What a NOS tube costs reflects its rarity, not necessarily its sound.

Whether you buy NOS tubes in Europe, the UK or elsewhere, choose sellers who publish test figures, explain how they test and accept returns of tubes that fail on arrival.

How to store NOS tubes

• Keep them in their original or padded boxes, away from knocks and vibration; heavy tubes such as the GM-70 and 845 need generous padding.
• Store them dry and at a stable temperature, not in a garage, an attic or a car. In a hot climate, an air-conditioned room is the right place.
• Label each tube with its test figures and the date of the test, and keep matched pairs together.
• Re-test spares stored for years before relying on them. Some owners run their spares for an hour or two every year or so as a check.

Our guide to tube life and care covers how long tubes last in use and what heat and humidity do to them.

NOS in DACs: a different meaning

In digital audio NOS stands for non-oversampling: a DAC that converts each sample as it arrives, without the digital interpolation filter that almost every modern converter uses. It has nothing to do with tubes. To add to the confusion, the classic multibit chips used in NOS DACs, such as the TDA1541 and PCM1704, are long out of production, so NOS DACs are often built from old-stock chips. Our article on NOS multibit DACs explains the converter side. The Reference Tube DAC is built on classic converter chips (NOS on its four multibit versions), and the tubes of its optional output stage are selected and matched before fitting.

Which Demograf Audio piece fits

• Vintage NOS transmitting triodes: Ether (GM-70 Triode / GK-71 Pentode) and Helios MkII (G-811).
• Vintage tantalum-anode tubes: Tantal (3C24) and Tantal Senior (4E27, 1930s–1940s production).
• Directly heated triodes with vintage NOS input tubes and rectifiers: Atlas (300B), Solaria (45) and Eurybia (2A3).
• A choice of output triodes with vintage AL4 input tubes: Endymion (845, 211 or 805).
• A DAC with an optional NOS tube output stage: Prometheus or Hyperion.

A reserve matched pair of output tubes can be ordered with each of our single-ended amplifiers. Every unit comes with a NOS tube declaration listing the manufacturer, country and production year of each tube, part of the document package described on our Atelier page. Every amplifier is made by Dmitriy Gubchenko. Tell us which tubes interest you, and we will tell you which of our amplifiers is built around them.