Below is an in-depth article about NOS vacuum tubes and why they are vastly superior to current production tubes. It was written by Eric Barbour, one of the world’s leading authorities on vacuum tube technology, and former editor of Vacuum Tube Valley Magazine. We hope you will find the article both helpful and informative.
Why are NOS tubes better than new ones?
By Eric Barbour (former senior editor, Vacuum Tube Valley magazine)
Tubes for audio equipment use oxide-coated cathodes. This type of cathode starts with a tube or sleeve made of a special nickel-iron alloy, with specific impurities added. On the outside of this sleeve was coated a mixture of barium and strontium oxides plus other chemicals. A small electric heater wire, coated with a special insulator made of aluminum oxide and binders, was inserted into the sleeve to heat it to about 900 degrees Celsius—red hot. When this is done, the sleeve’s oxide coating starts to emit electrons.
The oxide cathode is a strange thing. Its operation is VERY complex and remains poorly understood to this day. Essentially, it is like a semiconductor, with barium and strontium interacting with other materials and with the base metal (usually high-purity nickel or nickel-iron alloy) in ways that are not easy to study. The coating has billions of nano-scale sharp tips, which are believed to be the emitting elements. If you look at an oxide cathode under a very powerful microscope, it looks “fuzzy”. The tips wear down over time due to the electrons boiling off them, so they must be replenished by complex chemical reactions in the coating. Oxide coatings are prone to “sputter” onto other tube parts, possibly producing stray leakage currents (a real problem in poorly-processed tubes such as the low-cost Chinese ones made today). And oxides are not very resistant to high electric fields, which limits the tube’s operating voltages. This is why “cathode stripping” is a problem, especially if the tube is operated with its anode at more than 400 volts. Turning on the plate power when the cathode was still cold might damage the coating. This is why most large guitar amps have a “standby” switch, that is turned on when the tubes are warmed up to operating temperature. High-end audio amplifiers usually have a standby switch or a built-in time delay relay to apply anode power after the tubes are heated up.
Prior to the 1980s, most “receiving” tubes used cathodes that were made by specialty suppliers, to very high standards. This was a carry-over from the WWII era, when military equipment and early digital computers required tubes with high-quality cathodes that would last a long time, especially in severe duty such as computer circuits. Prewar tubes usually had a shorter lifespan due to impure cathode materials. The first tube designed to last a long time, the Sylvania 7AK7 pentode of 1947, used a cathode made of high-purity chemicals and metals. It was not meant for audio amplifiers, it was meant for use as a logic gate in MIT’s Whirlwind computer. Logic tubes in computers operated in “cutoff” for long periods of time, which was very hard on the coating. As the market demanded better and better tubes, cathode manufacturers concentrated on making the best cathodes they possibly could, with the same high-purity ingredients and special coating techniques to insure a consistent coating thickness and ruggedness.
Most oxide cathodes in modern power tubes last about 1000 hours in normal service, especially in guitar amps used in heavy rock, which undergo a lot of abuse. A really well-made oxide direct-heated filament, such as that used in the original Western Electric 300B power triode, can last 50,000 hours if operated conservatively. The 300B is an extreme case, as it was specially engineered for long life. Small preamplifier tubes like the 12AX7 are even more dependent on cathode quality. The low-quality cathode of today, usually having impurities that damage the electron emitting sites over time, might last 10,000 to 20,000 hours at best in a preamp tube. NOS preamp tubes routinely last 100,000 hours, especially if operated conservatively. (Compare this to the high-temperature thoriated filaments used in giant transmitting tubes, which ROUTINELY last 10,000 hours in VERY severe service. Radio transmitters run their output tubes very hard, to get maximum efficiency. The world record for lifetime of a power tube is held by a large Eimac transmitting tetrode. It was in service in a Los Angeles FM radio station’s transmitter for 10 years, for a total of more than 80,000 hours. When finally taken out of service, it was still functioning adequately. The station saved it as a spare. Yes, it had a thoriated filament. Audio tubes rarely use thoriated filaments because they don’t emit as many electrons as oxide cathodes, and use more power to heat to operating temperature.)
Why do NOS tubes sound better than current production? There are various complex reasons, primarily economic in nature. Modern factories are under great pressure to produce tubes as quickly and cheaply as possible, so they compromise the hardness of the vacuum in the envelope. A good NOS tube was vacuum processed for more than an hour (sometimes several hours) before being sealed. Modern tubes are shoved through the production line very quickly, so it’s not unusual to pump them for only half an hour or so. Residual gas in the tube can affect the tube’s operating linearity, and even its reliability. Gas molecules, especially oxygen, can bombard the oxide cathode and shorten its lifetime. A poor vacuum can sometimes cause arc-over between the anode and cathode, as the gas ionizes and conducts a lot of current. This kills the tube and can even destroy the amplifier.
Consistency counts in a mass-produced vacuum tube. Badly made tubes literally have higher electrical distortion than good ones. I did numerous tests in the 1990s for VTV magazine, and found that NOS tubes of a given type were usually lower in distortion than current tubes of the same type. Winding the control grids is an art, and requires a special lathe and quality control that is effective. NOS tends to be superior in this area because manufacturers tried to be consistent. Hi-fi manufacturers like McIntosh, Marantz and Fisher used the best, most linear tubes they could find, because they were making “high fidelity” amplifiers. This is why power tubes intended specifically for consumer hi-fi amplifiers were introduced in the 1950s and early 1960s. This includes the 6550, 7027, 7591, 7868 and 8417.
The cathode is extremely important, even more so than the vacuum. A really good cathode has plenty of reserve emission, so that signal peaks are handled well. Yes, this impacts the sound of the tube. In the “old days” cathodes were made by specialty cathode manufacturers, separately from the rest of the tube. These firms could concentrate on quality and consistency. Virtually all of those manufacturers went out of business in the past 20 years. The only remaining cathode producers concentrate on profitable, high-priced specialty products, such as electron emitters for electron microscopes or cathodes for big, costly transmitting tubes. Modern audio tube factories, the few that remain, must be “vertically integrated”, and make their own cathodes from scratch. And they must do it inexpensively. Since their customers (usually large guitar amp makers) are only interested in low cost and NOT in longevity, these tube factories are forced to make their cathodes quickly and cheaply. They are not rewarded for making long-lasting tubes, so they don’t bother.
If you would like to read more about tube materials, let me suggest a book: HANDBOOK OF ELECTRON TUBE AND VACUUM TECHNIQUES, by Fred Rosebury, American Vacuum Society (later reissued by the American Institute of Physics), 1964. ISBN is 1-56396-121-0. It has been out of print for some years, but is the best general textbook on the subject I’ve ever seen. This classic text will answer all of your questions, and can be trusted as an authority.