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Termoionica Applicata

A starting guide to NOS vacuum tubes

[Italian version]

People sometimes ask me to recommend a particular vacuum tube for audio applications. They don't simply mean the type of vacuum tube (for example, 12AT7 versus 12AU7) - they already know the type that they will be using. These people mean the brand name of the vacuum tube. Now, I already hear you asking what does the brand name have anything to do with the application. In a word - everything!

You see, not all brand names were created equal. Just as in fine automobiles there are differences (Porsche versus BMW), different manufacturers of vacuum tubes create different sound in the end result.
Since one of the things which I do is buy and sell NOS (New Old Stock) vacuum tubes, I can give opinions as to which tubes I prefer in certain audio applications.
I use vacuum tubes everyday in all my audio equipment - I have not used my solid state audio gear in years! - I listen to different types of tubes. I also collect NOS and rare vacuum tubes. Collecting tubes is both educational and fun.

I will limit this article to NOS tubes. Current production tubes are more economical, and in certain situations just as good as NOS, but I prefer to use NOS tubes if I have a choice (not all NOS are still available, so one is literally forced to use modern production tubes).

NOS stands for New Old Stock. These are brand new tubes of yesteryear which are no longer produced by certain manufacturers.
Manufacturers were either American (RCA, GE, Sylvania, Ken-Rad, Dumont, Tung-Sol), British (Mullard, Brimar, Mazda), Dutch (Amperex, Philips), French (RTC), and German (Telefunken). There were other NOS manufacturers, but these are the most important and most popular.
First I will summarize my findings for small signal (driver tubes). Then I will move to output tubes.

My favorite NOS 12AT7 tube is RCA black plate. RCA made a very high quality line of tubes. I believe that of all the USA manufacturers, RCA was the best - both in terms of sound and quality of manufacture. This tube was produced from the 50's to the 70's. The plates are jet black. The sound is super.

The best NOS 12AU7 tube is, in my opinion, Mullard. The plate (anode) construction was a box type. Mullard produced the highest quality tubes in the world. They are my favorite.

As for NOS 12AX7, Telefunken is the very best. The sound is very sweet in the midrange. The tube is manufactured to very high quality standards. Telefunken 12AX7 tubes have been known to last tens of thousands of hours - maybe even 100,000 hours. That is alot of hours on a tube. This longevity is testimony to the high quality of manufacture of Telefunken tubes.

Next to Telefunken, Mullard 12AX7 tubes are the best sounding. Some people even believe that they sound better than Telefunken. As always, sound comparisons are purely subjective. They depend on the hearing of the listener, and on the type of audio equipment used.

RCA 7199 tubes are, in my opinion, the best. GE is also good. Third place would be Sylvania.

EL 34 output tubes are expensive, especially the NOS variety. Expect to pay US$ 80-150 each! That equals US$ 320-600 just to retube a push-pull four tube amplifier. We are talking big bucks here.
But, the sound is worth it, and they last a long time. Hey, life is short so we might as well enjoy ourselves if we can afford it. Even if we can't afford it, let's enjoy ourselves anyway!! (just joking).

My favorite EL 34 tube is manufactured by Mullard. This manufacturer made four different versions over the course of the years of production. Look for type XF1 and XF2 tubes. They are the strongest in terms of construction, and sound the best.

My recommendation for EL 84 tubes is Mullard followed by RCA. Both sound great, and both last a long time.

Finally, the best 5AR4 rectifier tube ever made by any manufacturer is Mullard. Look for early versions of this tube. More recent versions are good, but the more vintage versions are better.
How do you tell which is more vintage? They had a double lined Mullard shield on the glass as opposed to a single lined shield - the single line one is more recent.

Rectifier tubes have an influence on the sound of the amplifier. So the choice of manufacturer is important. Mullard tubes give great sound. Mullard 5AR4 tubes seem to last a very long time. Some amps have been found with the same Mullard tube in it for 10 years or more!

Publications have tried to scientifically measure the differences in various NOS tubes in an effort to quantify the sound of these tubes. Characteristics such as distortion are measured. But in the final analysis, it is the human ear and the trained eye which determines which tube is best for a specific application.
The opinions given in this article are simply my own based on my own listening tests, and examination of the various manufacturing techniques of different Companies.
Your listening results and opinions may differ, and probably will differ to a certain extent. We may agree on certain points also. But, at least this article presents a starting point.

So have fun. Experiment with different tube manufacturers. And most importantly enjoy the music!

© Copyright 1999 Harvey A. Kader for TNT-Audio, https://www.tnt-audio.com

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