Shelf Life
How long do tubes last when kept in storage? Do they have sell-by date, like eggs or bread? Does the passage of time cause a degradation in their performance, even though they’re not being used? It’s a good question that I’m sure is of great concern to vintage tube enthusiasts. It’s really a question of entropy.
It’s well known that electronic components age. For instance electrolytic capacitors dry out (but not always, if they’re well sealed) and carbon composition resistors absorb moisture from the atmosphere, which results in a steady increase in their stated resistance year after year. However, there are other components, such as inductors and polyester capacitors which seem much more stable over time.
The reason some components age badly is because a detrimental change occurs within them. Fortunately tubes are very stable by nature of their design. Firstly, they operate at high temperatures. At high temperature chemical reactions speed up, so by design they have to be fabricated from chemically inert materials. Secondly, there’s no air inside a tube, which means there’s no oxygen, water vapour or other reactive gases in the envelope that can attack the metals within. And, if the pin-to-glass seals remain intact, atmospheric gases and moisture will never get into the glass envelope to breach the vacuum. This prevents any adverse chemistry which might affect the tube’s physical properties. The shelf life is therefore potentially indefinite, if the tube has been manufactured properly.
A good N.O.S. tube, such as a Mullard ECC83 can work wonders in the V1 position to ensure low microphony, hiss and hum. European tube manufacturers utilised a high grade magnesium-nickel alloy (see BS 3504:1962) to make their cathodes. Their magnesium-nickel alloy was quieter than the American cathode alloys—even better than ‘Cathaloy’ A30. These specialist alloys were developed to enhance electron emissivity and maximise the operational life of oxide-coated cathodes. This technology, coupled with Mullard’s precision-machined electrode geometry, meant that there has never been a quieter and more reliable audio tube than a Mullard ’10M Master Series’ ECC83, well, apart from the Telefunken ECC803s.
Mullard guaranteed their ’10M’ tubes for 10,000 hours. They weren’t the only manufacturer making exceptionally high quality audio tubes like this. The lifespan of RCA “Special Reds”, or Sylvania “Gold Brand” tubes, were specified in their datasheets at 10,000 hours too—and that’s a minimum figure—their usable life can far exceed this. Modern tube manufacturers don’t make such claims in their datasheets or sales blurb. Now why would that be?
Well, it’s simply this: modern manufacturers do not have the motivation or know-how to make tubes to this level of quality. Or, to put it another way, domestic consumers—that’s you, me and the amp manufacturers—want our tubes cheap. The trouble with that is: you get what you pay for. And over the decades tube producers adapted to meet the need for cheap tubes; not long-life, or high-reliability tubes. Along the way, manufacturers eventually forgot how to make a tube with a 10,000 hour cathode or a long-life rhenium-tungsten heater. They can’t make a high quality tube now, even they wanted to—the knowledge and the machines that made those tubes is lost.
The vast sea of technical expertise and superior materials available in the 1960s were light years ahead of anything that exists today. If you could travel back in time and see it with your own eyes, you’d think you were on another planet—our engineering efforts to make tubes today are pitiful in comparison. In the light of this, what might at first seem to be a hefty price tag of, say, $100 for a Sylvania 5751 tube, doesn’t seem quite so outrageous. Here’s a quick break down. One hundred bucks works out at one cent for each hour of use—the cost of your guitar strings, a cup of coffee or just lighting the room you’re sitting in far exceed this. So don’t be put off by investing in a good preamp tube for the V1 position on your amp.
There are other tonal benefits to be had by swapping the V1 tube out too, in guitar amps, and Effectrode pedals too. Any changes in tonal characteristics can be attributed to the following factors:
- Capacitance Larger plates do not always equate to greater capacitance. A quick look at the datasheets for a 1962 RCA 12AX7 and modern JJ ECC83 reveal the plate to grid capacitance for these long and short plate tubes are both exactly the same at 1.7pF. It’s more of case of computer tubes exhibiting greater capacitance than audio tubes. For example, the 12AV7 has a plate to grid capacitance of 1.9pF and a 5965 comes in at almost double the capacitance of a 12AX7 at 3.0pF. Tubes with longer plates have higher capacitance and depending on how the tube circuitry is implemented, substituting a short plate tube with a long plate type can give a gentle and welcome rounding off of the top end yielding a thicker, more bluesy sound. Technically what is happening is that the capacitance of the tube forms a low-pass filter with grid resistor in the circuit to filter out high frequencies at -6dB/octave.
- Gain Factor Some miniature 9-pin tubes have more gain than others. Substituting a 12AX7 for a tube with another of lower gain factor, such as a 12AY7 will lower gain in the preamp section. Again, the amount of gain reduction depends on the circuit implementation. In a typical tube preamp circuit where the plate resistor is chosen so that the 12AX7 is operating close to its full 33dB gain, a 5751 tube will reduce gain by a few dB and a 12AY7 substitution will knock about -15dB off this to open up the sweet spot for more bluesy sounds. The 12AX7 tube in the V2 position of the Effectrode ‘Blackbird’ and ‘Tube Drive’ pedals spot can be changed out for lower gain tube types to increase the amount of headroom for a bigger, bolder sound, an approach adopted by Stevie Ray Vaughan’s amp tech—he substituted the stock 12AX7 tube with a 5751 in the V2 spot on SRV’s Vibroverb and Super Reverb amps.
- Bias Point Biasing affects linearity and sensitivity. If you take a look at the RCA Receiving Tube Manual you’ll find biasing recommendations for the vasious 9-pin tube types. The idea is to set bias voltage of the tube with the cathode resistor to obtain maximum linearity before onset of clipping on the negative and positive signal excursions. However, guitarists are not usually seeking linearity. They want “colouration”, “texture”, “hair”, “complexity”, “richness”, in a word “distortion”—the thing they don’t want to hear is the “straight” sound of the guitar. If a 12AX7 tube is substituted with a 12AY7 then the biasing point will shift to a different point on the voltage gain curve. This not only affects the gain, but also the sensitivity of the amp. The “biting point” before the onset of “clipping” changes, and so does the “feel” and perceived experience of playing through the amp. Bias can be adjusted by either changing the resistance of the cathode resistor or the tube itself.
Modern tubes can be used for sonic experimentation too—this kind of fun isn’t the exclusive domain of those with tastes and pockets deep enough to afford old glass. In fact, tube swapping is a great deal more fun than fiddling around with the seemingly endless number of presets on the “menu” interfaces of digital gear—what a drag. Tube gear is more immediate, more tactile and real. The tubes get hot, and the vintage ones—like all vintage electronics—exude a gorgeous antique aroma. Using tube gear is an experience. Oh, and the best thing about tinkering with tubes is: the tones from the amp can be truly inspiring. For many guitarists, there’s nothing that compares.
