From the 1970s onwards, tube audio products and associated components experienced a huge revival. A thousand or so brands on the world market were enriched by the revival of a whole range of old tubes. This opportunity has given vintage equipment a new lease of life. But let’s be careful: the replacement of tubes by recent equivalents must be accompanied by multiple precautions.
Mythical brands: an almost irreplaceable know-how
A large number of brands, mainly American and European, have forged the history of the electronic tube in the world since the early 1900s. In each country, each brand has been able to develop and preserve its activities and its market share based on know-how that can be appreciated today in the form of “NOS” (New Old Stock), in other words, vintage tubes that are still new. Their visual inspection, their passage through the measuring bench, the lamp meter, as well as long comparative listening tests of models intended for high-fidelity applications, leave jealously guarded manufacturing recipes unseen. This remark applies as much to the famous ECC83 of Telefunken origin and the EL34 pentodes of the English firm Mullard as to the mythical WE300B of the American firm Western Electric.
Some technical parameters to remember
The main technical parameters generally retained by users for audio tubes can be summarized as follows
1 – Their specifications “in the field” and the differences noted in relation to the official specifications. Remember that tolerances of ±15% or more are common.
2 – To the life span. It is important to remember that the life of a tube varies according to each brand while remaining linked to each condition of use. The life of a tube depends as much on the degree of vacuum as on the possibilities of the emissive power of the cathodes, of the filaments with direct heating. A phenomenon known as “exhaustion” of their emissive power depends on the values of current and plate voltage in relation to the maximum values allowed by the specifications. The quality of the vacuum when a tube is new and during its use is an essential parameter. It is still linked to the metal degassing techniques, the sealing processes at the outlet connections and the bulbs. The latter use different glass with lower expansion at the bottom. Deficiencies in all three areas can significantly shorten the life of a tube. The final inter-electrode ignition is generally preceded by a generalized glow.
3 – To how well the specifications hold up over time. This depends as much on the origin of the tube used as on the conditions of use. Some tubes are known to be more robust than others. In each brand, depending on the type of use, an electron tube goes through three life periods: a break-in period spread over a few hundred hours, a so-called “cruising” period of between 500 and about 4,000 hours (this is only an approximate value). Then comes a period of more or less rapid decline in performance. Taking the example of the original WE300B triode, this tube, when properly used, is capable of operating perfectly for about 20 years. The same tube, when fully loaded, for example with a plate voltage of 450 V and a quiescent current of 80 mA, will have a life expectancy of no more than two or three years. In an audio application, it is advisable to replace the tubes after a certain number of hours of use without waiting for their filament to cut out. This time limit is not easy to determine. It would require either the use of an hour meter (rare on/off switches are equipped with one), or a visit to the audio measurement bench (measurement of the nominal power, bandwidth, distortion rate).
4 – The level of hum, background noise and the microphonic effect. These three parameters are of major importance on the phono stage of a preamplifier. It should be remembered, however, that the optimum sound qualities desired by the user do not necessarily go hand in hand with the tube model offering the best noise reduction. This is the case with the Noval ECC83 tube, which, in the original Philips and Telefunken brands (they differ from the others by their smooth surface plates), have great musical qualities without being the best in terms of residual noise. It also happens that, for simple questions of electrical insulation between the cathode and the filament, a specific tube model is more prone than another to noise phenomena, small hissing noises mixed with the background noise. A small hiss often occurs on the “SRPP”, a set-up in which the cathode of one of the double triode elements is raised to a high potential with respect to ground.
5 – The sound and musical qualities. Be careful because these parameters are often distorted due to the dispersal of tube characteristics. In most cases, the user does not have the possibility of sorting the tubes beforehand on the parameters of current and slope. On the same appliance, the replacement of a used tube by another, new but unsorted one. This may lead to different operating points than the original ones, to different measurement and listening results. On a preamplifier, an optimal plate voltage of 200 V for a quiescent current of 1.2 mA may thus change to 190 V and 1.8 mA on an unsorted tube of another origin. Under these conditions, the change in operating point depending on the tube used will make it impossible to compare listening performance. This also applies to power tubes. They are subject to tolerances that easily reach ±10% while working in much higher current ranges. The sound quality, the musical performance of the tubes remains unclear as it tends to vary over time, over months, in different contexts. The tubes belonging to the “SQ” (Special Quality) or professional series, those whose reference is usually accompanied by the suffix W or WA, have the advantage of a longer life. Their rather timid cathode emissivity during their break-in phase produces a sound that may lack depth and breadth. These subjective effects are misleading because they can disappear completely after a few weeks of use. Moreover, it is impossible to distinguish between tubes that sound “warmer” or more refined than others when tested on a bench. You must take into account your own experience and the opinions of other users.
Selecting sorted tubes as a priority
The replacement of a used tube with either a new vintage tube or a new tube should be pre-selected, sorted to ensure small tolerances from the original tube specifications. The method of sorting with the help of a lamp meter, with measurement of the slope, is fairly well suited to Noval or miniature voltage amplifier tubes, such as ECC81, 82 or 83 (preamplifiers, input stages of power amplifiers). On the other hand, this sorting can lead to significant dispersions when it comes to power tubes. Indeed, several lamp meters use a relatively low plate voltage, for example 180 V, for the measurement of the slope, the transconductance. However, it is important to remember that a true pair of tubes means the superposition of all families of Ip/Up curves for different grid polarizations, for plate voltages up to the maximum limits allowed by the specifications. In an audio application, the operating points vary on a case-by-case basis, from stage to stage, from device to device. The operating points are very rarely identical to those used on a given model of lamp meter. For example, two KT88 tetrodes considered “identical” on one lamp meter may, under real conditions of use, for example on the famous McIntosh MC275, in class B and under 450 V plate voltage, give glaring disparities. The use of a curve tracer is not within the reach of everyone, so a practical solution is to measure and adjust the quiescent current. However, these adjustments are not always easily accessible. They should preferably be entrusted to a specialist who is used to measuring high voltages, and who should therefore take the necessary precautions. One trick is to make your own plugs, male-female sockets, which are inserted between each tube and the instrument. In this way, the grid polarization, plate voltage and quiescent current values of each tube can be measured without having to dismantle the device. These external measurements require a milliammeter and a voltmeter. This test is complicated by the fact that you must make not one, but several plugs adapted to each type of tube. Not easy! As for the current and voltage values measured in this way, they only provide useful information when the specifications and curves of each tube are available. These considerations make us understand that the “quick and dirty” replacement of a tube by another to “boost” the performance of a device involves a certain amount of risk. This risk is increased on circuits combining direct links and stabilized power supplies, like certain models from Audio Research. Let’s take the example of two ECC83 tubes connected directly, without a link capacitor. An inadequate change of tube can modify in a very sensitive way the operating point, the cathode-grid bias of the following stage. For example, the plate voltage of the first tube, 200 V, can determine very precisely the grid bias value of the following tube: for example – 1.5 V. However, the direct link between the two stages requires that the cathode of the second tube does not have to be around 200 V, but exactly 201.5 V. That’s precise!
The mains voltage is important
Most tube devices, preamplifiers and power amplifiers use unstabilized power supply circuits. The high voltage values are therefore dependent on the mains voltage. It is therefore essential to check that the mains voltage for which the device was designed is observed. An amplifier designed for 220 V mains but connected to a 230 V mains will lead to an increase in the high voltage value. In the power stages of tube amplifiers, a higher plate voltage than originally intended, e.g., 430 V instead of 400 V, may force the output tubes to operate at abnormally high plate dissipations. This can lead to risks to the life of the tubes as well as to the reliability of the device. The beginning of reddening of the plates of power tubes, such as KT88, 6550 or EL34, is sometimes only visible in a dimly lit room. This is a clear sign that the plate dissipation is too high and therefore dangerous (risk of inter-electrode ignition, shortened tube life). It should be added that the good matching of power tubes used in push-pull mode can be recognized by the fact that, on a square signal of about 50 hertz at a moderate output power, the output transformer emits almost no parasitic vibration. This is a sign that the currents in the two branches of the push-pull output stage are well balanced.
Beware of vintage tube copies
Over the years, the demand for vintage tubes has grown steadily. The fact that they have not been manufactured since the 1960s, 1970s or 1980s has made them obsolete. This situation has prompted several brands and companies involved in the tube market to manufacture copies. Be careful because the best copies look like the original models. Again, this is a matter for the specialist. Let’s not forget to mention that this is nothing new. We all know that tube manufacturers in the USA and Europe have overcome shortages of certain tube references by using other suppliers. A typical case is that of Siemens who, because of their reputation, were forced to market ECC81, ECC82, ECC83 (and many others) from a source other than their own at the end of the 1970s, while retaining the Siemens logo and packaging. More recent examples include the EL34 version, a coveted pentode when it comes to the best of the British Mullard brand. The original Mullard versions can be recognized by their blue box with the Mullard logo printed in pink (not red), their tubes with a brown baseplate, other baseplate details and the presence of a double getter (an internal cup used to collect titanium-based metals used to perfect the vacuum when the tube is ‘flashed’ during manufacture. The real vintage Mullard EL34 tubes can be recognized mainly because of a unit price that can reach several hundred euros in the “NOS” version (new vintage tube). However, it should be noted that a copy of a tube bearing the logo of a mythical brand does not necessarily mean a poor-quality product. It’s just a replica of the original version, the rest is history.
From the end of the 1970s onwards, the gradual disappearance of audio tubes was clearly felt. For a few years, this shortage was partially made up for by various wholesalers around the world. However, the renewed interest in tube audio assemblies increased the demand, prompting Russian tube manufacturers to offer equivalents. The advantage of attractive prices was often offset by the impossibility of having tubes made that were identical to European or American models, as well as obsolete versions such as direct-heated triodes. The inflexibility of the Russian manufacturers made Chinese companies think twice and take advantage of this shortcoming. This led to the creation of many tube manufacturers. Some of them had the great merit of responding very quickly to the demands of the market, which was becoming increasingly demanding in terms of reliability, technical and subjective performance. Between the beginning of the 1980s and today, let us recall that a good fifty replicas of the famous Western Electric WE300B direct-heating triode were marketed in China and elsewhere. To list them one by one in this article would require too much space. Let’s just mention in passing some brands and tube manufacturers that deserve attention:
– Electro-Harmonix: The history of this company, founded in 1968 by Mike Mathews, can be found in the online dictionary wikipedia.org/wiki/Electro-Harmonix. Thanks to a significant investment, this company has succeeded in creating a catalogue of tubes that is highly coveted by audio tube enthusiasts.
– JJ Electronic: This company was founded in 1994 in Slovakia and inherited a great deal of know-how from the Tesla company. Today it offers a catalogue of around forty models, all with an excellent performance/price ratio. On the company’s website (www.jjtubes.eu), you will find both very competitive prices and models that are impossible to find elsewhere. This is the case of the ECC 83S, a version with gold pins, the ECC99 (both unique and precious when used as a floor driver), the Noval versions with a frame grille or the more classic 300B, mounted on a ceramic base and offered for direct sale at a unit price of less than € 100. Website: www.jjtubes.eu.
– KR Audio: A famous brand. A large choice of triode tubes. Website: www.kraudio.com
Other brands can be found on many distributor websites. Here are a few chosen at random:
Quality of electrical contacts and reliability of tube appliances
It is rarely mentioned that failure and reliability problems observed on tube devices, whatever their price, are often caused by imperfect contacts between the chassis supports and the tube pins. The tube spindles must logically comply with an international standard relating to their diameter, angular positioning, and dimensional tolerances. Details can be found at: https://en.wikipedia.org/wiki/Tube_socket.
The diameter of each of the pins of a tube of the type ECC83/12AX7 must be exactly 1.016 mm, that of an EL34 pentode pin 2.36 mm and that of the pins of a 300B triode 3.2 and 4 mm. The surface oxidation of the metals, the temperature variations to which the tube pins are exposed before, during and after start-up generate problems of expansion, oxidation, and resistivity of the contact points, of which one must be aware. Rods terminated with jigs, each with an exact diameter of 1.016mm, 2.36mm, 3.2mm and 4mm, highlight these electrical contact problems. In a conventional push-pull tube amplifier, it should be borne in mind that the number of contacts between the pins and the tube supports is of the order of a hundred. The jigs mentioned, inserted delicately into each of these tube supports, make it easy to see that, among a hundred or so contacts, several are far from perfect: insufficient mechanical tightening, electrical contacts that are too resistive or unstable. When the tubes are removed, a magnifying glass examination of the pins of the tube holders on the chassis allows to see the biggest defects. It is easy to spot pins where the upper part of the pin is too far apart and no longer capable of making reliable electrical contact. It is unfortunate to mention in this connection that the templates mentioned (four models would suffice) are not available on the market, at least to our knowledge. However, it is easy to make them yourself by using pins from defective tubes welded to the end of a smaller diameter rod. The quality of the electrical contacts at this point is of vital importance. As a result of these imperfect contacts, a tube can become “depolarized” by accidentally blowing out one of its pins. Poor electrical contacts over a period of one or two seconds are enough to cause a cascade of problems. It is easy to understand why specialists have put high-quality tube holders on the market. Examples can be found on the JAC Music website (www.jacmusic.com/sockets/Socket-Yamamoto.htm).