Can you test a vacuum tube with a multimeter

Can you test a vacuum tube with a multimeter

In the previous article in this series (issue 128), I looked at measurement methods that help determine the quality of electronic components, such as capacitors and resistors. However, most hobbyists are happy with the electronic components chosen by the designer of their equipment and do not want to make any changes. However, there is one electronic component that always requires special attention: the vacuum tube. Of course, not all audiophiles will want to use tubed equipment, but for those who do, the quality and condition of these tubes play an important role in the sound quality of their system. Tube quality can be determined by testing.

People often have preconceived ideas about tubes. Tube amplifiers have a reputation for having a "soft" or "colored" sound and for having poor frequency extension and weak bass. The fact that many of us have experimented with tube amplifiers confirms these preconceptions, but these characteristics are not due to inherent defects in the tubes themselves, but to poorly designed and/or inferior components.

For those unfamiliar with how tubes amplify signals or need a quick refresher: the heated component (usually the cathode) releases negatively charged electrons that are attracted to the anode or positively charged flat plate. A musical signal is applied to the grid between the cathode and anode, which usually maintains a negative voltage with respect to the cathode, thus varying the flow of electrons according to the musical signal (like a valve controlling the flow of water, hence the British name for the vacuum tube is electronic valve). This signal current induces a voltage across the plate, the level of which depends on the plate resistance. Since the plate voltage is usually higher than the signal voltage applied to the gate, the tube acts as an amplifier. (The diodes or rectifiers used for power supplies work differently.)

) The topology of electron tube circuits can be divided into self-polarized (cathode) circuits and fixed-bias circuits. Self-polarized amplifiers require a cathode resistor to increase the cathode voltage. The gate of the electron tube is held at ground potential, so there is a negative potential difference (Vgk) between the gate and cathode. The plate current decreases as the negative value of V gk increases (a more negatively charged gate will repel more electrons released from the cathode) and increases as the negative value of V gk decreases until the potential difference approaches zero and the gate begins to conduct. At this point, the input impedance of the valve drops rapidly, causing distortion unless the control stage is designed to deliver current to the grid (a configuration known as class A2). As the plate current increases, the current through the cathode resistor also increases, increasing the cathode voltage. This makes Vgk more negative, which in turn reduces the plate current. This is the negative feedback mechanism by which the plate current remains fairly constant.

Although this negative feedback is useful in the steady state, it also reduces the signal gain (known as degradation). For this reason, a bypass capacitor is often connected in parallel to the cathode resistor to prevent negative signal feedback. The resistor and capacitor combination forms a high pass filter, which means that the capacitor value must be large enough to avoid low frequency degradation. The quality of these components has a significant impact on the sound quality, as they are in the signal path. However, the capacitance required is usually very large, so electrolytic capacitors are often used. The equivalent series resistance (ESR) and dielectric absorption (DA) of these capacitors are several orders of magnitude higher than those of film capacitors, making them unsuitable for use in signal circuits. This is one of the reasons why this type of design can degrade performance.

To avoid cathode polarization problems, fixed polarization can be used. In this case, the cathode of the lamp is grounded and the power supply is kept at a constant negative potential. This means that a negative voltage power supply is needed, which increases the cost. This type of power supply must also be well regulated, as this has a significant impact on the sound quality. However, this type of design has a cleaner sound and better transient response than cathode biasing.

What does this have to do with the probe tests? This is because the circuit design determines the extent to which certain electrical characteristics of the tube will affect the sound quality. This is why some tubes may sound good when connected to a particular amplifier, but be terrible when used in another amplifier. What is the reason for testing vacuum tubes? First, we want to make sure the tube meets or comes close to its original specification. Secondly, we want to match certain characteristics of the tubes so that they work well together and we can determine how they will perform in different circuits.

Vacuum tubes are a handmade product. In the past, once assembled by a skilled technician, the tubes were baked (usually for at least 48 hours) and then tested. More expensive premium tubes are often subject to stricter quality control requirements. For example, premium valves such as E188CC/7308 and 6072a are specially selected low-noise copies of E88CC/6922 and 12AY7 respectively. For some difficult-to-manufacture tubes, such as some lattice constructions, the scrap rate is very high. It should be noted that tubes were a staple of electronic equipment until the late 1960s and were used in many mission-critical applications. Well-known companies such as Telefunken, AEG, Siemens, Valvo, Western Electric, STC, Mullard, Brimar.

For example, Western Electric's 437A raster tubes are used in submarine amplifiers for telephone lines crossing the Atlantic and Pacific oceans. These tubes can easily last 20 years in continuous use. Imagine having to replace a damaged tube in this case; renting a submarine would suffice. These tubes have extremely high transconductivity; it is even possible to build a power amplifier with only 437A per channel. I have a stack of these waiting for a retirement project ......

A 1950 Western Electric 437A is available from Tube Depot.com for $1295.

Today, tubes are made primarily for guitar amps, where quality control is not as stringent. As a result, many audiophiles are willing to turn to electronic tubes from a bygone era. New (NOS) stocks of these old tubes are becoming increasingly scarce and many of those sold as NOS may be "throwaway" tubes or come from old equipment.

It is still better to buy NOS or new production tubes (if you are a manufacturer, it is safe or nearly impossible to use anything but new production tubes due to quantity requirements). Most of my NOS tubes I bought in the early 90's from respectable electronics dealers who still had them in stock in their warehouses. Counterfeits do exist, with the exception of a few reputable dealers who specialize in automotive NOS lamps and have the knowledge to differentiate genuine NOS lamps from those that are not, and all of them are honest and only sell genuine parts. I would be very careful about buying from other sources. And you absolutely must test tubes before connecting them to your expensive amp, unless they come from one of the few dealers who test all the tubes they sell. The quality of tubes varies greatly even when they are freshly manufactured, and rough handling during shipping can damage the internal structure of the tubes. Tubes should be tested after purchase and if found defective should be returned to the distributor.

There are several issues to keep in mind when using NOS tubes. First, these tubes lose their vacuum over time. Air molecules that enter the tubes during years of storage become ionized when the tubes are first started up and these positive ions are attracted to the cathode, striking the tube surface and causing damage, shortening the life of the tube. When these positive ions hit the grid, electrons are released from the grid, causing the grid to become more positively charged. This grid current causes a noise voltage that is amplified by the electron tube; the higher the resistance of the grid circuit, the higher the noise voltage. The positively charged grid increases the panel current, which in turn increases the grid current and causes the grid to become more positively charged (called thermal decay) until the cathode is depleted. In this case, the panel starts to glow red ("red coating"), which looks very bad. Cathode circuits are less susceptible to this problem, as they are self-correcting.

Morgan Jones, author of the excellent book Valve Amplifiers, suggests burning NOS valves in an oven at 120°C for 12 hours and letting them cool before use. His experiments showed that the heat reactivated the residual barium in the getter, a structure inside the tube that absorbs the residual gas and maintains the vacuum inside the tube.

The metal oxide cathode tube must be handled with care. If current passes through the cathode before it has reached the proper temperature, its life is shortened. In the past, when tube rectifiers were used in amplifiers, this was usually not a problem because the time required for the rectifier to heat up and conduct was sufficient for the cathode to reach operating temperature; however, modern devices with semiconductor diodes require a delay in the circuit. (Some modern devices, such as some guitar amplifiers, still use tube rectifiers). Some modern amplifiers are considered to have NOS tubes and the user should only install current production tubes in them, unless circuit modifications have been made.

What should we check to make sure the tubes are working properly? An open or shorted filament can be detected with a multimeter and should be checked before wiring. Set the multimeter to measure resistance and connect the multimeter leads to the filament pins (pins can be found in the lamp manual online). (You will need a lamp tester to check the other parameters).

I have a George Kaye small-signal tube tester that I have found very useful for testing smaller tubes for power, gain, noise, and microphony (the tendency of a tube to literally behave like a microphone; you can sometimes hear a "hum" or "ring" when you play such a tube into an amp or preamp). The tester checks distortion when the tubes are overloaded; distortion increases as tube power is reduced by aging. Gain is only measured at one operating point, but this is adequate to adapt the tubes in the less critical areas of the circuit. The noise and microphone test is very useful: the tester allows listening through headphones and displays the noise level on a VU meter. I use it to rank (from lowest to highest noise) the tubes used in phono, input, drive or output stages, preamplifiers, integrated amplifiers and power amplifiers. Unfortunately, this tester is no longer manufactured and can only test small signal tubes, such as the 12AX7, but not large power tubes. However, I mention its use to explain what to look out for when testing tubes and later I will discuss other earlier and current tube testers.

Adrian George Kay's small signal tube tester.

Tube matching is important when using tubes at the output of push-pull power amplifiers, differential (balanced) circuits, and parallel devices. In all these cases, the tube output should be set as high as possible.

In a push-pull output design, where one tube (or group of tubes) amplifies the positive phase (or half) of the audio signal and the other tube (or group of tubes) amplifies the negative phase, the current imbalance between the two phases will cause the transformer core to saturate and the inductance to drop rapidly. If this situation continues, the transformer core will become permanently magnetized. In many amplifiers, the output valve plate current is adjustable. If this is not the case, or if the amplifier uses cathode biasing, the electron tubes will have to be adjusted. Amplifiers with parallel push-pull output devices usually do not have individual settings for each electron tube. Therefore, matched tubes must be used. In differential circuits, mismatched tubes can cause higher distortion, but the result depends on the circuit design. Many well-designed modern amplifiers use a constant current transistor source or heat sink to control the tube plate current. This provides more stable operation and greatly improves differential graded common mode rejection. These designs are less sensitive to drift of parameters such as plate resistance, gain and transconductance.

Schematic of a sine wave, showing the positive and negative parts of the waveform. Courtesy of Wikimedia Commons/AlanM1.

Schematic of a sine waveform, showing the positive and negative parts of the waveform. Courtesy of Wikimedia Commons/AlanM1.

Ideally, the electrons should match in terms of emission and transconductance (change in plate current per unit line voltage). However, most electron tube testers only measure transconductance at one operating point. To match tubes at different operating points, a curve tracer is needed.

Historically, the most popular valve testers were the TV-7 series manufactured by Hick and others for the U.S. military. These testers were often available on the aftermarket for very little money, but they used proprietary test tubes that had to be checked and calibrated to function properly. Audio clubs with the means and experience should invest in a lab tube tester for their members to use. Some of the best old examples are the British AVO VCM 163 and the German Neuberger RMP370. My recording partner bought two AVO 163s and used one as a parts donor to refurbish the other. The tester is a great piece of British engineering and a pleasure to use. Old valve testers were also made by B&K, Eico, Knight, Precision Apparatus Company, Sencore, Heathkit and others.

[JI Agnew of Miedźna rebuilt the lamp tester. Vintage Tube Electronics, based in the U.S., repairs and calibrates electron tube test equipment. - Editor's note].

The most famous electron tube curve tracer is the Tektronix 570, which is rare and expensive. It displays test results on a cathode display. However, it can only provide up to 300V DC and 150mA plate current, so some electron tubes cannot be tested at their typical operating conditions. More readily available is the Tektronix 575 transistor curve tracer, which can be modified to measure electrons. In recent years, audio enthusiasts have had a number of curve tracers available to them. These devices connect to a computer via a digital interface and all information is displayed on the computer screen. These devices can check for radiation, gas leakage, short circuits between electrodes, plate resistance, gain and transconductance, and produce tube curves showing plate current versus plate voltage at different supply voltage levels.

The Amplitrex AT1000 is a stand-alone instrument that can measure various tube parameters without the need for a computer via the LED display. However, it must be used in conjunction with a computer to generate the curves. The software is rather crude and still uses the RS232 interface. Another limitation is that the parameters are only measured at one operating point. Also, it has only a mains supply, so only tetrodes and pentodes connected as triodes can be measured. It has a headphone output that allows the user to evaluate tube and microphone noise.

Amplitrex AT1000 tube tester.

The RoeTest was developed by a German hobbyist and is a very flexible device. It has three power supplies and can measure tube parameters at different operating points. However, the designer can only supply the software, blank circuit boards, transformers and construction files to build the cabinet. The user had to buy all the electronic components, order the cabinet from a store and build the device himself. The design is quite complex and because of the voltages involved of up to 600V, the user must have experience in building and testing tube amplifiers to handle such a project.

The novelty is the eTracer. It was developed by a Taiwanese electronics engineer and can test test tubes with a maximum plate voltage of 750 volts at 300 milliamps. This means that high-power triodes can also be tested. It can measure cathode heater leakage, which is important for helmut, SRPP, and other topologies where one tube is stacked on top of another and the cathode of the top tube has a high voltage. If the heater line of the top tube is held at ground potential, there will be a large potential difference between the heater and the cathode, generating leakage currents and causing noise. The correct way to design such a circuit is to have a separate power supply to heat the top tube, fed at the same voltage as the cathode. This adds cost and complexity, and not all manufacturers do this.

The eTracer can also detect gas leaks by measuring the change in plate current when a resistor is inserted into the grid circuit. When a resistor is present, the grid current caused by ionized gas molecules causes an increase in grid voltage, which manifests as an increase in plate current. The software is very powerful and displays the parameters (plate current, plate resistance, gain and transconductance) at different operating points by simply placing the mouse pointer over the different areas of the graph. There is a curve fitting function to help find the most similar tubes. eTracer can also calculate distortion at different anode loads, which is very useful when designing circuits.

The unit can be purchased in kit form or as a fully assembled and tested product. The basic model requires the user to manually connect the various pins of the tube socket with a banana plug, which reminds me of an old telephone exchange. There is also an optional cable management module for the computer, but I don't think this is a necessary expense unless you plan to test many different types of tubes in one session. A fully assembled basic unit costs about $1200, which in my opinion is a worthwhile purchase if you use tube units regularly. By comparison, a pair of reissued Western Electric 300Bs cost $1499 on the manufacturer's website. Shouldn't you at least check that they work as advertised?

P.S. A word more about the output transformers.

The quality of the output transformer of a tube amplifier is extremely important. Since quality transformers are expensive to manufacture, any attempt to cut costs will severely degrade the sound of the amplifier. This is especially true for single-ended amplifiers, where air gaps in the transformer are required to prevent transformer core saturation, which can lead to audible distortion and other problems. However, the air gap reduces inductance, which affects low-frequency range response. To increase inductance, more windings must be added, which adversely affects the high frequency response. The design of these transformers is therefore a balancing act. The obvious solution is to use different amplifiers optimized for different frequency ranges, with active crossovers for the different speaker drivers (woofer, midrange and tweeter), but I would argue (and this is just one of the variables that make owning and operating tube equipment so fascinating).