Amateur radio measurements
Simple lamp tester
The device allows you to determine cathode emission, short circuit between electrodes and breakage of leads from the electrodes of lamps and screen.
The emissivity of the lamp cathode can be judged by the readings of a microammeter connected between the cathode and the first grid. Electrons emitted from the heated cathode charge the lamp electrodes, including the control grid, negatively. The microammeter works like a millivoltmeter and measures the potential of the first grid, which varies widely from 10 to 500 mV and depends on the types of lamps and the quality of their cathodes.
The readings from the device are compared with the emission of known good (calibration) lamps. Such calibration is carried out when setting up the device, and it is necessary to use as many types of lamps as possible. The data is entered into a table.
When checking diodes and kenotrons, a microammeter is connected between the cathode and anode.
Toggle switches Vk1-Vk6 connect all other electrodes of the lamp to the device. In the absence of interelectrode short circuits and broken leads, the instrument readings should increase. So, for example, when checking a lamp, the device "AVO-5m" (limits 60 and 300 µA) showed a current in the circuit of the first grid of 50 µA, when connecting the second grid - 70 µA and when connecting the anode - 90 µA.
When checking the kenotron, the device "School AVO-63" in the circuit of the first anode it showed a current of 4.9 mA, when connecting the second anode - 10 mA. In both cases, the lamps were taken from working equipment.
Switch P1 (with a neutral position) switches the measurement limits of the device; the values of resistances R1 and R2 are selected when adjusting the device according to the best radio tubes.
To manufacture the device, you need a step-down transformer with a power of 10...20 W, a microammeter of 50...300 μA and eight toggle switches.
The windings of the Tr1 transformer are wound on a core made of ShL-16 plates, the thickness of the set is 25 mm. The primary winding contains 1100 turns of PEL 0.35 wire plus 800 turns of PEL 0.27 wire. Secondary winding at 4.5; 6.3; 12.6; 20 and 30 V - respectively 48+12+18+78+84+120 turns of PEL 1.2 wire.
You can use a transformer assembled on a core from Sh-20 plates with a set thickness of 20 mm with a primary winding of 1360 turns of PEL wire 0.34+1000 turns of PEL wire 0.27 and a secondary winding of 43+11+13+63+74+100 turns of wire PEL 1.0.
The device can check the emission of picture tubes and oscillographic tubes.
Eng. V. Leonov. "Radio" No. 12/1965
Comments on the article:
Once upon a time, during the golden era of tube technology, receiving and amplifying radio tubes were used in military, metrological, navigation, and industrial equipment. Therefore, the quality in the production of radio tubes was brought to the appropriate level. Then the imperative of the equipment designer was to obtain the specified characteristics without selecting lamps and reducing the number of lamp parameters used in the design.
This approach will not work today. By definition, new-made lamps do not require serious use (but the fetishization of lamps is thriving), with all the ensuing consequences. Well, who takes a guitar amp seriously except the user and his quarrelsome neighbors? Few people check even the basic compliance of the output power (and it depends on the selection of lamps) with the rated value during equipment maintenance!
On the other hand, those original lamps (NOS - New Old Stock, which means “from old stocks”), which today can be obtained by hook or by crook, were not necessarily stored in the Pentagon warehouses (where the lamps had priorities far from sound), but could remain as unclaimed rejection or something like that. Who knows?
Thus, on the one hand, we have lamps whose characteristics have a significant scatter, and on the other hand, subjectivity, “taste” in assessing the performance of the equipment (aka sound). It is not possible to eliminate the last extra “degree of freedom”.
This means that lamps must be carefully checked and selected. Not writing on the lamp packaging one single, hastily taken, value of the anode current in who knows what mode - this is not selection! And give an adequate set of parameters. In fact, this is exactly what decent sellers do. Why are we any worse?
It would seem that lamp meter devices like the domestic L3-3 (and less accessible American ones, Hickok) exist and are quite accessible. These instruments allow you to perform a wide range of tests on hundreds of lamp types.
They also have their own limitations that do not allow us to solve all our problems. So, for example, it is impossible to “fry” a lamp of type 6550 on L3-3. And the excellent emission indicators of some small lamp, recorded using such devices, indicate the performance of the lamp, with which consumer equipment will be unsuitable for use due to the microphone effect or noise. Add to this the “delights” of reading on a multifunctional dial indicator scale. We are interested in specific, application-related tests of lamps of a limited range and in large quantities.
Test bench developed by Yuri Bolotov
Therefore, it is advisable to test lamps for sound equipment using specialized means that you have to make yourself.
I would like to note in this matter the importance of stabilizing supply voltages in equipment, be it filament, bias or high voltages.
Preamp tube testing
Most lamps used in audio equipment are double triodes with identical halves, in a finger design. Exceptions are rare and exotic and require individual consideration. This is where the specificity of mass testing of lamps for commercial purposes comes from.
In addition to rejecting unsuitable specimens, there is the task of identifying specimens with special properties:
Instances with a higher or lower gain (for example, high gain);
- low noise and non-microphone (V1, low noise);
- with identical gains of triodes in the cylinder (balanced).
The remaining specimens, not outstanding in terms of the listed properties, but undoubtedly suitable, form the corresponding group of lamps (without additional designations, standard, regular - I prefer the latter designation).
In principle, the static mode of triodes is of little concern to us (except for rare special cases), it is important that it more or less fits into the standards for lamps of this type and that the “swing” of the halves is within certain limits.
The test bench allows you to implement typical electrical modes most often found in audio equipment and conduct specialized tests for the range of lamp types of interest.
The lamp is installed on the stand, high voltage is supplied after the cathode is warmed up. Then the lamp is trained for some time (from 20 minutes), the voltage at the anodes is controlled. An alternating voltage from a generator is supplied to the input of the stand, and the voltage amplified by each triode is measured. Based on the result, one can judge the amplification capabilities of the lamp.
The insulation between the cathode and the heater is also tested, for which it is possible to introduce a constant voltage between the filament and the common wire of the circuit. A negative voltage is applied to this section within the limits of 100 V acceptable for most lamps. We judge the quality of the insulation by the amount of current flowing in this circuit (it is negligible). In general, lamps for serious use are subject to a more severe voltage test of about 250 V, which can also be achieved if you are willing to pay extra.
The next stage of the test is subjective. The stand with the test lamp is located approximately 1 foot in front of a guitar cabinet with a twelve-inch speaker connected to a high-gain guitar amplifier, configured so that the guitar produces a clear “j-j” and the volume at this point in space is about 110 dB. The outputs of the stand, of which there are two, as well as the triodes in the cylinder of the lamp under test, are connected in turn to the input of the guitar amplifier.
The lamp, which is prone to microphone effects, instantly reveals itself with a loud and joyful pig squeal. Additionally, by tapping a seemingly non-microphone lamp with a wooden stick, we find out the degree of its resistance to this evil. Well, the noises... you can hear them! Character, coloring, level - it is quite difficult to adequately measure. But some experience as a user of high-gain guitar amplifiers allows you to get an assessment in exactly the form that is required - in an emotional way, because this is what the meaning of using tubes ultimately comes down to.
Output tube test
Let's assume that the lamp is a pentode or beam tetrode; these are the lamps that are used in the output stages of the vast majority of tube amplifiers.
Testing the lamp begins by applying voltages to the electrodes in the proper order. At first, the lamp operates in light mode. If there are no signs of obvious unsuitability of this instance, we move on to the next stage.
Anode current;
- current of the second grid;
- current of the first grid;
An alternating voltage from the generator is introduced into the first grid circuit. The alternating component of the anode current is measured. From this value the slope is calculated using the first grid.
An alternating voltage is introduced into the circuit of the second grid, and the alternating component of the anode current is measured. From this value the slope is calculated using the second grid.
Then the installation is switched back to light mode. Anode current at reduced power dissipated by the anode (approximately 20% of maximum). This additional control point is of some importance for the selection of pairs of lamps that will operate in push-pull cascades of class AB or B.
Thus, we obtain a set of parameters sufficient to group lamps into pairs or quads. The basis for rejecting a lamp may be “outstanding” values of these parameters, especially an abnormally large value of the first grid current. The latter indicates, for a freshly baked lamp, the presence of too much residual gas in the cylinder, which for those types of devices that are prone to the occurrence of thermal current in the circuit of the first grid (primarily lamps with a high slope, for example EL84, EL34), further reduces Reliability of operation in fixed bias mode.
A new method for testing and selecting output tubes - the three-point method
When testing lamps for flux, the task of reducing the labor intensity of this process becomes particularly important. It is also necessary to maintain or improve the accuracy of measurements.
The measurement accuracy is influenced by both the measurement technique itself and the quality of stabilization of the voltages used in the circuit. Labor intensity is influenced by the need to control these stresses. It follows from this that in order to reduce the labor intensity of the process, it is necessary to minimize the number of voltages used in the circuit.
The minimum set of voltages sufficient to test lamps in a variety of modes of interest to us consists of filament voltage, high voltage and bias voltage.
A stable filament voltage is obtained from a winding wire of a transformer connected to a stabilized alternating current network, wound with a sufficiently thick wire (to avoid sagging under the load that varies depending on the type of lamp being tested). In our case, an electro-mechanical type stabilizer is used, which provides the specified output voltage with an accuracy of 1%. The remaining voltages are obtained from adjustable electronic stabilizers. The high voltage in our installation is limited to 450 – 500 V.
The lamp testing process begins... with cleaning the base. The fact is that even from the factory the lamps come dirty. Then our special designations are applied.
Next, the lamp is installed on the stand, the filament is warmed up (the bias voltage source is always on), and high voltage is applied to the anode and screen grid. For some time, the lamp is additionally warmed up and brought to the maximum permissible mode for the power dissipated at the anode, in which it is maintained for at least 2 hours. In this case, you can observe the glow of the electrode system and draw appropriate conclusions regarding the quality of this lamp. Upon completion of this stage, the anode current Ia1 and the control grid current are measured. After this, the high voltage is reduced by the amount dU2 at a constant bias voltage. The lamp switches to another mode, a new value of the anode current, Ia2, is measured. Then we reduce the bias voltage by the amount dU1 at a constant high voltage and measure the new value of the anode current, Ia3.
In principle, this ends the lamp testing program. The whole process takes 2.5 – 3 hours.
Estimation of the slope of the lamp characteristic using the first grid:
S1 = (Ia3 - Ia2)/dU1
Estimation of the slope of the lamp characteristic using the second grid:
S2 = (Ia1 - Ia2)/dU2
In the last formula we neglect the influence of the anode (high) voltage on the anode current. With this test method, a phenomenon such as thermal inertia of lamps becomes noticeable, which manifests itself during their slow transition from one mode to another. Therefore, when changing the electrical mode, measurements are performed only after the new thermal mode has been established.
The criterion for selecting pairs and quartets of lamps is that the spread of anode currents in each of the three measured operating points should be within 2%. It should be noted that this is a rather stringent requirement that guarantees the pairing of lamps in a variety of modes that differ significantly from the test ones.
Based on the values of the anode current at all three points and the slope of the characteristics on the first grid, the lamps are sorted into the categories Compressed Distortion - Dynamic Clean, the number of varieties depends on the volume of testing of lamps of the same type.
The proposed device is intended for testing radio tubes with an octal base and finger-type radio tubes with a seven- and nine-pin base, as well as low-power transistors of the p-p-p and p-p-p types.
When testing radio tubes, the device is powered from an alternating current network of 127/220 V and consumes up to 12 W, and when testing transistors from an internal DC battery KBS - L - 0.50 with a voltage of 3.7 V.
Radio tubes are tested for the integrity of the filament, the absence of short circuits between the electrodes, the emission current, the absence of breaks between the electrode terminals and the pins of the base. When testing transistors, the reverse collector current of the junction and the gain p are determined.
The schematic diagram of the device is shown in Fig. I The device consists of a lamp tester, a transistor tester, a measuring circuit and a switching circuit
The lamp tester circuit includes lamp sockets and P-G9 plug sockets. switch P1, power transformer, network terminals, fuse, signal light, switch P4b, P5, wire with cap, plugs for supplying filament to the lamp being tested, resistances R5, R6, diode D.
The transistor tester circuit includes cartridges for clamping transistor terminals, a KBS-L-0.50 battery, and Rl-R4 resistances.
The measuring circuit includes the M592 device, the R7-RI0 universal shunt and the GIA switch.
The switching circuit includes switches P2 and PZ, P5, B2.
To test the radio tubes most often used by radio amateurs, you can limit yourself to all three tube panels: octal, seven-pin and nine-pin.
Before checking, the lamp is installed in the appropriate socket, the PZ switch is set to the “p-p-p, lamp” position, the device is connected to the network, and switch B1 is turned on, and the signal light comes on. If the lamp being tested has an electrode connected to the cap, the tester's clamp 1 is placed on it, connected to the 9th pin of the finger socket. To check the integrity of the filament, it is necessary to set switch knob P1 to the number of one of the lamp filament terminals in accordance with the base, switch knob 414 to the “off” position, switch knob P2 to the short-circuit position and remove the filament plugs from the sockets. When ^gom, an alternating voltage of 25 V will be supplied to the lamp filament from the transformer through the limiting resistance R5, diode D and a measuring device with a shunt. All other electrodes of the lamp are connected to the body of the device. The deviation of the instrument arrow will indicate the integrity of the filament. When checking the integrity of the filament, the instrument scale is turned on to the limit of 2.5 mA.
When testing a lamp for the absence of short circuits between the electrodes, proceed in the same way as when checking the integrity of the filament. In this case, switch 111 is alternately set to positions I-9. The absence of instrument readings indicates that the electrode (the number of which is set by switch P1) is not connected to the other electrodes. The deflection of the instrument arrow indicates the short circuit. To determine which electrode there is a short circuit with, you need to check the remaining electrodes one by one.
The lamp tester allows you to conditionally measure the emission current of radio lamps. The emission current in this case cannot exceed 10 mA. Therefore, according to the measurement results
The preliminary test is aimed at determining the integrity of the lamp filament and the absence of short circuits between its electrodes.
This test is carried out with an ohmmeter or a neon lamp NL (Fig. 1). In this case, you only need to observe whether current flows if you connect the device to the terminals of the filament on the lamp base, and whether it is absent if you connect the device to other electrodes. Most static lamp testing instruments provide a convenient and quick preliminary test of this type.
Rice. 1. Preliminary tests of lamps.
a - for thread breakage; b - for a short circuit between the electrodes.
Static lamp test is a determination of all lamp parameters, but it requires rather complex equipment and is carried out only in laboratories. In workshops, simplified instruments called lamp testers or lamp testers are used for static testing of lamps.
Emission measurement. Most testers allow you to determine the cathode emission, i.e., the cathode current of the lamp at certain constant voltages on its electrodes, which are indicated for various types of lamps by the manufacturer in special tables attached to the tester: the tester device includes potentiometers and switches that allow for these tables to reproduce the required test mode. The anode current obtained under these conditions is considered a criterion for the suitability of the lamp.
The anode current indicator scale is often not graduated, but is divided into two or three sectors with designations: good, suitable and unsuitable. When testing lamps on a tester with a scale calibrated in percentages, lamps that produce at least 70% of the normal anode current are considered good; at 50-69% they are considered still suitable, and below 50% the lamps are rejected altogether. Determination of emissions in a simplified way can be carried out without the help of a special tester. To do this, it is enough to have on hand the source of the voltages necessary for testing the lamp and a milliammeter (Fig. 2 a).
Rice. 2
a - Simplified method for measuring cathode emission.
b - Measuring the slope of the characteristic
Measuring the slope of a characteristic. Constant voltages corresponding to its normal operating mode are applied to the electrodes of the lamp under test, including the grid bias voltage, which must correspond to the selected operating point. Having determined the anode current of the lamp using a milliammeter (Fig. 2 b), reduce the grid bias by exactly 1 V and again note the anode current.
The increase in anode current in milliamps determines the static slope of the characteristic in mA/V.
Vacuum test. To test a vacuum, the lamp is connected to a circuit similar to that for measuring emission or slope, with the negative voltage on the control grid corresponding to the selection of the normal operating point. Having noticed the magnitude of the anode current, introduce a resistance of 1 MOhm into the control grid circuit (Fig. 3) and observe the change in the anode current.
