Spectra of harmonic distortions in electron tubes. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment
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The author presents the results of measurements of the harmonic distortion spectrum in a cascade with a transformer load for a number of common triodes and pentodes in a triode connection. The article gives recommendations on the choice of vacuum tubes for a low-power amplifier.
The reason for studying the spectrum of harmonic distortions of the tone signal was the desire to choose the most suitable tube in the output stage of the headphone amplifier. The conceived amplifier required a triode (or a pentode in a triode connection) with a well-defined output signal spectrum. The purpose of the lamp determined the test mode: the anode current (lA) - within 20 ... 40 mA and the load resistance reduced to the primary winding of the transformer - 5 kOhm.
Having outlined several types of lamps, I assembled a cascade according to the circuit shown in fig. 1 and started testing. The spectrum analyzer was connected to the output of the stage at point A. Although the transformer contributes to the total amount of distortion, at a frequency of 1 kHz, the error due to its non-linearity is so small that it can be ignored.
After examining the intended lamps, out of curiosity, I examined the spectra of all more or less suitable lamps for this cascade.
The pentodes were tested in a triode connection, and for the convenience of comparing the results of the measurements, they were carried out at the same output power for all lamps, equal to 1 W, if the lamp was capable of delivering this power.
For lamps of low power, the anode current was chosen as high as possible, but not more than 40 mA, while a mode was selected that ensures the minimum level of harmonics.
The results obtained can be extrapolated with some caution to other operating modes of the lamps; this is especially true for high-power lamps, which were significantly underloaded during the tests.
For most lamps, the following considerations can be followed.
A decrease in the lamp load resistance with a simultaneous increase in current does not significantly change the nature of the harmonic energy distribution in the spectrum. A decrease in load resistance at a constant quiescent current leads to an increase in low-order harmonics and the appearance of higher-order harmonics. A similar effect is observed with an increase in the output voltage swing, but in this case, the growth of high-order harmonics occurs faster.
With some lamps, by adjusting the power mode, you can achieve the suppression of any harmonics. For example, for a 6P13S lamp, by choosing the position of the operating point, you can almost completely suppress the fourth harmonic.
On fig. 2-16 for various lamps shows the spectra of harmonic distortion of the output voltage with an indication of their mode of operation in table. 1.
(click to enlarge)
On fig. 17 shows a summary graph of the spectra of the lamps tested.
In table. 2 shows the numerical levels of harmonic components in percent.
Based on the results obtained, a number of conclusions can be drawn regarding the most promising or frequently used lamps.
For simple amplifiers with an output power of 1 ... 2 W, 6P14P, 6P1P lamps will be the best choice from domestic ones.
For symmetrical powerful driver stages, 6P1P and 6N6P are well suited. The possibility of using a 6S19P lamp is highly controversial. This tube has the smallest third harmonic level, but the longest harmonic tail. The 6P9 lamp is unstable at high power dissipation at the anode.
For more powerful single-cycle output stages, according to the results of measurements, the EL36 lamp is most suitable; it is perhaps the most linear of all lamps tested. The use of a 6PCD lamp in single-cycle cascades, in my opinion, is completely inappropriate, despite the fact that it has a minimum level of the third harmonic. It has a rather long harmonic tail and a high second harmonic level; its place is in push-pull cascades. The 6P41S lamp looks very attractive for use in single-cycle cascades. The 6H13S lamp, due to the increased level of even harmonics, "gravitates" more towards push-pull stages.
The recommendations set forth here for the use of some of the tested lamps should not be taken unambiguously due to the significant deviation of the measurement mode from the optimal one for high-power lamps. The measurement results should be considered as input for reflection; the presented materials will help readers in choosing the output stage lamp.
The author's conclusions regarding the applicability of the most powerful lamps in various stages do not entirely coincide with the recommendations found in the literature.
Author: E.Karpov, Odessa, Ukraine
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