Single-cycle tube amplifier on triodes. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Tube Power Amplifiers

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First, a few general points to clarify the choice of amplifier circuitry, which I will talk about, the radio components used in them, etc.

The range of direct-incandescent lamps, moreover, relatively affordable, is limited to several types. These are 300B, 2A3, 6C4C, 6B4G, GM70. The choice of indirectly heated triodes, mainly intended for voltage stabilizers, is also not very large. These are 6S19P, 6S41S, 6S33S, as well as double triodes 6N5S and 6N13S. Despite the fact that there are a number of single-cycle designs on 6H5S, 6H13S lamps, it should be noted that the current-voltage characteristics (CV) of these lamps are less linear, and the coefficient of nonlinear distortion (THD) is high (reaches 10% at rated power and Ra / Ri =4), while in 6С19П, 6С41С, 6С33С it does not exceed 3% under similar conditions. Therefore, 6H5S, 6H13S are best used in push-pull cascades.

Each of these lamps has its own unique sound, so it is very difficult to describe it in a nutshell. I will state my perception, and agree with it or not, your right.

GM70 - breadth and scale. On this lamp you can create an amplifier with an output power of more than 20W!!! The voltage at the anode of the lamp can reach up to 1000 volts, the anode current - up to 125 mA, so the output transformers must have high dielectric strength (about 3 kilovolts). The sound is very powerful and, in my opinion, a bit straightforward. Small nuances of a piece of music seem to be suppressed by this power and pressure, but I like a more delicate sound. In general - for an amateur.

2A3, 6S4S - very beautiful, detailed and melodious sound. I would call it "cozy and homey", but at the same time - accurate. The lamps are dual-anode designs with a common jumper and differ in voltage and filament current. In 6C4C, the filaments inside the cylinder are connected in series, and in 2A3, in parallel. As you understand, this affects the background level. In the case of using 2A3, it is possible to power the filament circuit with alternating current, but in the case of using 6C4C, it is better to use direct current.

6B4G - western analogue of 6С4С. It has a slightly more analytical sound. Since 6C4C and 6B4G have the same pinout, you can reveal your preferences by simply replacing one lamp with another. By the way, the Saratov "Reflector" also produces a single-anode version with the same CVC and parameters.

300B - is considered the "queen" of direct filament triodes. In my opinion, the lamp occupies an intermediate position between GM70 on the one hand, and 2A3, 6C4C, 6B4G on the other, combining (to a reasonable degree) the advantages of these two types of lamps. Judge for yourself. The output power of a single-ended amplifier on a 300B tube is 8,0W, versus 2,5-3,0W for 2A3 and 6C4C, with a fairly detailed and full sound.

Unfortunately, the sound of straight filament triodes, especially the 300B tube, is very dependent on the year of manufacture and manufacturer. I was able to listen to several modern amplifiers on this tube. To put it mildly, I was surprised and disappointed. They reproduced classical music without problems, but modern and dynamic, inexpressive and dreary. The reason (from my point of view) is that the 300V tubes were turned on in auto-bias mode, and this tube sounds best fixed. And only one of the amplifiers showed a decent sound. I was not allowed to remove the casing (apparently the developer was afraid of divulging his company secrets), but, according to him, the 300B lamps were imported, made in 1958, and the offset was fixed. The amplifier coped well with any musical material, providing a full-fledged sound.

6S19P - from the family of indirectly heated triodes, the lowest power (Pa = 11W). There are no foreign analogues. Therefore, when using one such tube in an amplifier, you have to be content with three watts of output power. But if you install two lamps, turning them on in parallel, the output power will increase to 6W. The sound is quite beautiful and detailed, so you can safely use these devices in the output stages of amplifiers. Naturally, in this case, it is necessary to select lamps in pairs or take measures to equalize their parameters.

6С41С - also a triode with indirect heating (Pa = 25W), has an approximate foreign analogue of EC360, moreover, with an octal base. On the Internet in various forums, I had to meet a variety of assessments of the sound of this lamp, and absolutely opposite. I will not quote the authors of these statements, since, in my opinion, most of them did nothing on this triode, since no one discussed operating modes or switching circuits. My experience of using a 6S41C lamp in the output stage of a single-ended tube amplifier, as well as the experience of A. I. Manakov, D. Andreev, V. A. Starodubtsev, allows us to say that the 6S41C is a great-sounding lamp, and with any type of bias.

Excellent, well-articulated bass and very voluminous and detailed sound reproduction are the hallmarks of the 6S41S sound. In addition, you will be surprised, the power of a single-cycle cascade on it is about 7 watts! The sound of 6S41S is somewhat similar to 300V with a fixed bias, and not one of the worst specimens. But the 300V lamp loses a little to the 6C41C lamp (this is not only my opinion) in dynamics. The disadvantages of a purely constructive nature can be considered the need to purchase special (not cheap) lamp panels and a high filament current. Some designers also consider a longer time to "enter the mode" (approximately 20-30 minutes) as a disadvantage, compared to direct filament lamps. However, I do not consider this fact a disadvantage, rather a feature, because any tube amplifier starts to sound better after a 20-30 minute warm-up. Such obvious advantages as excellent sound, high output power, no hum problems inherent in direct filament lamps, a simpler output transformer (Ra = 800 ohm is enough) due to the low internal resistance of the lamp (which is also good), etc. - more than compensate for these shortcomings.

6S33S (6P18S) - a very powerful triode of indirect heating (Pa = 60W). It has no Western analogues. The tube has been used in amplifiers for a long time, many circuits have been published in various publications and on the Internet. It must be said that this instrument is best used in auto-bias mode due to time and temperature instability and a tendency to self-heat. The sound of a tube in a single-ended amplifier I would describe as somewhat mundane and heavy, with a lack of air, but this is just my opinion, so I leave the choice to you. I emphasize that we are talking about a single-ended tube amplifier with an output transformer. At A. Klyachin's house, I listened to a 6C33C amplifier, made according to the scheme without output transformers (OTL), and so, that amplifier sounded great.

The output power of the amplifier when using 6S33S (6P18S) will be about 12W. The lamp "enters the mode" for an even longer time, compared to 6C41C.

Now let's talk a little about output power in general. For analysis, I will allow myself to introduce the term "comfortable power". This, as a rule, is the power at which the device operates for a long time, its sound does not irritate and allows for the most expressive performance of all the nuances of a piece of music. So, it turned out that for me in a room of 18 square meters, "comfortable power" was about 0,5 watts per channel. The vast majority of my friends who own single-ended tube amps have confirmed this fact. Someone had 0,4W per channel, someone had 0,7W per channel, in general, the numbers were similar.

Feel what I'm getting at? Considering that the maximum output power per channel of 2,5-3,0 W is more than sufficient for our apartments, as well as the great scarcity and high cost of good 300V lamps, the choice fell on the use of 6C4C, 2A3 or 6B4G direct-heated triodes in the output stage. If you need a more powerful amplifier, use indirectly heated triodes 6S19P, 6S41S.

Move on. One of the disadvantages of triodes is considered to be a large buildup voltage. Let's consider this moment in more detail. We open our favorite SE Amp CAD program and model the cascade on the 6B4 lamp. With a supply voltage of about 300 volts and a current of 55 mA, the output power when using a transformer with Ra \u4d 2,44 kΩ will be 40 W at an input voltage of about 2,0 volts. It would be foolish not to take into account the fact that the output voltage of modern CD players with delta-sigma DACs and operational amplifiers at analog outputs is 02 volts nominally (my Rotel RCD-100S has an output impedance of 2,0 ohms and a nominal output voltage of 2,8 volts , respectively, amplitude - 40 volts). Therefore, 6 volts to drive the output triode can be obtained from a simple preliminary stage on resistors, using a lamp with the gain you need. In my case, this condition is fully satisfied by lamps 5S6S, 2S6S or 8NXNUMXS.

They are very linear and have a deep opening of the anode characteristics when biased on the grid down to -24 volts. In addition, these types of lamps are perfect for working with direct-heated triodes, mutually compensating for each other's distortions.

If the output voltage of your signal source is small, then you can do the following. Firstly, you can use a lamp with a high gain, for example 6N9S, 6N2P, ECC83, E41CC. Secondly, apply an isolating transformer with a ratio of 1:2. Thirdly, use a pentode (tetrode) as a preliminary stage lamp. To opponents of the use of pentodes, I can say that the best examples of single-ended tube amplifiers of the last century had a pentode in the input stage, and their sound is still considered a reference. A little lower I will give diagrams of preliminary lamp stages on a pentode and a circuit that uses an isolation transformer.

(click to enlarge)

Let's move on to the diagram in Fig.1. We use it as a base, and by applying various lamps and changing their operating modes, we will try to create an apparatus that meets your specific tastes.

As you can see, the circuit is very simple, and consists of only two stages, preliminary and final. I always adhere to the principle of the minimum possible number of gain stages, since adding unnecessary elements in the signal path leads to a deterioration in sound.

The preliminary amplification stage is resistive. Since there are calculations for the cascade on resistors in almost any literature and the Internet, I do not give them. I think that in our case it would be more useful to talk about the sound of the preamplifier tubes. When discussing the amplifier circuit with A.I. Manakov, he proposed the 6S5S lamp as the most linear, having a cylindrical design of the electrode system. In second place - 6S2S. If you open the reference book, you will see that the parameters of these lamps are almost the same, which cannot be said about the internal design. This explains the difference in sound. Despite the individual differences (and they are), both lamps sound very good. I did not notice any shortcomings (I do not consider one triode in a cylinder a disadvantage, rather an advantage). I suggest you try both options and decide which one you like best, especially since you don’t need to redo anything. If you could not find these lamps, use a 6H8S double triode (we connect both halves in parallel). The features of such an inclusion are described in my last article "Single-cycle tube ..., returning to the printed one", so I will not repeat myself. You can also use a 6H8C lamp without connecting the halves in parallel, in which case one lamp will work on both channels (there is a space saving).

I consider it necessary to tell you about one more thing. A 6C2C lamp is not half of a 6H8C lamp (as many "experts" on Internet forums mistakenly believe). Reference data are similar, the design of the electrode system is similar, but there are differences. Due to the larger anode area in 6C2C, the steepness of its characteristic is higher, and the real internal resistance is lower than that of the 6H8C half. The gain is the same (about 20). Traverses for mounting the electrode system 6S2S and 6N8S are the same, however, in the case of 6S2S, they attach one triode, not two. This explains the almost complete absence of the microphone effect in 6C2C. As you understand, because of this, the difference in sound (though not very big) will be necessary. The same must be said about the 6C41C lamp, which is not half of the 6C33C lamp, as many believe. Look carefully at the passport values ​​​​of the parameters of these lamps, as well as the volt-ampere characteristics. It is clear that the difference in sound will be significant.

In addition, you must remember that the real dynamic gain of the cascade on resistors is always less than the static gain of the particular lamp used. In order not to clutter up the article with formulas, we can assume that it is 25 percent. Thus, when using a 6C5C (6C2C) lamp, the dynamic gain of the real cascade will be 15-16. This moment must always be taken into account when calculating the lamp cascade on resistors.

You can use a choke instead of a resistor in the anode of the input lamp. According to some radio amateurs, the choke stage sounds better. Unfortunately, I cannot agree with them. I understand that everyone has different tastes, but I must express my (and not only) opinion about the sound of such cascades.

If you like to listen to symphonic or jazz music, then a choke-loaded cascade is not the best option. It sounds harsh, I would even say annoying. The overtones of string and wind instruments are emphasized very strongly. Reed instruments (saxophone, etc.) sound unnatural, with some unpleasant overtones. If you have the opportunity to listen to both stages (resistive and choke) at the same time (naturally with the same final stage), then put on a good recording of Dizi Gilespie (trumpet) or David Sanborn (saxophone). I think you will immediately hear the difference in sound.

The advantage of such cascades can be considered the maximum dynamic gain, close to the static one of the lamp used, but the minuses need to be said in more detail.

As you know, the inductor is an inductance, the preliminary stage lamp (driver) has an output capacitance, and the final stage lamp has an input capacitance, respectively. As a result, we have a resonant circuit tuned to a frequency, which is determined by the sum of these capacitances and the inductance of the inductor. F=1/2P multiply by the square root of the product LC. You should be aware that with a large inductance of the inductor, the resonance will move from the ultrasonic region to the audio frequencies and, despite the fact that the circuit is shunted by the internal resistance of the driver tube, and is significantly attenuated, it is still present. At the resonance frequency, the rise can reach up to 10 dB.

And one moment. The inductor resistance increases with increasing frequency, as a result, we get an uneven gain of the cascade (it increases with increasing frequency). Naturally, this lengthens the spectral "tail" of the harmonics, which does not have the best effect on the sound.

Since we are talking about preliminary cascades, it should be noted that there are many schemes whose authors use batteries or accumulators to organize the bias. Many people think that electrochemical current sources in bias circuits are preferable to the traditional resistor and capacitor, which adversely affect the sound. It must be said that batteries or accumulators can stand both in the grid circuit and in the cathode circuit.

I have tested seven types of batteries and three types of batteries from various manufacturers available in stores. Of the lamps, the following were tested: 6N1P, 6N2P, 6S2S, 6S5S, 6N8S, 6N9S, 6S4P, 6E5P. Accumulators in cathode circuits are preferable as there is no need for recharging (they are charged by lamp current). The only thing, so that there is no overcharging, you need to choose their capacity of at least 20 * I of the lamp. In my case, I chose the battery capacity in the range of 700-1000mA / h.

The first impression was very good, but as I listened, a small flaw was discovered. In my opinion, the sound acquired some "rigidity" (regardless of the type of electrochemical current source), which was not there when using a resistor and a capacitor. The best results were obtained when using NiCd batteries, and those standing in the cathode circuit, and not in the grid.

Of course, it must be said that I use Black Gate Rubicon electrolytic capacitors in the cathodes. Perhaps a cascade with a battery or a battery sounds better than the traditional one, especially in the case of poor quality Chinese capacitors and resistors taken from computer boards and power supplies. I don’t have such radio elements, so I suggest you listen to both options yourself and choose the one you like best.

Further, the signal through a separating capacitor is fed to the input of the final stage, made on a straight-line 6C4C triode. I wrote about the types of isolation capacitors many times, so now I will only talk about one nuance. When using lamps with a low gain in the input stage, it is best to use capacitors such as FT-3, K-77, K-78 as a separator, but if a tetrode or pentode is used as a driver, then paper in Jensen, K40U- 9, K42U-2, etc.

The final stage has no features. The lamp is on in auto-bias mode. In previous articles, I described the advantages and disadvantages of fixed and automatic offset types, so it makes no sense to repeat everything again. Choose yourself. Let me just say that when using Black Gate electrolytes (on the C6 and C9 diagrams), there is practically no difference in sound, but there are much fewer disadvantages inherent in a fixed bias.

To avoid problems with the background when using 6C4C, I powered the glow with direct current. In the case of using KD226 diodes, the heating voltage under load is 6 volts. If you use other diodes (necessarily "fast"), it may be necessary to adjust the filament voltage using an additional 0,3-0,5 ohm resistor. And one moment. For a direct-heated triode, the cathode and filament are the same, so the connecting wires of the filament circuits must be of high quality (unlike lamps with indirect filament). If you use a 2A3 lamp, then its incandescence can be powered by a "change", its background level is initially lower (I repeat, due to the parallel connection of the filaments of both triodes inside the cylinder).

It must also be said about why I used a transformer with Ra \u4d 6k. The fact is that many in their designs have already used the Audioinstrument TW4SE transformer, and it has Ra \u100d 10k. In order not to spend extra money on buying a new transformer, use the one you already have. Of course, it is better to use a transformer whose overall power is 6W, for example TW20SE, low frequencies will be reproduced even better in this case, but with TWXNUMXSE you will not be disappointed, since the overall power of the output transformer is selected within XNUMX*Pout or more.

In general, the maximum output power is achieved when Ra=2Ri, where Ra is the AC resistance of the primary winding of the output transformer and Ri is the internal resistance of the lamp. Unfortunately, in this case, the non-linear distortions are too high (about 6%). Therefore, the resistance of the primary winding of the transformer Ra is chosen within 3-5Ri (sometimes up to 7Ri), as a compromise between the magnitude of non-linear distortion and output power. But it must be taken into account that the power of the cascade decreases linearly, and the coefficient of nonlinear distortion (THD) exponentially, with all the ensuing consequences, therefore, there is the concept of reasonable sufficiency. In addition, an excessive increase in the anode load reduces the dynamics of the cascade. In our case, when using 6C4C or 2A3, with internal resistance Ri = 800 ohm, this condition is met.

To illustrate the above, I give data on the output power of the amplifier and the coefficient of the second and third harmonics at various values ​​of Ra (at 40 volts of alternating voltage at the lamp input, an anode current of 60 mA and 250 volts of anode voltage). I cited these current and voltage values ​​​​as an example not at all by chance. In the textbooks of Tsykin and Voishvillo, it is these modes that are recommended to achieve the best sound quality.

Ra=4,0kΩ, Pout=2,22W, 2nd harmonic 3,1%, 3rd harmonic 0,2% Ra=3,5kΩ, Pout=2,4W, 2nd harmonic 3,4%, 3rd harmonic 0,1% Ra=3,0kΩ, Pout=2,54W, 2nd harmonic 3,8%, 3rd harmonic 0% Ra=2,5kΩ, Pout=2,7W, 2nd harmonic 4,4%, 3rd harmonic 0,1% Ra=2,0kom, Pout=2,9W, 2nd harmonic 5,3%, 3rd harmonic 0,3%. I hope comments are unnecessary.

The quiescent current, as always, is controlled by the voltage drop across the cathode resistors. If you use the details indicated in the diagram, then it will be 55-60mA for a 6S4S lamp and 5-6mA for a 6S5S lamp.

Now let's move on to cases where the input voltage of the amplifier is less than two volts, or when a lamp is used in the output stage that requires a large buildup voltage (for example, 6C33C). Figure 2 shows a diagram of a pre-amplifier on a tetrode 6E5P in a triode connection, and in Fig. 3 in a standard tetrode connection.

You may ask why 6E5P? The fact is that while experimenting with various pentodes (6Zh4, 6Zh52P, etc.), I could not get a sound that would completely satisfy me. In some cases, transparency disappeared, in some cases, dryness appeared, etc. etc. And only 6E5P provided the necessary sound quality. The general impression is that the sound is very similar to the triode, only a little brighter. Deep, well-articulated bass, transparent highs and very detailed mids are hallmarks of the 6E5P's sound. My rating is excellent! In any case, it's up to you to choose and listen, and I will give the parameters of the lamp in triode and regular switching.

Triode connection: Ri=1,2kom; S=30mA/V; Kus=30-35. Tetrode connection: Ri=8kom; S=30mA/V; Cus=200. Well, how impressive? Naturally, having such parameters, the lamp will be able to freely "rock" any triode, be it 300V, 6S41S, 6S33S, GM70, etc.

It should be noted that the broadband tetrodes 6E5P, 6E6P with low internal resistance were "discovered" for audio applications by AI Manakov. They are successfully used by many designers in drivers (triode and tetrode mode) and as output lamps. On the same lamps at the end of 2003 A.I. Manakov also developed a resistive ultra-linear cascade, which also has a very good sound.

Now consider a variant of the circuit using an interstage transformer. The advantages of such inclusion are considered to be:

1. possibility of coordination with any load
2. maximum possible amplification
3. high efficiency
4. lower stage supply voltage
5. more dynamic sound.

However, not everything is so smooth. The disadvantages of the scheme are:

1. large dimensions and weight
2. the need for shielding
3. high price
4. high price

If these problems do not scare you, then Fig. 4 shows a diagram of a preliminary stage using an interstage transformer with a transfer ratio of 1: 2. The features of such cascades are repeatedly described in various sources, so I do not consider it necessary to consider them in detail.

The article would not be complete if you do not give an amplifier circuit in the output stage of which an indirect filament triode operates. I chose 6S41S, because there are very few circuits using this lamp, unlike 6S33S.

I strongly recommend that you give this design a try. You will be simply surprised by the sound. Compared to a 6C4C or 300V amplifier, I would describe it as more versatile. The amplifier equally well and naturally reproduces both classical music and modern, with a large number of impulse components.

The circuit using a 6E5P lamp in the input stage is shown in Fig. 5. As always, it's fairly simple and repeatable, so you shouldn't have any trouble making this variation. You can try different tubes in the input stage and choose the one that sounds best to you. The 6E5P lamp is turned on by a triode, so the sensitivity of the amplifier will be 1,8-2 volts. If this is not enough, apply the circuit in Fig. 3 or Fig. 4. The sensitivity of the amplifier in these cases will be 0,35-0,4V and 0,8-1,0V, respectively.

I will say a little about the choice of lamp modes 6S41S. The anode-cathode voltage is 165-175 volts, with a current through the lamp of about 93-95mA. This means that the dissipation power will be about 16 W, which is one and a half times less than the passport value (that is, the lamp operates in light mode).

Offset -70 volts. If you also look at the volt-ampere characteristics, you will see that the operating point of the lamp is in a linear region. The total current consumption of one amplifier channel is about 110 mA. Thus, if you are making a stereo amplifier, then it will be enough to use one 5Ts3S (5U4G) kenotron in its power supply. The rated rectified current of this kenotron is 220-230mA (reference value). If you decide to increase the current (which is quite acceptable), then you will need to use two kenotrons connected in parallel in the power supply of the amplifier, or make the amplifier in the form of two monoblocks. Naturally, the primary winding of the output transformer must also be designed for this current.

On forums on the Internet, I once saw a discussion of the power supply of an amplifier using television damper diodes, for example 6D22S. I must warn you that when using these lamps, the sound of the amplifier loses volume and detail, the depth of the stage disappears, it seems that the musicians are on the same line. This sound does not suit me, but you yourself have the right to decide this issue. If there is no desire to make a power supply on kenotrons, it is more expedient to use "fast" semiconductor diodes - "fasts" and "ultrafasts", designed for the corresponding current and voltage, shunting each of them with K78-2 capacitors with a capacity of 0,01-0,022 Mkf, to eliminate switching noise when switching.

The power supply circuit is similar to the circuit shown in Fig.1. Since the incandescence of the 6C41C lamp is powered by alternating current, diodes D1-D8, as well as filter capacitors C12-C15, must be excluded. Remember that the filament current of one lamp is 2,7 amperes, so the filament windings of the power transformer must be designed for it.

The cathode resistor of the 6C41C lamp gets very hot, so its dissipation power must be at least 15-20W.

The output transformer used in this circuit is made by "Audioinstrument" and has the following parameters: Ra=1kom; Ktr=12,5; Pgab=100W; I=150ma. The resistance of the primary winding to direct current is about 150 ohms.

Even better sound quality was obtained when using output transformers wound on OSM-0,16 cores, made at my request by Dmitry Andreev, for which special thanks to him. The parameters of these transformers are as follows: Ra=1kom; Ktr=10,05; Pgab=160W; I=200ma. The resistance of the primary winding to direct current is about 50 ohms. In both cases, the bias was -70 volts, and the dissipation power of the 6C41C lamp in the second case increased by only 1W. The sound has acquired even greater volume and detail, the band of reproducible frequencies has expanded (up to 70 kHz) and the depth of the stage has increased.

The installation of all the amplifiers that I talked about was done in a hinged way, using a Kimber TC series copper stranded cable. I like the neutral sounding character of this connector, as well as the resistance of its Teflon insulation to heat. The cost is about $30 per meter. But by purchasing 1 meter of this cable, in fact, you get 8 wires of 1 meter each (4 blue and 4 black). Agree that $ 4 per meter of good wire is not so much.

The wiring of the "earth" is made by a "star", in the last article I described this method in detail. AC hum is only audible if you bring your ear close to the speaker system. If this is not the case, you need to tinker with the relative position of the radio elements. In my case, the power supply chokes are in the basement of the chassis, and the power and output transformers are on top.

Well, that's about all. In conclusion, I would like to thank my friend A.I. Manakov, detector(dog)surguttel.ru for constant consultations and assistance in editing this article (all the circuits were personally tested by Anatoly Iosifovich long before me), as well as for the 6E5P and 6S41S lamps sent to them.

I must also tell you that the peculiarities of the perception of music are very individual, so you should not get hung up on any individual circuits or lamps. Not only straight-wire triodes provide high-quality sound. Both pentodes and indirectly heated triodes, with proper circuit design, the right choice of operating point and modes, are no worse. So learn, try, listen, experiment. We must not forget about the theory of electrovacuum devices and the construction of amplifiers on them, so that there are no empty "influences" and "revelations from above." Only in this case you will be able to create an apparatus that will fully correspond to your musical tastes.

Author: V.V. Puzanov, caravan@online.bryansk.ru, Bryansk; Publication: radioradar.net

See other articles Section Tube Power Amplifiers.

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