The effect of many of my friends' actual achievements is not ideal. There are two main reasons:
1. Since the knowledge of the electronic tube circuit and its application is the fifth century. Only in the sixties textbooks, there was basically no teaching of electronic tubes. So younger enthusiasts don’t know much about tube knowledge.
2. Many of my friends who make hands-on amplifiers are now copying them according to some reference circuits. Many of the technical parameters in the reference circuit are not clear to the mind. Does not necessarily achieve the desired effect. Based on my own little knowledge and experience, I discussed with you some issues in the design and production of the amplifier. If you do not want people to criticize and correct me. This article mainly discusses some problems in single-ended small power amplifiers. Because Class A single-ended amplifiers are one of the best circuit forms of timbre, they are also one of the circuit types that enthusiasts make more.
First, on the issue of output power 1, Class A single-ended tube amplifier This form generally uses a single power tube to amplify, by the power amplifier to limit its own maximum power dissipation, the output power is generally not very large, common circuit The output power is generally between 1W-15W. Table 1 shows the output power and some common parameters of Class A single-ended power amplifier circuits formed by some common power amplifiers.
The output power value in Table 1 has a great relationship with the working voltage of the panel and the load impedance (primary impedance of the output transformer). Any change of data will cause the output power value to change. Appropriate use of the occasion and the sensitivity of the speaker used, the higher the sensitivity the greater the use of the area.
Tube Type Filament Voltage Filament Current Maximum Screen Dissipation Power Pin Form Power Transformer Power Output Power Applicable Occasions KT88,65506.3V/1.6A40W8 Foot Stand 150W15W 30 Square Meters Above Room EL34,6CA76.3V/1.5A25W8 Foot Stand 120W11W15-30 square meters of room 6L6G,6P3P6.3V/0.9A19W8 feet tube seat 100W8.5W15-30 square meters of room 807,FU-76.3V/0.9A25W5 feet tube seat 100W10W15-30 square meters of room 6P14,EL846.3V/0.76 A12W small 9 foot tube 80W5.4W15 square meters below the room 6P156.3V/0.76A12W small 9 foot tube 80W5W15 square meters below the room 6V6, 6P6P6.3V/0.45A12W8 foot tube seat 70W3.8W15 square meters below the room 6P16.3V /0.5A12W Small 9-pin socket 70W5W15 square meters The following room 2, the output power calculation method There are many different versions, the calculation results of each version are basically the same, but the calculation of the required parameters are different. A relatively simple calculation formula is provided for your reference: I2×R/2. Where I2 is the square of the quiescent current and R is the primary impedance of the output transformer, also called the load impedance. After a lot of practice the result of this formula is more accurate and practical.
Second, on the working voltage of the plate in the electron tube manual we can find the typical application parameters of the power amplifier tube, generally have a plate working voltage this parameter, for example, 6P1 electron tube screen voltage recommended on the manual is 250V, there are many Production drawings and audiophiles actually select the AC output voltage of the power transformer according to this parameter in actual production. In fact, this is not good, and the performance of the power amplifier tube cannot be well played because output transformers are cascaded in the screen-level loop. . The primary coil of the output transformer is DC resistance. When the quiescent current flows through the primary coil, it will produce a voltage drop. At this time, the DC working voltage applied to the screen of the electron tube will decrease. Other parameters also change with the screen voltage. Corresponding changes, I use the following Figure 1 and Table 2 to explain.
From Figure 1, we can see that after the output voltage transformer is connected to the plate working voltage provided in the manual, the working voltage actually supplied to the plate electrode is 22V lower than the typical operating voltage in the manual, and the entire power tube is decreased by 22V. What are the changes to other parameters? See the comparison in Table 2.
Table 2 Parameters in the manual with parameters for output transformer Screen voltage V250 Screen voltage V228
Curtain grid pressure V250 curtain grid voltage V228
Negative gate voltage V12.5 Negative gate voltage V11.4
Screen quiescent current mA44 Screen quiescent current mA38
Maximum plate current mA88 Maximum plate current mA76
Curtain current mA7 Curtain current mA6
Screen resistance KΩ42.5 Screen resistance KΩ44.5
Transconductance mA/V4.9 Transconductance mA/V 4.67
Output power W5 output power W3.97
From Table 2, we can see the changes in its data, because now the frequency response of the output transformer is higher than the earlier requirements, the number of turns of the primary coil is also more, and the DC impedance is larger. Therefore, we must consider the DC voltage drop generated by the primary coil of the output transformer when we use the panel-power supply voltage. This can meet the application parameters provided in the manual.
Third, on the issue of the output transformer Output transformer is the soul of the amplifier, if there is no output transformer there will be no so-called bile taste exists, in the case of all components and parameters unchanged, replace the output transformer of different manufacturers, its The sound of replay is not the same. The design of the output transformer in the self-made tube amplifier determines the result of the final playback sound. There are many versions of the output transformer design. Here are two examples of calculations for comparison (see Table 3 and Figure 2). We will use the calculation of the inductance (L) as an example.
Second Calculation Method Formula Remarks Second Calculation Method Formula Remarks Primary coil inductance L unit (H) RL is the best load impedance of the tube (Ω) fL is the minimum reproduction frequency (Hz) primary coil inductance L unit (H )
Core minimum cross-sectional area S units (cm2) Ip is the plate static DC current (mA), L is the primary inductance (H) Core minimum cross-sectional area S units (cm2) P is the transformer rated power primary inductance coil turns Nplave is the average magnetic line length of the core, S is the cross-sectional area of ​​the core, L is the inductance, the number of turns of the primary inductor is Npu is the permeability coefficient of the core permeability (k) Ro is the secondary load impedance (Ω), and n is the transformer efficiency. 75% at low power, RL is the primary load impedance (Q) The relevant correction factor K is greater than 1
The number of secondary windings NsNs=K×NpK is the transformation coefficient, Np is the number of primary inductance coils The number of secondary coils NsR is the secondary impedance, n is the transformer efficiency Air gap length G Units (mm) Ip is the electron tube plate Static DC Current (mA), Np Primary Inductor Winding Number Air Gap Length G Unit (mm) G = NpLo/600Io Primary quiescent current (mA)
Initial and secondary coil diameter units (mm) calculated according to the current density of 3A/mm2. Initial and secondary coil diameter units (mm) Calculation of the current value based on the current density of 2.5A/mm2 A calculation method is: L is the inductance (unit H), RL is the best load impedance of the tube (in Ω), and fL is the minimum reproduction frequency (in Hz). Another calculation method is: where RL is the optimal load impedance of the tube (unit Ω) fL is the lowest reproduction frequency (in Hz). 3.14 is the constant at the lowest frequency response of -1dB. The constant 0.159 in the first calculation method is based on the data when the lowest low frequency response is -3dB, so the calculation formula should be selected according to the need of the lowest frequency response. From the above two calculation formulas, we can see that the final results of different versions of the formula are not the same.
The output transformer design information that we can now see in various publications is mostly many years old, and some are not complete. Each manufacturer's data on the output transformer is confidential. This gives some friends who want to do it yourself. There is a lot of trouble, so the success rate of self-made single-ended output transformers in amateur conditions is not high. Which can produce high-quality output transformer under amateur conditions? The answer is yes, I have provided some of my own output transformer experience for reference, there is no detailed calculation formula. However, the output transformers that have been made in this way have performed very well.
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