6AS7 tube amplifier

The author found many tubes, among which there is 6AS7, which is a double triode tube, often used as a voltage regulator in the electronic tube power supply. It has a large output current and a very low internal resistance (470Ω), and it is "transplanted" into the low-frequency amplifier as a power amplifier tube, and the effect is very good.

The circuit is shown in the figure (only one channel is drawn), and each channel uses three tubes. G1 uses high μ dual triode 6N2. Half is used for pre-voltage amplification, and the gain of this stage can reach 32dB. The other half is used for screen and cathode split phase inversion. This circuit has good balance, low distortion, and no voltage gain. G2 uses medium μ dual triode 6N1 as a driver, and its gain is about 24dB. The final power amplifier G3 (6AS7) is connected in series push-pull circuit, and the voltage gain is -1dB, but the power gain is 22dB. The open-loop gain of the whole circuit is about 40dB.

In order to improve the amplitude-frequency characteristics and simplify the circuit, the following measures are taken. 1. Direct coupling is used between the preamplifier and the inverter, and between the excitation stage and the final stage. 2. The secondary winding (110V) of the power transformer is rectified by D5-D8, and directly reaches the screen and cathode of 6AS7 after passing through C8, C9, and π-type filtering, and is superimposed on the power supply of about 300V (230V winding). In this way, the negative gate voltage of 6AS7 can be directly taken from the screen of 6N1, thereby simplifying the bias circuit of 6AS7. 3. The power supply of the output stage adopts a floating power supply.

The resistance values ​​of R5 and R7, R8 and R9, R12 and R13 in the circuit should be as consistent as possible. The electrolytic capacitor is the Peace brand CD15 type (axial single welding piece), and the shells of C8 and C9 must be insulated before installation. Only two resistors need to be debugged. Adjust R10 (when static) to keep the two-screen current of 6AS7 between 20mA and 30mA, then input a 460mv-1kHz sine signal at the IN terminal, and adjust R15 so that there is an undistorted voltage of about 9.3V at the speaker (can be connected to a dummy load). The static voltage value is indicated in the figure. If the deviation is not large, no adjustment is required. The table shows the production data of B1, B2, and B3 for reference.

Through adjustment and listening, I feel that the tone is more beautiful than that of the tetrode (6P1) and pentode (6P14).

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┃Power transformer>100W ┃Output transformer┃Low frequency choke┃

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┃35mm (tongue width) × 35mm ┃22mm (tongue width) × 30mm ┃28mm (tongue width) x 35mm ┃

┃(Stacking thickness) 0.35mm thick height ┃(Stacking thickness), 0.35mm thick height ┃(Stacking thickness), 0.35mm thick height ┃

┃Silicon wafer cross-insertion┃Silicon wafer, cross-insertion┃Silicon wafer, forward-insertion┃

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┃Primary. Φ0.47mm enameled ┃All four primary windings are used ┃ ┃

┃Wire, wound 880T, after winding┃Φ0.25mm wound 275T, its┃ 0.2mm air gap┃

┃An electrostatic isolation layer should be added┃The middle and lower windings are reversely wound┃ ┃

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┃Filament (2A): Φ1.16mm ┃ ┃ ┃

┃26T winding, filament (5A): ┃Secondary winding with Φ0.8mm ┃Φ0.44mm winding 1400T ┃

┃ ┃100T. ┃ ┃

┃Φ1.2mm double wire parallel winding 26T ┃ ┃ ┃

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┃Secondary (110V) Φ0.45mm ┃ ┃ ┃

┃440T winding. Secondary (230v): ┃ 1kΩ: 8Ω ┃ Resistance about 27Ω ┃

┃Φ0.15mm winding 920T ┃ ┃ ┃

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