The production of a fully direct-coupled gallstone hybrid power amplifier circuit

This all-direct-coupled tube-stone hybrid amplifier made by the author combines tubes and transistors together. The bipolar transistors only play the role of constant current and transition, without current gain, effectively avoiding odd harmonic distortion. Under 10V output, the frequency from 10Hz to 110kHz is almost a straight line.

1. Principle Introduction

The electron tubes VE1, VE2 and transistors V5, V6 form a common cathode and common base voltage amplifier stage. V3, V4 are constant current sources, but they are no longer the loads of VE1, VE2. Instead, they cooperate with V5, V6 to change the direction of the dynamic current caused by the input signal of VE1, VE2. The constant current source loads of V1, V2 are V8, V9. A 200kΩ resistor is connected to the ground of the collector of V9 to improve the open-loop frequency response. The AC signal is short-circuited with a capacitor between the collector of V8 and the negative power supply. V9 no longer contributes to the gain, but becomes a pure load and cooperates with V8 to stabilize the zero potential working point of the output stage and suppress the common mode signal. In this way, the current gain of the entire voltage amplifier stage is completed only by the electron tube, and V5, V6 also have no current gain. The bipolar tube in the channel has no possibility of generating odd harmonics, and the timbre of the electron tube is maintained.

Since the output internal resistance of the voltage amplifier stage is very high, VE3 and VE4 use electron tubes in parallel to form a buffer and final stage driver stage, which drives the two pairs of field effect tubes in the final stage (only one pair is shown in the figure) with very low internal resistance and large current. In order to make this amplifier circuit have perfect "courage", the output stage uses power field effect tubes with a sound close to that of electron tubes, and even the static current bias tube between the two gates of the final stage tube uses a low-power VMOS tube.

2. Component selection and debugging
V5, V6, V7 should use medium power tubes with high frequency, low current and withstand voltage greater than 180V. 4 red light-emitting diodes are used as voltage regulators, and their tube voltage drop is about 1.9V~2V. Two pairs of K1529 and J200 are used for each channel of the final stage. This circuit uses two transformers, one of which is used exclusively for the filament and delay circuit.

Production and debugging: After checking that everything is correct, do not install the final power tube first. Use a 10kΩ resistor at the output end as a dummy load. After about 20s to 30s after power-on, the relay will pull in and the main circuit will be powered on (if the delay time is not appropriate, the capacity of the 4700μF capacitor can be changed). Adjust the lower bias 2kΩ resistor of the final static current bias tube to the maximum value. Adjust the variable resistor between the two emitters of V8 and V9 so that the cathode potential of the electron tubes VE3 and VE4 can float up and down, and finally set it at about 0V. At this time, check that the static current of each level except the final stage should be roughly consistent with the value marked in the figure, otherwise the emitter resistance value of the corresponding constant current tube should be adjusted. After confirming that there are no problems, turn off the power and install the final power tube (avoid static electricity when welding), change the dummy load at the output end from 10kΩ to a power resistor of 10Ω~20Ω. After powering on, first adjust the adjustable resistor between V8 and V9 to make the output end at zero potential, and then adjust the variable resistor between the gate and source of the static bias tube to slowly generate current in the subsequent stage. Note that it should be small at the beginning, and then adjust it to about 200mA after it stabilizes.

Note that the static current of the VMOS tube used in the final stage cannot be less than 150mA. Don't forget to readjust it after a few days of burn-in.