6P3P parallel 20W power amplifier experiment

In an audio amplifier, there are three ways to transmit the output signal of the previous amplifier to the next amplifier: resistor-capacitor coupling, transformer coupling, and direct coupling.

Since capacitors and (transformer) inductors are nonlinear components, the audio signals transmitted to the next stage will be distorted at both the low and high frequency ends, i.e., distortion will occur. This will reduce the undistorted audio frequency range, i.e., the frequency band will be narrowed. Direct coupling can transmit the audio signal to the next stage amplifier intact, so that all frequencies can be transmitted without distortion, thus the frequency band is very wide and the distortion is very small. The figure below shows a typical transistor and tube direct coupling amplifier.

BG1, BG2, and BG3 (all three tubes are green dots) are PNP semiconductor triodes. The audio signal is input to the base of BG1, output from the collector after amplification, and directly coupled to the base of BG2; BG2 is an emitter follower, and the signal after current amplification and impedance transformation at this stage is directly added to the base of BG2 from the emitter output: the 18V excitation voltage formed at the 1 and 3 ends of the autocoupled input transformer B1 after amplification by BG3 is directly added to the gates of the power electron tubes V1 and V2, and outputs 21.6W audio power through the output transformer B2.

The two electron tubes V1 and V2 are used in parallel and work in the Class A1 amplification state. The screen voltage is 350V, the screen grid voltage is 250V, the grid bias is -18V, the input audio voltage (effective value) is 18V, the output power of each tube is 10.8W, and the total output of the two tubes is 21.6W. In order to prevent self-excitation, a 47kΩ anti-vibration resistor is connected in series to the grid of each tube. The negative grid bias is directly taken out from the -18V voltage on the collector of BG3 through the B1 winding 2-3, and then added to the grid of the two power amplifier tubes through R10 and R12.

This circuit has three negative feedbacks: the first one is connected to R9 of the BG3 emitter circuit to take out the signal voltage and add it to the BG1 base through R2 and C2; the second one is taken out from the BG3 collector and added to the BG1 emitter through R15 and C6. The third audio signal is taken out from both ends of the speaker Y and added to the BG1 emitter through C15, C16 and R20. After the above three negative feedbacks, the circuit works more stably and has less distortion. The input signal is obtained from the output end of the MP3, etc., and the volume and tone adjustment are completed in the preamplifier.

Components production

1. The primary of power transformer B3 is wound with φ0.59mm wire for 660 turns, the secondary filament winding is wound with φ1.2mm wire for 990 turns, the secondary high voltage winding 1-2 is wound with φ0.31mm wire for 990 turns. 2-3-4 is wound with φ0.59mm wire for 81+81 turns, 4-5 is wound with φ0.31mm wire for 990 turns. Core: tongue width 40mm. stack thickness 75mm, E1 type silicon steel sheet, core plugged.

2. Input transformer B11~2 end is wound with φ0.55mm wire 274 turns. 2~3 end is wound with φ0.13mm wire 1920 turns. Core: Tongue width 19mm, stack thickness 28mm.

Insert the core in the same direction and place a 0.5mm thick paper sheet in the magnetic gap.

3. The primary of output transformer B2 is wound with 4000 turns of 0.23mm wire, and the secondary is wound with 266 turns of φ1.0mm wire.

Core: Tongue width 34mm, stack thickness 35mm. El type silicon steel sheet, core inserted in the same direction, 0.5mm thick paper pad at magnetic gap.