Single-ended Class A amplifier using gallstone composite tube design

The circuit is shown below.

This circuit has only two stages of amplification. The front stage is a 6N3 common cathode amplifier. 6N3 is a medium μ (35) amplifier tube with a small gate negative voltage. The cathode is connected to a 300Ω gate bias resistor. The working point is appropriate, and the bypass electrolytic capacitor is cancelled to form a negative feedback of the current at this stage. The feedback amount is not too large or too small. The final output is a composite of a 6N6 tube and a high-power transistor. The 110Q resistor is the emitter resistor of this stage feedback, which makes the working point of this stage very stable. Because it is a current negative feedback, the driving ability is strong, it has a constant current characteristic, and the nonlinear distortion is low. The voltage drop of the composite output of the tube is less than that of the tube triode at the maximum current, and it has the charm of the tube triode but the output efficiency is higher than that of the tube triode. The circuit is simple, the two channels share the same power supply, and the front stage does not need a decoupling circuit, so there is no worry about stray coupling. If the filter electrolytic capacitor is replaced with a non-electrolytic high-quality capacitor, this machine is an amplifier without electrolytic capacitors. 'In addition, since the output internal resistance of the composite tube is low, the requirements for the output transformer are greatly relaxed, and an ordinary output transformer can achieve excellent results.

When the collector current of the final stage tube is 0.115A, the power consumption is 25 watts and the output power is 10.3 watts. If the cathode resistance is reduced, the working current becomes larger and the output power can reach 15 to 20 watts. At this time, the output transformer needs to be redesigned. The heat sink of the Class A amplifier must be large enough to facilitate heat dissipation.

The reverse withstand voltage of the high-power triode should be greater than 500V, and the β value should not be too low. , The 6N3 and 6N6 can be used, and the large eight-pin hairpin 6N8P can be used instead. The power of the power transformer Tl is 160 watts; the power of the output transformer T2 is 10 watts. , S=9.6cm2, the primary wire diameter φ=0.27mm, wound 2100 turns, the secondary wire diameter φ=0.8mm, wound 140 turns.

The primary-to-secondary impedance ratio is 1.8kΩ:8Ω. The filter inductor L has a S=8.2cm2 wire diameter φ=0.35mm, and is wound until full. T2, the inductor L core has a gap of 0.22mm to prevent magnetic saturation.