300B Vacuum Tube Hi-Fi Amplifier

The direct-heated 300B electron tube has a history of more than 70 years since its introduction. Due to its reasonable internal design, moderate tube consumption, low internal resistance and excellent linearity, the single-ended Class A power amplifier made with it has a pure, delicate, clear and transparent tone. It is known as the highest level of Hi-Fi sound quality and is deeply loved by audiophiles at home and abroad.

After analysis and comparison, the author finally decided to use a triode voltage amplifier as the first stage, then use a power tube as the driving stage, and finally use 300B as the power amplifier. The circuit is shown in the figure.

Pre-voltage amplifier stage.

The pre-voltage amplifier stage V1 adopts the domestic medium-amplification low-noise dual triode 6N10, which is famous for its rich musical flavor and has been used by many famous machines in the world.

Using dual triodes in parallel in the circuit can reduce their internal resistance, which is beneficial for matching with the input signal. The transconductance is doubled that of a single triode, ensuring that the previous stage has sufficient voltage amplification factor.

Intermediate drive stage

Direct-heated triode power amplifier tubes are generally less efficient. When 300B is at full power output, the gate variation generally requires 60-70Vpp, so the characteristics of the driver tube are more important. The commonly used 6N8p and 12AU7 cannot achieve satisfactory driving. For this reason, this stage uses power amplifier tube 6P6P, which has low noise and low frequency band, and the effect is better after using triode connection.

In fact, Class A amplification does not require driving power. The main purpose here is to solve the voltage amplitude problem of the driving stage. This stage is connected as a common cathode amplifier circuit, with a negative gate bias voltage of more than 20V, a wide input range, a voltage amplification factor of about 6, and a maximum undistorted output voltage peak of more than 200V. It can not only drive 300B with an input amplitude requirement of about 70V, but can even drive right-characteristic triodes such as 211A and 845 to full power output.

Power amplifier stage

This stage adopts a self-supplied bias circuit, which can suppress the vicious growth of screen current to a certain extent and ensure the safety of the expensive 300B.

A 300mA ammeter is connected in series with the cathode resistor, just like two eyes monitoring the working condition of the power amplifier tube 300B at any time.

The cathode resistor uses a 900Ω, 40W high-power resistor, and the cathode capacitor uses an 85uF/230V oil-immersed capacitor. Compared with the electrolytic capacitor, the sound quality has a new leap.

power supply

1. High-voltage circuit

The power transformer has two 360V windings on the secondary side. Each arm uses two 1000V 1N5408 silicon rectifier diodes in series for full-wave rectification. A 0.033uF/400V organic film capacitor is connected in parallel to each diode to prevent electrical interference from grid noise and ensure the safety of the rectifier diode. The rectified pulsating DC high voltage is filtered by two 2000uF/250V large electrolytic capacitors in series, and then sent to two 5Z4P electron tubes for secondary rectification. The filament heating is synchronized with the power amplifier tube filament heating to avoid sudden impact of high voltage on the tube. A 100Ω/15W current limiting resistor is connected to the cathode of the rectifier tube to prevent the impact of the filter capacitor surge current on the rectifier diode.

2. Filament power supply

The filaments of the pre-voltage amplifier stage and the driver stage are powered by AC, and a DC voltage of about 40V is connected to the center tap, so that the filament winding is positive to the cathode. The cathode cannot absorb the electrons emitted by the filament, so there is no AC voltage drop on the cathode resistor iRk, and the AC voltage drops on (Rg) can also offset each other, thereby eliminating the AC noise caused by the filament power supply circuit.

The filament is powered by AC, the tone is vivid, the low frequency is strong and more flexible. However, DC power supply produces some stone, which makes the sound quality hard. The cathode (filament) of the direct-heated 300B power tube is an equipotential body. When heated by AC, it is easy to produce subtle AC sound, which makes people feel very uncomfortable. For this reason, the power amplifier tube is still powered by DC. The rectifier circuit uses a 15A bridge pile, which is 10 times the filament current. The filter capacitor is 27000uF/20V Moroli electrolytic. Resistors are connected at both ends of the filament to form a balanced bridge circuit to balance the current value through the distributed capacitor coupling through the Rg loop.

Component Selection

1. Transformer

The power transformer and output transformer were ordered from Sichuan Guanghan Kaili Electric Factory. The cross section of the bull core is 35mmx67mm, the secondary high voltage winding is a set of double 320V, 360V, 400V/0.4A, a set of 40V/0.2A, the filament winding is two sets of 5V/3A, and a set of 3.15V-3.15V/3A, and the weight is 6.5kg.

The output transformer model is KSC50A, using Japanese imported H10 high silicon stamping, high purity oxygen-free copper wire, layered and segmented winding according to high fidelity design, primary impedance 3.5kΩ, secondary taps 4Ω, 8Ω.

Choke coils are selected from ZL-10-40 type from Shangwu Erqi Factory, 10H100mA, two are used in parallel.

2. Capacitor

The coupling capacitor pre-stage uses MKP capacitors, the driving stage uses imported blue SIEMENA MKP capacitors, and each is connected in parallel with a domestic 0.05uE/400V MKP capacitor.

The front-stage cathode capacitor uses a manon brand 470uF/50V electrolytic capacitor, and is connected in parallel with a ROE 0.1uF/100V organic film capacitor.

3. Resistance

The resistor does not have to be restricted to a certain brand. The main considerations are the resistance to voltage, noise, power consumption, and stability. As long as the quality is good, it will be fine. For resistors below 2W, use domestic Dahongpao resistors, and for resistors above 2W, use RX tube-type oxide film wirewound resistors. For 300B cathode resistors, use RXYD type moisture-resistant glazed wirewound resistors. The required resistance value can be obtained by series and parallel connection, and the power can reach 40W.

4. Other components

Switches, connectors, terminals, etc. should be of high quality as much as possible. The power switch should be the old KN3 double-throw double-knife toggle switch. The audio source switch should be the KCT medium-sized insert-type rolling step-structure porcelain band switch, which has strong insulation performance and large contact area. For connecting cable plugs and sockets, use CB2 rectangular plugs and sockets, which have low contact resistance and long life. The mA meter should also be the professional-grade 91C4 type. Although the wire material is not high-end such as Monster, Teflon, and silver-plated wire, it must be guaranteed to have sufficient capacity and strength. In short, components must be high-quality products from regular manufacturers.