Using 6P15 to create an ultra-linear push-pull amplifier

The video tube 6P15 was a special tube for television in the 1970s, used for video voltage amplification. Later, integrated circuit televisions became popular, and the 6P15 tube also withdrew from the field of televisions, and audiophiles rarely paid attention to it. The author believes that the electronic tube televisions at that time were high-tech electronic products. It can be imagined that the manufacturing process and quality of tubes such as 6P15 (or 6P12, 6P13) are not inferior to general power tubes. It is very close to EL84 and 6P14 tubes, and they are both pentodes. The author decided to use uncommon tubes such as 6P15 to make push-pull power amplifiers, which should be able to achieve better results. The production steps are as follows:

1. Design according to the basic electrical performance parameters of the tube

Since the anode voltage of 6P15 is 300V to 330V, the current is 30mA, and the transconductance is 14.7mA/v, the output power is close to that of 6P14 and EL84. If this tube is used to make a single-ended, the output power is small, and it is difficult to drive speakers with more than two frequencies, which is obviously not suitable. Therefore, the author designed it according to the ultra-linear push-pull circuit, which can obtain an output power of 14w, driving a two-way speaker or a three-way speaker with more than 90DB, which can meet the listening needs in a living room or bedroom of about 15 square meters. The author uses a simple three-stage amplification, as shown in Figure 1. The first stage voltage amplification uses direct coupling to obtain a wider channel frequency, and the second stage is reverse phase and driving.

The author has also adopted some technical measures to make 6P14, EL84 and 6P15 universally interchangeable, so that future maintenance and tube replacement are not restricted by the tubes.

2. Device Selection

The voltage amplification and inversion parts use ordinary 6N1, 6N3, 6N4 series dual triodes (the author uses 6N1). Use 1/2 of the 6N1 dual triode for voltage amplification, and the other 1/2 for the screen-shaft split inversion of the driver stage to drive the power tube 6P15 at the end. The resistors can be selected according to the power requirements shown in the figure (if not specified, select 1/2w). The coupling capacitors C1 and C2 use oil-immersed types above 0.2uF/400v, or metallized paper dielectric capacitors. The power decoupling capacitor uses oil-immersed capacitors or metallized paper dielectric capacitors, which is conducive to beautiful sound.

3. Selection of push-pull tube gate voltage

There are generally two ways of negative gate voltage, the first is self-sufficient gate bias, and the second is fixed gate bias. These two methods have their own advantages. Self-sufficient bias almost does not need to be adjusted, and there is no need to increase the winding of the transformer and the gate negative voltage regulating power supply, but the troublesome things are transferred to the next step, making the selection or pairing more difficult. The difference of fixed gate bias is that it requires the transformer to wind an extra set of 70v-100v windings as a gate negative voltage regulating power supply for the bias voltage of the push-pull tube. Under the same screen voltage, the self-sufficient bias is equal to the screen voltage minus the gate voltage, while the fixed bias is a bit like the positive and negative power supply of the OCL circuit, that is, the screen voltage plus the gate negative voltage. Obviously, the fixed bias can increase the supply voltage and increase the output power. When selecting the tube, there is no need to pair, and the balance of the two tubes can be adjusted easily and freely. Especially for amplifiers with low power, if you want to increase the output power, it is more reasonable to use a fixed gate negative voltage, which is also the reason why the author chose it.

4. Power Requirements and Processing

The power supply seems simple, but it is not a simple link. It will directly affect the performance of the whole machine. The author uses the total screen consumption current of 4 6P15 tubes 120mA to determine the current of the secondary high-voltage winding of the power supply.

Small, and required to be greater than 15% to meet the requirements of this circuit.

Filament power supply treatment. In order to eliminate AC noise, the middle tap of the 6P15 filament power supply is grounded. The 6N1 filament power supply is not directly grounded. The potential is raised by connecting two resistors (R17: 3D0kΩ, R18=68kΩ) across the high-voltage power supply and the ground wire, dividing the voltage to obtain a voltage of about 70v, and then connecting it to one end F1 of the 6N1 filament to eliminate the AC noise. Another point that is easily overlooked is that the AC-powered filament power line is prone to generate magnetic field radiation and AC noise. In this regard, all AC-powered wires must be routed with a twister, and the direction of the twister should be opposite to the winding direction of the transformer. This is an indispensable step to reduce AC noise.

5. Power Transformer and Output Transformer

The power transformer power is 120w. The output transformer core section

The area is 22mm×40mm. The primary is wound with Φ0.14mm enameled wire for 900+900+900+900 turns. The secondary is wound with Φ0.83mm enameled wire for 120 turns (4Ω) and 160 turns (8Ω). The silicon steel sheets are cross-inserted without leaving any air gap.

6. Machine debugging