Using FU-50 high transconductance pentode to realize the design of AB1 push-pull amplifier circuit

In recent years, class AB1 amplifiers have become more and more popular, and almost all foreign amplifiers use class AB1. The primary advantage of AB1 type is high efficiency. For amplifiers, high efficiency is not only saving several watts and tens of watts of electric energy, but the key is to make the electron tube with a certain anode dissipation power to output more power.

The KT88 with an anode power consumption of 42W has a class A single-ended output power of only 12W, while the maximum output power used for class AB1 push-pull can reach 100W. Secondly, the AB1 class gives full play to the complementary advantages of the push-pull circuit, which not only improves the input and output dynamic range of the tube, but is also very beneficial to improve the nonlinear distortion. Each output tube of Class A push-pull amplifies the entire waveform of the signal. The anode current-grid voltage characteristic of the positive half cycle is close to the nonlinearity of the saturation region, which causes distortion of the input signal in the positive half cycle. When the anode current is close to the cut-off point in the negative half cycle, distortion will also occur. The distortion of the two tubes is bidirectional, and the distortion component is still retained after synthesis.

This is not the case with AB1 push-pull. First, the AB1 output stage electron tube anode power consumption can be fully utilized. The static operating point current is small, and the anode voltage can be increased without exceeding the anode power consumption. As a result, the linear region of the tube is extended to the direction of large current. Half-cycle signal input will not produce nonlinear distortion caused by characteristic bending. As a result of the downward shift of the static operating point, the negative grid voltage is more negative than that of Type A, which naturally increases the input nonlinear distortion in the negative half cycle of the signal.

The distortion of the AB1 class amplifier is mainly asymmetrical distortion components, but when the push-pull is used, the signal phase of the two tubes is opposite, and the negative half cycle of the upper tube is exactly the positive half cycle of the lower tube. Therefore, the output stage distortion is obtained in the AB1 push-pull Mutual compensation, this is that the nonlinear distortion of some models of tube AB1 push-pull and class A push-pull is almost the same, or even smaller (typically the European KT88).

FU-50 is a high transconductance pentode, used for AB1 to increase the output dynamic range, the anode power consumption of 40W can be fully utilized, even if the anode voltage is below 500V, it can output power of more than 30W. The parameters for making AB1 push-pull amplification are as follows.

Application data of Gr50 (same as FU-50) in AB1 class amplification:

Filament voltage/current 12.6v/700mA, anode power supply 450V, static anode current 2×60mA (two tubes), second grid voltage 250V, second grid current 2×10mA (two tubes), fixed grid negative voltage -30V , The best load impedance is 8000Ω, the gate drive signal voltage is 2×30Vp-p, and the output power is 34W.

The circuit is shown in Figure 1. The output stage adopts two Gr50s from RCA Company, B1 is 450V, B2 is 250V with regulated voltage, and a 50kΩ potentiometer is used to adjust the negative voltage of the fixed grid to 30V. The front stage of the circuit uses 12AU7 as a symmetrical drive amplifier, 12AU7 load division phase inversion and pre-amplification. It is not difficult to see that the previous amplifier circuit is of the same kind as the Williamson amplifier, but the 6SN7 is changed to 12AU7. The closed loop gain of the previous three-stage voltage amplifier is 20 times. The input signal voltage is 1.9Vrms in the Williamson amplifier and the output phase is Opposite two sets of 38Vrms drive signals. The machine uses a pentode Gr50, and the output stage only needs 2×30Vp-p, that is, 2×21Vrms, so the front-stage input signal voltage only needs 1Vrms. Since the output stage uses a pentode, the gain of this stage is greatly improved. In order to achieve a negative feedback of -20dB, RNF: select 6.8kΩ, and the amount of negative feedback can be adjusted by changing the value of RNF.

Using FU-50 high transconductance pentode to realize the design of AB1 push-pull amplifier circuit

After the output stage adopts the pentode, its internal resistance is higher, and the ripple rate of the anode power supply is increased to 2%, which is also allowed. Therefore, the use of 100Ω resistors and 220μF large capacitor filtering for 450V power supply is sufficient, eliminating the need for bulky and expensive choke filtering, which cannot be achieved by triodes and ultra-linear output stages.

The second grid of Gr50 uses a gas-filled voltage regulator tube to supply power to protect Gr50. Among them, vr150 can be substituted by domestic WY-4P and Soviet Cr-4C, and VR105 can be substituted by WY-2P and Soviet Cr-2C. If used Semiconductor voltage stabilizer tubes need to be connected in series to form a 250V voltage stabilizer circuit, and the maximum Zener current is not less than 45mA. The AB1 type fixed gate negative voltage has no load current and can be filtered by large resistors and capacitors. The filter resistance in this machine is 47kΩ and 50kΩ. The voltage divider doubles as a voltage divider, so that the negative voltage of about -45V at both ends of the potentiometer is adjusted to -30V, so that the static cathode current of the two Gr50 tubes (the sum of P+G2 current) is 140mA.



The output transformer of this machine adopts finished products of 8kΩ/8Ω, 16Ω/45W, its frequency response is 20Hz~100kHz±0.2dB, the primary anode is 8kΩ, and the maximum current of the push-pull tube is 140mA. When the quiescent current difference between the two tubes is less than 5mA, the total inductance of the primary is 120H and the maximum power is 45W. According to this requirement, self-made can be circumvented according to the following data:

The iron core is divided into two pieces of EI interlaced, no air gap inserting method using E28×40, equipped with a double-slot flame-retardant frame.

The primary windings P1～B～P2 are wound with φ0.2mn enameled wire with 2000+2000 turns, the secondary winding 0～8Ω is wound with φ0.8mm enameled wire with 120 turns, and 8φ～168Ω is used for winding ∞. The 64mrTl enameled wire is wound with 48 turns.

In order to meet the high-end frequency response of 100KHz, a multilayer clamping winding method divided into two sections can be used. The schematic diagram is shown in Figure 2. The three-level level is divided into two sections, a total of six sections. After each section has 20 turns, the partition is slotted to pass through the other section. The upper opening of the slot is open, and the wire passes through the gap to avoid shearing.

The primary is divided into two layers and two sections, and the forward and reverse windings are cross-connected. All secondary and primary positive windings except the primary are in the same direction to avoid frequent removal of the windings and reinstallation of the winding machine in the opposite direction. Attention should be paid to insulation at the leading end or connecting end of the winding, and at the same time try to avoid crossing the coil laterally. In order to make the tail end of each layer at both sides of the frame, pay attention to the lateral spacing when winding the wire. It is better to wind a few turns, and not to cross the coil and lead it out laterally. It should be noted that the thickness of 0.05mml polyester film and a piece of cable paper are used for insulation between the primary and secondary layers of each layer. After winding it, check it slightly (use a multimeter to check the winding structure), and not leave it in the air for a long time, heat it and dry it immediately, and impregnate the paint.

It is cumbersome to wind a high-quality output transformer, but as long as you are clear-headed and mark the beginning and the end, it is still extremely easy.

When the output power of this machine is 34W, the input signal voltage is 0.8Vrms, the frequency response of the whole machine is 16Hz~45kHz-1.5dB at the 8Ω end, and the THD is .7% at the rated output power.