Tube amplifier made with FU-29

Tube amplifier made with FU-29

FU-29 is a double-beam quadrupole power electron tube with a high withstand voltage, the limit value is 750V, and it has large plate power consumption and plate current, good linearity, low saturation voltage drop and high transconductance.

The difficulty in making FU-29 lies mainly in the high working voltage and the production of the output transformer with special impedance. Of course, 600V is nothing for amateur masters who have made 211, 845, 805 and other tubes, but it is a challenge for inexperienced beginners. The output transformer with an impedance of 13.75 kΩ is almost impossible to find on the market. The solution is to find a business that can provide customized services, or to wind it yourself if you have the ability to do it yourself, but due to its high working voltage, the insulation problem is worth noting. The output transformer parameters of this machine. A high-quality output transformer is the basic guarantee for the success of making a tube power amplifier.

This is a fairly "classical" circuit form, which highlights the overall balanced characteristics. Transformer B1 input and phase inversion, sharp cutoff pentode 6J4P for voltage amplification, transmitting tube FU-29 for power amplification, transformer B2 for output impedance conversion.

Among various forms of phase inversion, transformer phase inversion has excellent performance and simple circuit, small waveform distortion and good symmetry. In the early years, tube amplifiers were widely used. Nowadays, due to factors such as frequency response, sound coloration and price, this form of phase inversion is rarely used, but it is still one of our favorite circuit forms. In fact, within the audio range, it is not a problem to design a transformer with an ideal bandwidth. Moreover, due to differences in design, materials and manufacturing processes, different input transformers have different sound performances, which provides a method for calibration. We use a pair of second-hand disassembled input transformers with an impedance ratio of 1:1+1. Of course, readers with hands-on ability can also wind them by themselves.

The voltage amplifier tube 6J4P is a sharp-cutoff pentode commonly used in medium-frequency voltage amplification. It is rarely used in the audio field, but due to its high transconductance and large screen current and screen loss, and its low price, it performs quite well here. It is also one of the electron tubes we like to use. Its parameters are shown in Table 2. R1 and R2 are the grid resistors of 6J4P, which are also the load resistors of the input transformer and determine the input impedance. R3 and R4 are the cathode resistors of 6J4P. The voltage drop of the cathode current on R3 and R4 is used to provide a negative bias voltage for the grid. Since there is no parallel capacitor on the cathode resistor, the two resistors R3 and R4 also play the role of current negative feedback in this stage. The screen grid voltage is obtained by R9 stepping down and C5 decoupling. In order to obtain a larger screen current, the screen grid voltage can be higher. Since this stage has a strong current negative feedback, and the resistance of the screen resistors R7 and R8 is not large, although the pentode amplification factor is large, the gain of this machine is not too high, about 38dB. For power tubes like FU-29, the required driving voltage is not high, so the gain of this stage is not only sufficient but also has margin.

C1 and C2 are coupling capacitors between the voltage amplifier stage and the power amplifier stage. This capacitor has a great influence on the tone. If conditions permit, high-quality oil-immersed capacitors from home and abroad should be used as much as possible. Since FU-29 is a high-frequency power tube, its stability should be paid special attention to when used in the audio band. Here, R16 and R17 are set on the grid, and R19, R20 and other vibration-eliminating resistors are set on the screen. Negative voltage is added to the two grids of FU-29 through W1, R12, and R13 to ensure that FU-29 is at a normal working point. R10 and R11 are two negative feedback resistors, which together with R3 and R4 form a loop negative feedback, which can stabilize the circuit and reduce distortion. C3 and C4 play the role of high-voltage isolation here to avoid the influence of the screen high voltage on the working point of the voltage amplifier stage.

6P1, R21, C8 and two ion voltage regulators WY2P form a simple series voltage regulator circuit for the curtain grid of FU-29. Under normal circumstances, the power pentode and beam power tube have a relatively clear sound quality when the curtain grid voltage is working. Since the curtain grid voltage of FU-29 is far away from the screen working voltage, the curtain grid voltage regulator is generally used, and high-power crystal voltage regulators are mostly used, and the sound performance is refreshing and open. Perhaps the "tube amplifier stone sound" of FU-29 is also named for this! Here we use a voltage regulator circuit composed of ion voltage regulators and electron tubes to supply voltage to the curtain grid, with the purpose of adding charm to this refreshing feeling. Under the premise of maintaining the control and speed of FU-29, the thickness and density of the intermediate frequency are increased to achieve tube amplifier, tube sound, and tube charm. The voltage amplification factor of this power amplifier stage is about 60 times, which is 35.5dB. The total gain of the two stages is about 73.5dB. After adding the loop negative feedback, the gain is 47.5dB, with a total negative feedback of 26dB. In this way, the input transformer gain is 6dB, the two-stage amplification plus negative feedback has a total of 47.5dB, and the output transformer also has a -32.5dB gain. The actual gain of the whole machine is 21dB.

The power supply is also shown in the figure. The 600V high voltage of the screen is directly obtained from the 220V AC mains after being rectified by D1 and D2 and filtered by C12, C13, C14, C15 and C16. The 300V voltage obtained after the half-wave rectification and filtering composed of D2 and C13 is used for the voltage amplifier stage and the screen grid of FU-29. R22 and R23 are the discharge resistors of the power supply, which provide a way for the filter capacitor to release energy. After shutting down, these two resistors will release the remaining electric energy in the capacitor. Otherwise, after the cathode of the electron tube cools and stops working, the capacitor will still have a high voltage and maintain it for a long time, which is not good for safety and the life of the electron tube.

This power supply method saves a high-voltage and high-power transformer, which not only reduces costs but also obtains a very low internal resistance of the power supply. The power supply energy without transformer isolation is undoubtedly abundant and responds more quickly. In static and large dynamic conditions, the high voltage fluctuation is very small, and the excellent performance of the machine in the medium and low frequency bands is due to this! But without the isolation of the power transformer, safety becomes a problem again. It is unimaginable to have an amplifier with electricity throughout the body. Here, the input transformer B1 and the output transformer B2 complete the isolation function, so that the casing does not carry AC mains power. It is worth noting that the grounding of the casing and the "grounding" of the internal circuit in the figure are separate, and must not be connected together. Since B1 and B2 have to complete the dual functions of transmitting signals and isolation here, the quality of these two transformers cannot be taken lightly.

This power supply method has rarely been used in the past, mainly for safety reasons. In fact, as long as two transformers are used, the isolation effect is exactly the same as using a power transformer. This tube amplifier has been used for many years and is very safe. This machine has not gone that far in filament power supply, and still has an ordinary power transformer B3.

A power transformer B3 is used to provide 6.3V for three groups of filaments and 36V for one group of grid negative voltage. Since the required power is not large, a 60W small power transformer can be used. The 100Ω potentiometer W3 connected to the voltage amplifier stage 6J4P filament is used to adjust the AC interference of the filament to the minimum. It is OK to ground one end of the filament of FU-29. The filament can also use 12.6V by changing the pin position of FU-29. But please note that the filament winding connected to 6P1 cannot be grounded. Here, the cathode potential of 6P1 has reached 220V. If the filament is grounded, the voltage between the cathode and the filament will far exceed the 100V limit value of 6P1, and it can only be powered in a floating state. The AC 36V is rectified by the silicon bridge, filtered by C9, R21, and c10, and stabilized by the voltage regulator D3, and then adjusted by W2 to provide a fixed negative voltage to the grid of FU-29.

The withstand voltage of FU-29 is very high, up to 150V. In order to improve efficiency and increase output power, the electron tube must work in a high-efficiency state of high screen voltage and low screen current. Here, the screen voltage of FU-29 is Ua=600V, and the maximum screen current IAMAX=60mA. From the Ua-Ia curve of FU-29 (Figure 2), when Ug=0V, Ia=160mA, the voltage drop between the screen and the cathode is about 40V. In this way, the output power of this machine can be calculated as P=(600-40)×0.16/2=44.8(W)

Calculated based on the 90% efficiency of the output transformer, the actual output power is 40W. This is already in the golden power range for tube power amplifiers, and the actual driving force is not inferior to that of a 100W transistor amplifier.