Amplifier made with FU-29
FU-29 is a dual beam quadrupole power tube, with high withstand voltage, the limit value reaches 750V, and has large screen power consumption and screen current, good linearity, low saturation voltage drop and High transconductance.
The difficulty of making FU-29 mainly lies in the output transformer with higher working voltage and special impedance. Of course, 600V is nothing for amateur amateurs who have done 211, 845, 805, etc., but it is a challenge for inexperienced junior enthusiasts. The output transformer with an impedance of 13.75 kΩ can hardly find a finished product on the market. The first solution is to find a merchant who can provide customized services, and the second is to do it by yourself. However, 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 tube power amplifiers.
This is a fairly "classical" circuit form, which highlights the overall balance. Transformer B1 input and reverse phase, sharp cutoff pentode 6J4P for voltage amplification, launch tube FU-29 for power amplification, and transformer B2 for output impedance conversion.
Among various forms of phase inversion, the transformer has excellent phase inversion performance and simple circuit, small waveform distortion, and good symmetry. In the early years, tube amplifiers were used a lot, and nowadays they are restricted by factors such as frequency response, noise, and price. This type of inversion is rarely used, but it is still one of the circuit forms we prefer. In fact, in the audio range, designing a transformer with an ideal bandwidth is not a problem at all, and due to different designs, materials, and manufacturing processes, different input transformers have different sound performances, which just provides more sound correction. a way. We use a pair of second-hand disassembled input transformers with an impedance ratio of 1: 1 + 1. Of course, readers with hands-on capabilities can also wind them by themselves.
The voltage amplifier 6J4P is a sharp-cut pentode commonly used in mid-frequency voltage amplification. It is rarely used in the audio field, but due to its high transconductance and large screen flow screen consumption, and its low price, it is also used here. Very good, this is also one of our favorite tubes, and its parameters are shown in Table 2. R1 and R2 are the gate resistance of 6J4P, which is also the load resistance of the input transformer and determines the input impedance. R3 and R4 are the cathode resistance of 6J4P. The voltage drop of the cathode current on R3 and R4 is used to provide a negative bias voltage for the gate. Since there is no parallel capacitance on the cathode resistance, the two resistors R3 and R4 are also in this stage. Play the role of current negative feedback. Curtain grid pressure is obtained by decoupling R9 and C5. In order to obtain a larger screen flow, the grid pressure can be made higher. Because this stage has strong current negative feedback, and the resistance of the screen resistors R7 and R8 is not large, so although the pentode magnification is very 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 the coupling capacitors between the voltage amplifying stage and the power amplifying stage. This capacitor has a great influence on the sound. If possible, high-quality oil-immersed capacitors at home and abroad should be used, as FU-29 is a high-frequency power In particular, when using it in the audio section, pay attention to its stability. Here, the grid is provided with R16, R17, and the screen is provided with R19, R20 and other anti-vibration resistors. Negative pressure is applied to the two grids of FU-29 through W1, R12, and R13 to ensure that FU-29 is at the normal operating point. R10 and R11 are two negative feedback resistors. Together with R3 and R4, they 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 high voltage of the screen on the working point of the voltage amplification stage.
6P1, R21, C8 and two ion voltage regulators WY2P form a simple series voltage regulator circuit for FU-29 curtain grid. Under normal circumstances, the power pentode and beam beam power tube under the curtain grid voltage stabilization working condition, the sound quality performance is relatively good, because the screen voltage of FU-29 is far away from the screen electrode operating voltage, so the curtain is generally used The method of grid voltage stabilization, and the use of high-power crystal voltage regulators, the sound performance is refreshing and open. Perhaps FU-29's "Stone Sound of Amplifier" is also named for this! We are here using a voltage regulator circuit composed of an ion regulator and an electronic tube to supply pressure to the curtain grid, the purpose is to add charm to this refreshing . On the premise of maintaining the control power and sense of speed of FU-29, increase the thickness and density of the intermediate frequency in order to seek the amplifier, the sound and the rhyme. The voltage amplification 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, and there is a total of 26dB of negative feedback. In this way, the input transformer gain is 6dB, two-stage amplification plus negative feedback, a total of 47.5dB, the output transformer also has a gain of -32.5dB, the actual gain of the whole machine is 21dB.
The power supply part is also shown in the figure. The 600V high voltage of the screen is directly obtained from the 220V AC mains through D1, D2 voltage doubler rectification and C12, C13 and C14, C15, C16 filtering. Among them, the 300V voltage obtained by half-wave rectification and filtering composed of D2 and C13 is used for the voltage amplification stage and the curtain grid of FU-29. R22 and R23 are the bleeder resistors of the power supply, which provide a way for the filter capacitor to release energy. These two resistors will release the residual energy in the capacitor after shutdown, otherwise the capacitor will still be very high after the tube cathode cooling stops working Voltage, and keep it for a long time, which is detrimental to safety and the life of the tube.
This power supply method saves a high-voltage high-power transformer, which not only reduces the cost, but also obtains a very low power supply internal resistance. There is no doubt that the energy of the power supply without transformer blocking is abundant and the response is more rapid. In static and large dynamic conditions, the fluctuation of high voltage is very small, and the excellent performance of the mid-to-low frequency band of this machine will benefit from this! But without the isolation of the power transformer, safety has become a problem again. The amplifier is unimaginable. Here, the input transformer B1 and the output transformer B2 complete the isolation function, so that the cabinet does not carry AC mains. It is worth noting that the grounding of the chassis and the internal grounding of the internal circuit in the figure are separate and must not be connected together. Because B1 and B2 have to complete the dual role of signal transmission and isolation here, the quality of these two transformers should not be taken lightly.
Historically, this kind of power supply has been rarely used, mainly for safety considerations. In fact, as long as the two transformers are checked in tandem, the isolation effect is exactly the same as that of the power transformer. This amplifier has been used for many years and is very safe. This machine has not gone so far in the filament power supply, or a common power transformer B3 is set.
Three sets of 6.3V for filament and one set of 36V for grid negative pressure are provided by another power transformer B3. Since the required power is not large, a 60W low-power transformer is sufficient. The 100Ω potentiometer W3 connected to the filament of the voltage amplification stage 6J4P is used to adjust the AC interference of the filament to a minimum. One end of the filament of FU-29 can be grounded, and 12.6V can also be used to change the position of the filament of FU-29. But 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 of 6P1 and can only be suspended for power supply. The AC 36V is rectified by a silicon bridge, filtered by C9, R21, and c10. After the voltage regulator D3 is stabilized, it is adjusted by W2 to provide a fixed negative voltage to the FU-29 grid.
The voltage resistance of FU-29 is very high, up to 150V. In order to improve efficiency and increase output power, it is necessary to let the tube work in an efficient state of high screen pressure and low screen current. Here, the screen pressure value of FU-29 is Ua = 600V, and the maximum screen current IAMAX = 60mA. Obtained from the FU-29 Ua-Ia curve (Figure 2), when Ug = 0V, Ia = 160mA, the voltage drop between the screen and the cathode is about 40V, so that the output power of the machine can be calculated P = (600- 40) × 0.16 / 2 = 44.8 (W)
According to the 90% efficiency conversion of the output transformer, the actual output power of 40W is also sufficient. For the tube power amplifier, this is already in the golden power section. The actual driving force is not lost to the 100W transistor amplifier.
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