FU-29A single-ended power amplifier, Fu-29 Vacuum Tube Amplifier
Keywords: FU-29A single-ended power amplifier
Many audio enthusiars have a unique lift of the A-class single-ended output, using Fu-29 to make a single-ended output stage and quite attractive effects. According to the two limit conditions of Fu-29, PAMAX ≤ 20W (each quadrupole), UG2MAX<225V, the anode power supply, the second gate supply is 200V.
As can be seen from the Fu-29 characteristic curve, the UA = UG2 = 200V, the anode current cut-off point gate negative pressure is about -25V, that is, the anode current still has about 20mA when the gate is instantaneous, so the anode current still has about 20mA, so select -10V As a static work point for a Class A, when the input signal 10VP-P meets the requirements of Class A state. From the curve family, it can be found that the single-stage quadrupole is 100mA, which can calculate the anode power consumption to PA = 200V × 100 mA = 20W, which is in line with the requirements of safe use. Calculated according to the above state, the output power of the FU-29 single four-pole portion is 4.3W, and the optimum load impedance is 1200Ω. FU-29 is a composite tube, which is a matter of succession of the two groups of beam tetles. At this time, a FU-29 group four-pole parallel, single-ended Class A state operating conditions are:
Wire voltage / current: series connection 1 ~ 7 foot is 12.6V / 0.95A, connected in parallel 1 ~ 7 foot connecting to 5 foots 6.3V≈2A, the anode voltage 200V, the second gate voltage 200V, gate negative pressure -10V , The operating point anode current is 100mA + 100mA, the second gate current is 9mA + 9mA, the optimum load is 600Ω, the output power is 8.6W, the open loop nonlinear distortion. 1881.9%.
As can be seen from the above results, the FU-29 is combined with a single-ended output stage, the output power exceeds 6 L6GC, and its low voltage high current characteristics, the internal resistance is lower than the common beam output tube, the optimum load impedance is only 600Ω, The result is that the structure and process of the output transformer become extremely simple, and the requirements for primary inductance are vigorously reduced. It can be calculated that even if the output tube low-end turning frequency is set to 10 Hz, the primary maximum inductor is only 8 h, so that the frequency of the amplifier is extended easier.
The FU-29 parallel A-class single-ended output stage amplification circuit is shown in Figure 1. V2 is composed in parallel to Fu-29, which can be directly connected to the holder, and 6 foot is connected to G1, 3 foot is two sets of second gate, and it has been connected in the tube, 4 foot is two group cathodes shared lead, 1, 1, 7 feet are two sets of filaments 12.6V lead-out ends, 5 foot is tapped in the center of the filament, in order to supply power with 1, 7 foot 6.3V, and the two sets of anode can be directly connected. The two groups of beam tubes are connected in parallel, and the static cathode current reaches 220 mA (P, G2 current). When the self-grating negative pressure is used, the value of the cathode resistance RK should be RK = 10V / 0.22A = 45.5Ω, Two 91 Ω / 2 W metal film resistors can be used in parallel. The value of the CK can be used with an electrolytic capacitor above 1000 μF / 16V, while simultaneously accessing an O.47μF thin film capacitor.
The FU-29 is amplified in Class A, and the drive signal can output 8W power as long as the drive signal can output 8W, so the front stage voltage amplifier does not require too high gain. The V1 in this machine is used as a SRPP circuit with a double tripode 6SL7 (the same domestic 6N9P and the same) to reduce the output impedance, improve linear and frequent sound. The voltage gain is about 45 times, which is far exceeding the design sensitivity, so the negative feedback of -14dB is added between the pre-stage and output transformer secondary, and the high gain is controlled while improving the nonlinear distortion of the five-pole output tube. When the output power is 8W, the input signal voltage is 70mvrms.
In order to avoid the FU-29 second gate voltage overrun, the circuit is adjusted to select 100 Ω / 2W resistance, so that the supply voltage is between 200 to 220V. The front-level SRPP circuit, the two-diode is supplied in series, in order to obtain a 300V supply voltage, a set of multiplier stream circuits to provide a small current 300V voltage. Here the load current is in milla-level, there is no problem with voltage load adjustment rate.
The power transformer 6.3V windings are used for the forefront power supply. The front-level V1 cathode has 145V DC voltage, in order to avoid breakdown 6SL7 cathode, the filament is insulated, and the 6.3v winding cannot be grounded. In this machine, the 300V voltage is divided into + 150V (Figure H point) with two 100kΩ resistors, which can effectively avoid the SRPP circuit breakdown V1 tube cathode. At the same time, this connection allows the cathode potential to be slightly lower than the filament, and the filament heat electron radiation is absorbed 50Hz noise. 12.6V filament voltage supplies the voltage to the FU-29, 7 foot, and the center of the FU-29 filament is pumped 5 foot to reduce the interference of the filament alternating electromagnetic field.
The output transformer of the machine is only 600Ω, but the FU-29 is used in parallel, and the DC magnetization current of the transformer reaches 200mA. Therefore, the iron core section is slightly better, otherwise the effective magnetic flux will be significantly reduced after 0.2 mm air gap, so that the primary inductance is too low. The output transformer data is as follows:
The core section is 35mm high silicon steel sheet with a single tank flame retardant, and the core EI sheet is separately paired, and 0.2mm (4 kraft paper) is clamped after the air gap is left. Winding data: The primary use φ0.31mm enameled wire by 1500; the secondary is 0 ~ 4Ω, with φ0. 71mm enameled wire wiring around 120: 4 ~ 8Ω, with φ0.64mm enamel wrap around 50. The transformer does not have to segment, only the secondary is around the primary winding 1/2. The secondary sides and 0.05 mm polyester film and one layer of kraft paper are insulated from the primary side. The whole machine THD = 7% at the rated output power of 8W, and the output is 1% when the output is 5W. The above indicators have been item Class A power ampass, and its linear distortion is mainly harmonic.
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