Tube Line Preamplifier Project      

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The interest for circuits of tubes, remains big. Thus I will give completed in enough big degree, circuit of preamplifier. It is mainly constituted by the department of main preamplifier, the department of input selector, delay of application voltage and connection output of preamplifier with the final amplifiers and the department power supply. The department of mainly preamplifier appears in the Fig.1. Deliberately it does not have something revolutionary in the designing. In the entry exists a simple circuit constituted from relay that is drive from input selector [S2]. The signal afterwards drive to the next stage that is constituted by a circuit cascade S.R.P.P (shunt regulated push–pull) and a circuit cathode repeater, in the exit. As S.R.P.P used the double triode ECC82 (U1a-b), in connection ‘’series ‘’ of two triode, with characteristically pentode . The circuit was used as department of input R.F, in the old televisions with tubes, offers big input impedance and very good response in the high frequencies. In the circuit preferred use capacitors with big capacity in various points and capacitors of polypropylene or MKT. in the exits of tubes, for better quality of sound and response in low frequencies. Exists a simple circuit for the VOLUME and BALANCE potesometer, that is should be very good quality. In the exit exists a circuit Cathode Repeater, that is constituted by the half department of tube ECC81 (U2a). Here exists a particularity. Whoever wants it can use the one of half tube for a other channel and half for the other channel. We can however use his other half tube of channel, leaving him inutile, placing a output tube for each channel. This becomes in order to we avoid his effect of channel in the other, if we use the same tube ,for two channels. It’s known that the tubes, do not have very good separation, when find itself the two departments, in the same integument. The choice is yours. In this stage are found also the contacts of RL6, who connect the exit of line preamplifier in the final amplifier, with such certain time delay, so that are not heard annoying noises, when open or close the preamplifier. The RL6 closes after have been stabilised and applied, all the voltages of circuit. All the relay of SEL. and OUTPUT, should be very good quality. The circuit of control appears in circuit SEL - DELAY, (Fig.2). .

Part List

R1-7=1Mohm R12-13=100ohm C9=4.7uF 250V*
R2=100Kohm 1W C1=220nF 250V* U1=ECC82
R3-8=1Kohm 1W C2=220pF ceramic or mylar U2=ECC81
R4=1.2Kohm 1W C3=2.2uF 350V* RV1=2X100Kohm log.
R5-11=5.6Kohm 2W C4=1000uF 16V RV2=2X100Kohm lin.
R6=100Kohm C5-6=2X50uF 450V RL1....6=Relay 12V 2X2 [1A]
R9=22Kohm C7=1uF 250V*
R10=100Kohm C8=220nF 630V *polypropylene or MKT

Input Selector - Delay Drive

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In the Fig. 2, you see the department that concerns the control of input sel. relay and the circuit of time delay. The drive of inputs choice Relay becomes from selector S2, that drive the corresponding Relay (Fig. 1), via the JF3/1…..6. Simultaneously we have optical clue of choice from corresponding Led (D1…. D5). The circuit is exceptionally simple. In the second department of Fig. 2, exists the circuit of time delay, round the IC1. The first delay time (10’’), with R6, C2, concerns the control of RL7 that give the high voltage, in the circuit of line preamplifier. This becomes in order to is applied the high voltage, after first, heat the filament of tubes. The control of RL7 becomes from the Q1, JF3/9-10 and the optical clue becomes from LED (D10). After close the RL7 and is given the high voltage then, after a time delay that is determined by the R8, C3 (20’’), the Q2 conduct and drive the RL6 via the JF3/7-8, in order to it connect the exit of preamplifier in the final amplifier. Optical clue of this operation becomes from LED (D11). With switch S3, when it needs, drive the base of Q2 to 0V, it open the RL6, so that is cut the sound signal to the exit, without close preamplifier (MUTE). The circuit is supplied by two voltage. The voltage + 12V, that supplies the Relay and the voltage + V DELAY, which supplies the IC1. This segregation will be explained in the circuit power supply.

Part List

R1.....5=1.2Kohm C2=10uF 25V Q1-2=BD679
R6-8=1Mohm C3=22uF 25V IC1=4081
R7-9=10Kohm D1......5=3mm Red LED S2=SEL 1X6 step
R10-11=1.2Kohm D6......9=1N4148 S3=1X2 mini switch
C1=100nF 100V D10-11=3mm Green LED

Power Supply

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In the Fig. 3, appears the circuit of power supply. He’s also simple in the design. We can him separate in the department that are reported in the benefit of high voltage and the department benefit of low voltage, for the filament of tubes and the circuit of delay and control Relay. Exist two transformers, for high voltage and low voltages (in plate). This solution was preferred for reasons of tumor. In the department of high voltage, we have the separation of voltage in two parts. To the line preamplifier and the other to preamplifier PHONO. This becomes, because somebody it can want use in his preamplifier unit PHONO. In line with the high voltage supplies exist contacts of RL7, what are drive by the circuit of delay and they close that exists the essential time delay in order that filament reach in good temperature. In the department of low voltages, exist and here separation of control of filament, with regulated continuous voltages [ IC1-2 ]. This becomes clearly for reasons of resolution of provided power. The IC’s that regulates the voltage of filament, it should they are placed without fail in heatsink. Their regulation should become first without load, in the exit (after we place a resistance 1.5K 1W as load, which we remove afterwards the end of regulation) and afterwards with complete the load of filament. In the department supply of Relay and delay exist resolution of voltages, in two parts. A part via the regulator + 12V [IC3 ], supplies the Relay. A other sector via the D7, R7, D8, C20 supplies the IC1 [ Fig. 2 ], with a voltage + 12V, stabilised from zener D8. This resolution becomes because the voltage [+ V DELAY ], should '' fall '' very fast, in order that cut also the Relay fast, when we close the main line of supply. This is the reason that the C20 is small in capacity. The block diagram of connections of mainer voltages, appear in the Fig.3. A point that it will be supposed you are careful they are, that the lines of 0V, are separated and are not met between them, in no point, neither are drive to the earth, only that the 0V of high voltage. The choice of unit of preamplifier PHONO can become from you.

My proposition is: Tube PHONO Preamplifiers with ECC83

Part List

R1=2.2Kohm 5W C8-13=10uF 16V IC3=7812  [1A]
R2-4=100Kohm 2W C10-15-19-20=47uF 25V T1=230Vac / 200V 0.15A
R3=680ohm  5W C16=470uF 25V T2=230Vac / 12V 3A
R5-6=220ohm C17-18=100nF 100V F1=0.2A slow
R7=470ohm  1W BR1=4 X 1N5408 F2=0.1A fast
TR1-2=4.7Kohm trimmer BR2=4 X 1N5408 S1= Switch 2X2 10Aac
C1=33nF 630V BR3=4 X 1N4007 RL7=12Vdc Relay 2X2 10A
C2.....5=2X50uF >350V D1.....7=1N4007 EMI RF Filter 230Vac/6A
C6=4700uF 25V D8=12V 0.5W zener * with heatsink
C7-9=100nF 100V IC1-2=LM317*

Sam Electronic Circuits 12/01