Tube
Line Preamplifier
Project

Fig.1

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 pushpull)
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. Its 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

Fig.2

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

Fig.3

In
the Fig. 3, appears the circuit of power supply. Hes 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 ICs 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

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