Part 6: The universal amplifier of
The Vanderveen Project
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|The circuit diagram contains
three sections: the tube section, the amplifier wire board and the output
transformer. The blue colored connections V0, V1, Vn and ground-1 plus the
filament f-f refer to the power supply, as discussed in part 5.
section: the tube B1 at the input is a differential amplifier
and phase splitter at the same time. The cathode resistor R6 in combination
with the Zener diode Dz functions as a current source ( I = 39 V / 15
k = 2.6 mA; each B1 triode gets 1.3 mA). The anode resistor R7 is a little
larger than R5 to let B1-a and B1-b amplify equally. Pay notice to the
bottom side of R7 which is connected to the upper side of R5 ( = V2).
The capacitors C3 and C4 bring the amplified and phase inverted signals
to the power tubes. Mount C4 in such a way that it can be easy disconnected,
because the amplifiers 1 to 9 in the twenty amplifiers figure of part
3 do not need C4. The control grids of B2 and B3 get an adjustable (P12
and P13) negative voltage through R11 and R14 in order to trim the quiescent
currents through the power tubes. Use 10 turns trim pots for easy adjustment.
Mount P12 and P13 in such a way that when turned fully counter clockwise
the control grids are at a maximum negative voltage. The quiescent currents
can be measured over R17 and R18 with a DC voltmeter, see the Test Point
connections which are 4 mm panel sockets in my amplifier. When for instance
the meter reads 0.50 V over R17, the quiescent current through B2 equals
50 mA. There are two grounding points. Place ground-1 close to B2,3 and
ground-2 close to B1 for minimum hum and hiss.
More news to follow soon!