is 42dB so that the signal level at this point is –8dBV for a reading on the meter of –
40VU. The level at this point is unaffected by AOT8. VT11 is an amplifying stage,
the current in which is adjusted by AOT19 and AOT20 to give the required potential
at the collector, and the gain of which is adjusted by AOT22 to give the required
overall gain. D6 and D7 are 7.5V zener diodes which prevent the bottoming of
VT11 which would otherwise occur in overload. Bottoming of VT11 would both
reduce its input impedance which would upset the gain of VT8, and prevent
instantaneous recovery from overload which is essential for correct meter indication.
This collector is connected to the bases of two complementary transistors VT12 and
CT13, the emitters of which are connected to the bases of VT14 and VT15. The
sum of the currents of VT14 and VT15 is held constant at about 3 mA by VT16. The
meter is connected in the collector circuit of VT14 and is shunted by a large value
capacitor C11. AOT21 is selected to set the meter current, in the absence of signal,
to 70
µ
A. AOT19 and AOT20 are selected so that the collector potential of VT11 is
the average of the values at which VT12 and VT13 begin to conduct. When the
signal is sufficient to cause these two transistors to conduct the current in VT15
decreases and that in VT14 increases causing deflection of the meter.
12.18.3.3
Forward Path B
This is similar to Forward Path A. The major difference is that the feed to the
inhibitor circuit must in this case be from the negative supply.
The B input stage is identical with the A input stage except that its negative
supply is taken through an emitter follower VT31. In the first limiting stage the diode
D10 is connected in the reverse direction and so limits the positive peaks and a
pnp transistor VT33 limits the negative peaks and, when it is non conducting,
current flows from the inhibitor circuit through R8 and R89 to the negative line.
The second limiting stage is identical with the second stage of the other
channel except for the reversal of limiting functions of the diode D11 and the
transistor VT35.
The third limiting stage is identical with the second except for the provision of
means for returning the earthy and of D12 to a potential slightly positive of negative
if necessary in order to compensate for any difference in the limiting voltages of D12
and VT37 so as to minimise meter deflection as limiting takes place. If a positive
potential is required AOT12 is made 20k
Ω
and AOT14 is omitted. If a negative
potential is required AOT14 is made 20k
Ω
and AOT12 is omitted. In both cases the
optimum potential is obtained by selection of AOT13.
The next stage VT38 is an amplifier stage similar to VT8 but with difference
component values but it is not followed by a limiting stage. It is a.c. coupled to the
base of VT39, the emitter of which is connected to the gates board through
terminals numbered 5 on each board. The correct potential required at the gates
board is realised by selection of AOT10 and AOT11 in the bias chain R106 and
R107 which holds the base of VT39 through resistor R105.
The gain of this forward path from the input to output 5 is adjusted by AOT9
to 58dB.
12.18.3.4
Low Level Meter B
The circuitry of this is identical with that of Low Level Meter A, its input being
in this case derived from the emitter of VT38.
