sheet 6
The above may seem like a horrible jumble of lines, but in practice
is is a simple
bias detection condition monitor and bias fault detector network where
there are 12 output tubes.
The circuit was easily constructed using a board with hooked solid
wires
for tracks and components were surface mounted and soldered between
wire
tracks. Longer wires from the board to each cathode are are easily
done single strand hard wiring taken from some multi-colour multi-wire
telephone cabling.
All the voltages involved are lower than 50Vpk.
The signals from the 12 output tube cathodes are fed into the 12
input
points at the top of the schematic.
The signal consists of any ac or dc voltages on at the cathodes.
From each of these input points there is a test point in the form of
a phillips head screw in the circuit board
which is accessible from outside the chassis through a two rows of
6mm holes corresponding to the output tube positions.
A test probe from a cheap multimeter can connect from the to the test
points to the chassis to
obtain the dc voltage at each cathode, and so in a minute one can
rapidly
check
all the bias voltages at each cathode.
Such dc bias measurements are always made at zero signal voltages.
From each test point, the dc voltage is divided by resistances R1 and R13, R2 and R14 and so on across the board, so that if there is 35mA flowing to 0V from each cathode, there will be about 18V dc at each cathode test point and about 4.4vdc at each junction of the two R which have values of R1-47k and R13-15k.
During normal operation, we want this 4.4Vdc to remain free of signal voltages because the following fault circuit ( on sheet 7) has been designed to react only to Vdc changes. So 220uF has been strapped across each of all the R13 to R24, and so the output from each RC filter is largely free of all signal voltages.
Each of the 12 points where 4.4V is to appear are all
connected
through 12 diodes D1 to D12 to a common rail
shown as point Z.
The voltage at point Z is fed to the input of the circuit which works
an SCR which can only turn on if the input voltage at point Z is 7.0Vdc
which could occur if too much
dc flowed in one or more cathode bias resistors.
If 7.0V were to appear at point Z, it means that one or more cathodes
must be at about +32V,
indicating that there must be approximately 65mA dc of tube current
flow, or nearly twice the
usual idle bias amount, which is a fault condition at idle.
Should one of the 12 output tubes suffer a fault and conduct too much
current, then its cathode voltage
will be above the rest and its voltage will be divided by its own
voltage
divider to give a voltage
at point Z which is is then applied to the SCR circuit on sheet 7
below.
Due to the direction of the diodes, one tube's excessive bias voltage
cannot create a current flow
to other cathodes at lower voltages, so point Z voltage is controlled
by whichever cathode is at the highest
voltage level.
To ascertain which cathode is the cause of a bias problem, one has
to
be quick with the
dcV meter to check the tube cathode voltages as they rise and before
the faulty one trips the
SCR and turns off the B+ supply.
Alternatively, one can temporarilly disconnect the wire to point Z and
allow all the cathode voltages to rise without a faulty one causing the
active protection to work. The faulty tube will easily be
spotted.
The amp may be reset by turning it off at the mains, and back on
after 20 secs, and measuring
the tubes that could not be measured
during the first attempt to measure cathode voltages.
The 12 diodes D12 to D24 shown uppermost in the schematic are in two
groups of 6 diodes each.
Each of the two lots of 6 diodes are taken from the cathodes of the 6
tubes on each side
of the push pull cathode circuit
which works with oppositely phased signal voltages.
D13 to 18 each have a separate 100ohm 5 watt R in series ( R25 to R30
) and then all R connect together to give point Y which is
connected
to the collector of Q1 on sheet 5.
Similarly, D19 to D24 have separtae 100 ohm R in series R31 to R36
) and then all connect together to give point X
which is connected to the collector of Q2 on sheet 5.
Current in the 100 ohms plus diodes can only flow when the current
shunting
transistors are turned on by heavy
dynamic signal currents flowing in the output tube cathodes. Since
the transistors Q1 and Q2 remain turned off
for most of their life they do not conduct any current through the
D13 to D24 unless the escessive ac current
in the grouped cathodes causes them to turn on.
sheet 7
In this sheet 7 schematic, a dc voltage appearing from point Z is
applied
to the high impedance
input of a darlington pair emitter follower Q4 and Q5 and the emitter
voltage is applied across the 5.6V zener
diode.
If VZ = +7V, current will flow in the ZD, and through
R7, and R8 and raise the voltage at
the gate of the SCR and it will *latch* on and its anode voltage will
be pulled from 17V down to
near 0V, therefore turning on a relay in the remote PSU, and thus
turning
off the B+ supply.
The SCR also turns on Q3, the transistor which also operates the clipping indicator led, and the led indicates that no B+ is present; ie, that a fault condition exists.
The clipping indicator is fed a current through a 1M resistor R2
from
the anode of the input triodes
from a point C on sheet 1.
This low current signal has no effect on the audio signal flowing from
V1 to V2 and V3 on sheet 1.
The signal from the 1M R2 is cap coupled via C1 to the high impedance
base input of darlington pair
Q1 and Q2, and Q1 and Q2 turn on when the V1 signal is over a certain
signal level of 10Vrms
which indicates that clipping must be occurring, since the signal at
v1 anode rises
above 10V rapidly because it is the internal "error" signal within
the amp after NFB is applied.
So when you see the clipping indicator flashing sometimes, the music is probably too loud, and maybe you need to ask junior to turn it down lest the police make a visit to ask you.
On the far LHS, the blue ON led is fed a signal from the heater circuit.