Sheet
3
The above is the schematic
for what is within the 25Kg separate power supply unit supplied with each mono
bloc 300W amplifier. Basically it is a linear type of supply with a large power
transformer with a GOSS core rating for 1.9kVA, and a winding rating for 700VA,
so that temperature rise is negligible, and fault conditions can be sustained
for lengthy periods.
There is excellent natural regulation for the B+
anode/screen supply.
A voltage doubler rectifier is used with a pair of 6
amp silicon diodes with PIV = 1,000V.
There is provision to reduce the B+
should anyone wish to operate different octal output tubes in different
conditions.
It is shown with the 200Vrms tap on the HT winding in use,
but should the 157Vrms tap be used the B+ would be about +390V and the amp could
have higher plate current for less AB power but more pure class A operation into
a lower value RL.
The 135Vrms tap would produce about +340V, and allow the
use of 6V6 as the cheapest option to obtain about 90 watts of power.
There is a CLC type of
filter for the main anode supply, arranged so that caps C1 to C8 form two caps
of 470uF each side of L1, which is a 1.8H choke. With a dc draw of 500mA, there
is 2.34Vrms of ripple voltage at the bottom of C7,C8. The C1 to C4 capacitance
plus choke of 1.8H form a second order filter with an attenuation factor of
0.003, so the voltage ripple at the top of C1 and C2 is 0.007Vrms, or only 7 mV.
The resonant frequency of the LC filter is 5.5Hz.
The power supply has
been designed to work on 50Hz mains, so operation with 60Hz results in lower
heat losses. I have found that after being on for 4 hours, the transformer has a
temperature rise of less than 5C above the ambient room temperature, and despite
the size, operation is silent.
The choke filter is arranged
so it is placed in the 0V rail line instead of the B+ line so that there
is
no constant strain on the insulation within the choke.
The choke's dc
winding resistance = 9 ohms, so little dc voltage drop is wasted across the
choke.
There is a +17V dc supply to
operate relay 1 and 2.
At turn on the HT winding current surge is limited by
R3 for 4 seconds. In that time the B+ will have risen to 2/3 of its final value
and then Rel-1 is closed to shunt R3. There is a second current surge as the B+
reaches its final value of over +500V
in about a second. This stepped B+
voltage turn on results in about 1/2 the mains inrush current than if there was
no stepped
turn on, so a lower value mains fuse can be used. The two PN100
bjts and 15k feeding C10 act as the 4 second delay circuit.
Relay-2 is used to interrupt
the HT winding on the transformer when a fault occurs and normally is closed to
allow the ac from the HT winding to charge the caps via the two diode
rectifiers.
However, should one or more output tubes become faulty and
conduct more than 40% of the normal idle current for longer than about 3
seconds, the active protection circuit will detect the excess tube current and
trip an SCR seen on sheet 7,
thus grounding the Rel-2 coil and it opens to
interrupt the B+ anode supply.
The amp will still have its heaters on, but a
red LED will indicate it is in fault mode, and it will stay like this until
someone provides service. The amp can be "reset" by turning it off, waiting 20
seconds, then turning back on.
Should the amp protection circuit again trip,
there *is* a problem which needs fixing.
It should thus be impossible to
wreck an output transformer or other parts due to prolonged overheating.
No owner serviceable parts
are within the amplifier, and only qualified and well experienced technicians
should ever service such an amp.
There two umbilical cables
from each power to each mono bloc amp.
The cables are easily flexible cables
each with 5 wires within and each wire is a multi-stranded and unlikely to
become unserviceable in the foreseeable future. The type of cables used are
normally meant for many heavy industrial applications where 415Vac 3 phase power
is applied to mobile gantry cranes etc.
The heater currents from each of the
two 12.6Vac heater supplies for the tubes is 5.4A per phase.
One of the
umbilical cables is devoted to the heaters only and its current rating 40 amps
per phase,
and so there is little possibility of ever experiencing fried
cables if a short circuit ever occur.
A mains fuse will blow.
The other
umbilical cable has 15 amp ratings for each of the 5 wires within, and has
excellent insulation resistance since one of the wires carries the +500V B+
supply. It is no more dangerous than a normal flexible mains cable.
If you
keep a pet lion who likes chewing on cables, then try to wean him off the habit;
I cannot guarantee that such cables will not sustain damage due to your private
household arrangements.
Welded steel stands for the amps can be provided so
the likelihood of stretching cables due a fall.
At the end of each cable
hard wired into each amp chassis there is an octal tube plug which has been
reinforced to prevent easy breakage of the locating spigot.
The two such
cable plugs have been painted red and black, and the cable sockets are also red
and black so you know which cable goes into which socket.
Should the two
plugs be reversed by accident, the B+ supply will be shunted through the heater
windings to 0V, and a mains fuse will blow. The power supply enclosures have a
plywood block which is fixed to the casing by and external
and 2 internal
screws. The block has neatly sized holes to suit the external dia of the cable
plugs thus assuring that sideways force on the cables will not damage the plugs,
an once plugged in, little fingers cannot
reach behind the plug to touch
exposed pins if the plug isn't fully inserted.
It is impossible to remove
the case covering the power supply chassis without first removing the fuse cap
and IEC mains cable.
The umbilical cables
:-
The on-off switch is
mounted on the cases of the power supplies, so they should be mounted within
easy reach to turn the system on or off.
S1A is the mains entry
switch on the above schematic.
Two values of input fuse are used depending
on the applied mains voltages which can be altered over a wide range from
100Vrms to 240Vrms.
An 8 amp slow blow fuse is needed for 100V to 120V
mains, and a 4 amp slow blow fuse
is required for 200V to 250Vmains.
S1B is 1/2 the mains switch
which connects a 1K resistance from the B+ to 0V while the amp is turned off.
This discharges the supply capacitances quickly should the amp be turned on,
then off quickly by a service person without heated tubes which can quickly
drain the B+ supply rail which will charge up to +520V without a load. It also
allows testing the power supply safely without the amps being plugged
in.
There is a second fast discharge resistance and relay within each amp
chassis, see sheet 4.
The +17V supply in the above sheet3 is taken out of
the supply on pin six of the octal socket containing the B+ and OV and error
signals, ( painted red on the amp PS and cable plug ), so that +17V can power
the relay in sheet 4.
So at turn on, the relay opens, allowing the B+ to
rise, but at turn off, the 820 ohm x 10w R shunts the B+.
But should anyone
remove the red power supply cable from the power supply, the electrolytic caps
will take some time to discharge through the output tubes, and during this time,
the pins 4 & 5 will have the B+ voltage on them. If there was no plate
current or tubes installed, this could pose a danger for anyone touching the
pins of the plug.
So the the use of an 820 ohm resistance will discharge the
effective 300uF caps from a maximum of 500V to 68V within 0.5 seconds, and
so a shock to anyone is extremely unlikely.
Under normal operations with
tubes all fitted, the discharge of the power rails to less than 68V will occur
even faster should anyone remove the red cable with the amp turned on.
With all cables correctly plugged in, the rails will be discharged and
voltage reduced from +500V to +68V
when the amp is switched off within 1
second, even without tubes plugged in, and thus danger to service people working
is reduced. With all tubes conducting, rail discharge at turn off is even
faster.
The fast draining of the B+
rails at turn off also prevent the slow subsidence of signal levels with large
amounts of distortion which could damage tweeters if the music is left running
when the amps are turned off.
However, mains power interuption will not cause
the fast discharge to occur. In practice, such power interuptions are
unlikely to ever damage a speaker because the rails will discharge fairly
quickly because of the high current
draw from the B+ supply to the output
tubes.
The supplies run cool and do not need any special ventilation.
The power amp chassis run quite warm.
They DO REQUIRE GOOD
VENTILATION!!!!!!
Sheet 4 
The above part of the power supply is located in the amplifier chassis.
It has a solid state
regulator for the screen supplies to the 12 output tubes. It consists of an
MJE340 and BU108
mounted on a heatsink under the chassis.
The two easily
available discrete BJT devices are connected as a simple darlington emitter
follower pair. BU208 is also usable.
Should excessive screen currents ever
flow, R5 will develop a voltage across it reducing the
voltage across the
BU108, and it will stop regulation when current reaches 150 mA and allow the
screen voltage to fall.
Excessive output currents are also limited by R6, so
that if too much current should suddenly flow in R6,
then the 4 diodes from
the top of C5 to MJE340 base will conduct and the base voltage
reduced
quickly, and thus output voltage. The circuit is protected against reverse flow
of supply
voltages applications by the diode from the output to collectors.
But under normal operation, the screen voltages held very steady between low and
high power when dc screen currents vary considerably with signal levels.
The supply to the input stage is shunt regulated by the string of zeners by C1.
The lower RHS of the
schematic shows the entry points for the two umbilical cables from the
remote power supply.
I cannot recommend having 12 bias adjustments per chassis, 24 in all in a stereo system, with all tending to be slightly interactive while you set them. And many owners fail to ever adjust their bias correctly, often leading to early tube failures and expensive repairs. So a mixture of fixed and cathode bias is used.
The main power supply
capacitors for the B+ applied to the CT are located in the remote PS box
so
that should anyone remove an umbilical cable from a power supply without turning
off the amplifier at its switch, then the B+ rail voltage appearing on pins 4
and 5 of the red cable will fall rapidly due to plate current in the output
tubes discharging C1, C2, and C3 within a second or two.
This is assisted by
the use of a relay shown on sheet 4 whose action is explained above.
R10 and
R11 on sheet 4 bias the contacts of the DPDT relay so there is only 1/2 the B+
across the pairs of contacts while the amp is running, so less likelihood of an
arc.
The R9 limits the peak discharge current to 0.6 amps, so the relay
contacts will last well.
Do not allow pet lions or
your pet dog near the amps, do not set up the amps near swimming pools,
and
please do carefully think about where and how one keeps such devices to ensure
domestic harmony.
sheet 8 
This is the layout of the
heater supply within the amp chassis.
Two phases of 12.6Vac are brought into
the chassis from the remote PS to be applied
as shown, and to minimize hum.
rectifiers derive +/-17Vdc supplies to be used for the input tube to reduce
hum
and to provide a -14V fixed bias for the output tube grids.