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.