SHEET 3, Remote power supply chassis for one 300W amp channel.
Sheet 3 shows the schematic for a 400VA x 25Kg power supply.
Further down this page there's a picture of rear of PSU and with pair of octal output sockets for two
umbilical cables to suit two octal plugs on ends of umbilical cables hard wired into amplifier chassis.

PT1 has GOSS E&I lams and core rated for 1.9kVA and windings rated for over 600VA.
Temperature rise is less than 7C after several hours, noise is low, and there is good natural regulation of
anode B+. The B+ is filtered with CLC filter with 8 x 470uF x 350V elcaps ( C8 to C15 ) and choke for
1.8H at 550mAdc, with Rw = 9r0 and weight of 5Kg. The caps are arranged series and parallel to make a
CLC filter 470u + 1.8H + 470u, all rated for +700Vdc, and giving less than 8mV of 100Hz ripple at B+ output.
Resonant Fo of 1.8H + 470uF = 5.54Hz.

Rectifier B+ diodes are 4 x PX6007, each 6A x 1,000V piv. They are used in a voltage doubler in two parallel
pairs to increase the current rating to 12A.
To help keep diode currents equal, each has 1r2 series R. I measured very little difference of Vac across
each 1r2, ( R9+10, R13+14 ).
The doubler works from a nominal 200Vac HT winding to give +512Vdc at at about 500mAdc at idle when
mains are at 245Vac. The settings of HT taps give many possibilities :-
Table 1. HT and B+ for different tubes.
HT tap Vac
B+ approx
Ia, mAdc each tube
approx idle
Ek approx +Vdc
Rk = 500r
Eg1 bias
12 x octal based tube type
Ea =
B+ -Ek
Pda idle
each tube Watts
Max AB Po
Screen supply B+
ohms r
This page deals only with HT setting at 200Vac, suitable for ONLY 12 x 6550, KT88, KT90, KT120.
If anyone were to try to use say KT66, 6CA7, EL34, 6L6GC, they should employ a technician to change
the HT tap on PT1.
The technician MUST ensure that idle Pda for each output tube does not exceed 2/3 of the maximum Pda rating.
Pda = Ea Vdc x Iadc in Watts. Other changes are required :-
Reduce value of series R between B+ and screen Eg2 regulator, SHEET 4, Reduce zener diode string in
Eg2 regulator from 5 x 75Vdc to 4 x 75Vdc, SHEET 4, Zener diode voltage in protection circuit, SHEET 7.
Reduce fixed grid bias voltage.

In theory, other octal based beam tetrodes could be used such as 6CM5, 6V6. Unfortunately, many technicians
will have ZERO idea about how to alter an amp to comply with MY ideas of best practice. So I suggest leave an
amp like this alone unless you were to want to re-tube with say 6L6GC which are a lot cheaper than 6550, and
then make the other changes needed. The arrangement I have chosen gives very long tube life and good music.

There is R8 4r7 x 10W series R between HT winding and caps to slightly limit peak charging currents in diodes
at all times. In addition, I have R5 15r0 x 30W which is in series with the mains Neutral line input to PT1 primary.
This limits the very high mains inrush current after turn on.
The initial current for first 1/2 second is many times the idle state current and occurs due to 940uF being charged
up from having no charge. After 3 seconds, the B+ charge has reached 70% of full idle value, and initial heater
currents have lessened, so R5 is then shunted by Relay 2 by a 3 second R&C delay circuit around Q1&2 and
+17Vdc rail derived from rectified 12.6Vac heaters.

When R5 is shunted, there is a second inrush surge, but not more than twice the steady idle level when tubes
conduct Idc and all heaters have warmed up. The B+ rectifier circuit worked OK when only 2 x PX6007 were
used, but I have now used 2 pairs of PX6007 so diodes work well within diode ratings, and they will cause fuses
to blow before diodes fail to become a short circuit. If the amp is fully warmed up it can be turned off then back
on again and the delayed Relay 2 always works to limit charge currents.

At turn on when cold, each 6550 filament heater is 1/3 of the R value when hot, about 1r2, so the initial heater
power at turn on for so many tubes is about 420VA. This reduces in about 15 seconds to near the normal
constant level of 145VA.

Initial power input at turn on = approximately 500VA for HT and + 420VA heaters = 920VA total, so with 240Vac
mains the average input current in first second 3.8Amps. I found a 4A slow blow fuse could be used without
nuisance blows. Without the delay for B+, the fuse value might have to be 6A to 8A.

The delay R&C uses R12 8r2 and C6 470uF which have time constant = 3.8Secs. The 10V zener diode x 1W
rated is there to allow the Vdc across C6 to rise to +10.5Vdc before suddenly turning on Q1&2 which turn on
Relay 2 with audible click which always should be heard after mains turn on. R11 limits current from +17Vdc
rail to 12Vdc relay coil. The nearby 1N4007 rapidly discharges C6 when amp is turned off at mains and +17Vdc
rail rapidly reduces to 0V.

The power supply has been designed to work with 50Hz mains, but operation with 60Hz results in less core
heat losses. The tube heater windings are arranged as 4 x 6.3Vac windings in series to give 25.2Vrms with a
CT at 0V so output tubes are arranged to use two phases of 12.6Vac x 5.7Amps. The heater windings generate
a +17Vdc rail to drive Relay 2.

The output power of the PSU is from 2 recessed octal chassis sockets. A red plug on amp chassis is for B+
and other voltages at fairly low currents, and black plug used for heater power, with 3 parallel pins for each
5.7A phase of current.

I used red and black sockets and plugs but I now realize some ppl are colour blind so black and white seem
more sensible colours. Numbered labels would even be better.

There are 2 cables from each amp chassis. Both have octal plugs which I have reinforced with copper wire
inside hollow pins, well soldered, and with steel rod inside the centre key spigot. I searched everywhere for
something better but found nothing with say 16 pins which are all 2.5A rated and at least 5mm apart.
The cable wires enter a short PVC pipe which surrounds the reinforced octal plug and all soldered connections
in PVC pipe were then filled with casting resin.

Each 1.2M cable has flexible multi stranded wires normally used for 415Vac 3 phase power to mobile gantry
cranes and other industrial use. Each wire to "red" plug is rated for 15A, each to "black" plug rated for 20A,
so total current ratings are huge. I have found the arrangement always gives a good connection. At rear of PSU,
a 20mm thick plywood block surrounds the plug entries so that plug damage is avoided if a cable is yanked

The mains switch is located near top of front of PSU, so that if PSU are on the floor (where they should be) then its
not a big reach down to turn on the system. If all is well when amp is turned on then green LED on PSU and blue LED
on amp chassis should light up, both driven by the +12Vdc rail generated by 7VA PT2 protection transformer.

The +12Vdc "protection" rail is taken to amp chassis to power protection circuits seen in SHEET 4 and 6.
If the red plug is not plugged into PSU at turn on, then SCR2 in PSU turns on which turns on Relay 1 and turns
off power to PT1. A red LED lights up to tell an owner something is wrong. He may find he has has forgotten to
plug the umbilical cables into the back of PSU. If he gets the red and black plugs mixed up, a mains fuse will blow
at turn on.

If one or more 6550 conduct too much Idc because of bias failure or other fault, then SCR2 in SHEET 7 turns on
which turns on Relay 1 in PSU above, thus turning off PT1 by opening the mains Neutral line to PT1 primary.
So PSU and amp chassis are both turned off internally and both red LEDs at amp and PSU are alight.

But with normal operation, Relay 1 will rarely ever have to work. At mains turn on, a green LED lights up at
PSU and blue LED on amp chassis, indicating all is well, protection circuit rail is active, and mains power is on,
and tube heaters will glow red-orange. But if the amp is turned off due to a fault, such as high audio power applied
to a shorted speaker cable, the amp may be reset by turning audio volume level down, turning off at mains switch,
waiting 2+ seconds, then turning back on again. If the cause of the fault is not identified and fixed, eg, the shorted
cables repaired, the cycle of automatic turn off will re-occur.

If the amp protection works with no signal, there is a problem which needs fixing. Because there are 12 output
tubes, there is 12 times the probability that one tube will fail to hold bias. We may say that from a batch of 5,000
x 6550 tubes made by Russians, 20 might last only 3 weeks, another 20 last 3 months, 20 last 3 years and 4,000
last 5 years with the rest lasting up to 12 years. I am not familiar with real rates of tube failure, but my experience t
ells me it is common to get at 6 years from 8 x 6550, KT88, KT90 etc if amps are used each day for 3 hours.
That's 6570 hours, and 2,190 on-off cycles. This is very favourable, and could exceed the reliability of an
equivalent fancy high end solid state amp.

While working to repair many amps over 18 years in the industry, my bench always had a pile of SS amps to repair.
My active protection measures are not fitted to any other brand-name amp and it should be impossible to wreck an
output transformer power transformer due to prolonged overheating.

If you keep a large pet lion or dog who likes chewing on cables, then try to wean him / her off the habit. The cables
will taste bad if chewed, but if teeth penetrate insulation, the cables taste +235dB badder.
All cables are no more dangerous than ordinary common 240Vac mains figure 8 cable for 101 lights and other
appliances. DO NOT have umbilical cables placed where
they are where people walk and trip over, or yank PSU or amp chassis off a bench. PSU should be on the floor
with amps on a bench above.

The 2 umbilical cables are hard wired into the amplifier chassis at a plywood block well fixed with wires soldered
to screws. This means the umbilical cables should never be lost, but it also means the cables must be coiled up
and tied to amp when moving moving the amps.

>At the PSU, the plywood block with 2 holes to suit the cable plugs from amp will prevent anyone touching bare
metal pins for safety reasons.

It is impossible to remove the PSU box cover without first removing the fuse cap and IEC mains cable.

The umbilical cables :-
umbilical cables

The on-off switch is mounted on the top and front of PSU cases.

Two values of input fuse are used depending on the applied mains voltages which can be altered over a wide
range from 100Vrms to 245Vrms.
100V to 120V use 8 Amp slow blow.
200V to 250V use 4 Amp slow blow.

The power supply units run cool and require no special ventilation. The amplifier chassis with many output tubes
will run quite warm and MUST NOT be placed on thick carpet which stops natural ventilation up through bottom
cover and up around output tubes, and MUST NOT be placed inside a closed cupboard.
DO NOT place anything immediately above the amp chassis.
SHEET 4, screen supply B+, input and driver B+, and filament heaters in amplifier chassis.300W-amp-sheet4-Eg2-heaters-2014.gif 
Sheet 4 shows a solid state regulator for the screen supplies to the 12 output tubes.

Also shown are AC heating circuit for 6550 and EL84, and DC heating for input 6CG7.
A negative -17.7Vdc is used for fixed bias for 6550.

At top RHS, you can see where B+ from PSU feeds the single diode ahead of R19, 20, 21, each 10r, to allow
easy measuring of Idc flow to OPT CT, screen supply, and input and driver tubes. Each of 3 B+ Idc circuits has a fuse.

The single 6A x 1,000V diode acts as a safety measure to prevent stored Vdc in rail caps ever being able to flow
OUT of the amp chassis to something earthy. Thus if someone were to remove red plug from PSU and grab hold
of bare pins of plug, the diode will prevent an electric shock. SAFETY FIRST!

The regulator It consists of rugged Q1 BU108 plus Q2 MJE13003 as a Darlington pair emitter follower pass bjt
mounted on a heatsink under the chassis.
The regulator is used for several reasons.
The amp is configured for 20% cathode windings very similar to my 8585 amp and basically similar to Quad-II.
For high power output with 6550 the B+ may be at +512V, and Eg2 may be a lower fixed Vdc at +386Vdc applied
to all 6550 screens. In these amps, at idle, Ek = +23Vdc with cathode biasing and -17.6Vdc is applied to grids as
fixed bias.
So total grid bias Vdc = -40.6V.
This is a conveniently low Vdc and screen currents at idle are only 4mAdc. Using B+ = +500Vdc applied to
anodes AND screens require total grid bias of about -55Vdc with much higher Ig2 input at idle, and at high audio
power levels screen currents can rise alarmingly. The 6550 are very happy with my arrangement. Mains voltage
rise can cause tube heat problems if screen Vdc also rises. With a regulated B+ screen supply, the change in Pda
of tubes is kept constant, and is much aided by the use of R&C cathode biasing.

The regulator uses a string of 5 x "75V" zener diodes which give a "reference Vdc" at base of Q2. Zener diodes
can be a bit tricky and I found most 5W types gave 78V to 80V, not 75V, when their current was only 5mAdc.
The use of screen Vdc applied at +387V and Ek at +23Vdc means Eg2 = +364Vdc.

Between the B+ applied to regulator input and bjt collectors, there are 3 x 270r in series rated at 10W each,
R12, 13, 14, to make 810r.
When audio power exceeds 300Watts the Ia increases and anode B+ will fall from +512Vdc to about +450Vdc,
and Ek will rise from +23V to about +27Vdc, so Ea = +423Vdc. If the high audio power is a continuous sine wave,
screen current will increase nearly 3 times from 4mA to 12mA on each 6550. The screen Pdg2 can reach the rated
Total max Ig2 for 12 x 6550 is about 144mAdc. Voltage across 810r = 117V, and regulation has ceased and Eg2
supply behaves as if there is a simple feed to screens of 810r with 150uF cap, and C7 filters out audio F and
harmonics. With 117Vdc across 810r, the Eg2 falls to 333Vdc. With Ek at +27, Eg2 to cathode = 306Vdc, and this
reduction of screen Vdc voltage tends to bias the tubes into class B operation, and for expected full power there
is class AB2 operation with considerable grid current. It is not a happy picture.

But continual high audio power could endanger tubes. Fortunately with music the average power level without
any clipping of peaks is perhaps a maximum of  1/9 the rated power of the amp, say 33W. This is because the
highest Vo peaks are about 3 times the average levels. Nobody notices the clipping of peaks in music until
average Po rises to about 1/4 maximum at 75W. The time constant of screen input resistance + 150uF means
that average screen current never increases enough to cause a large drop in Eg2, and biasing remains ideal,
and there is no grid current and low THD.

The arrangement I have means that if absurdly high audio power is used, with much wave clipping, then screen
Vdc is allowed to fall with tends to bias the tubes off, and keep them cool, and unlikely to damage themselves,
and OPT, or anything else.

One way to observe real world behaviour in any amp built using my principles here is to use a pink noise
test signal which resembles music that is constantly busy. Signal content in pink noise 20Hz and above
20kHz should be filtered out with R&C filters, lest extreme LF and HF overload the amp. The oscilloscope,
(CRO), will show when peaks of test signal just begin to clip, and the clipping output voltage can be measured
by using some other signal source to produce a sine wave at 400Hz to the same level seen on the CRO.
For example, my 300W amp can produce 32Vrms into 3r0 load at clipping with a sine wave and with Eg2
sagged to 333Vdc. This is 341W. But if Eg2 could be forced to stay at regulated +387Vdc then Vo could be
about 35Vac for 408 W with a sine wave. If a pink noise signal is used, or even some heavy rock music,
peaks of signal would reach 408W.
I might add that while I tested the 300W amp I was using a 3r0 dummy load. This load is less ohms than
should be used, and OPT transformer losses are 10% so while I measure about 400W, in fact the tubes are
producing 440W, with 40W lost as heat in the OPT windings. Using such an amp for continuous sine wave
power might raise OPT temperature. But music is never a sine wave, and average output power is always far
lower then when using a sine wave and OPTwill remain very cool.

If the output from Eg2 regulator is shorted to 0V, bjts are turned on hard and act as if they are a short circuit,
so they remain cool. The 810r conducts maximum possible current = 610mAdc. The 250mA fuse before
regulator input will blow. Amp will become silent, but unharmed, and fuse must be replaced.

I had thought of using a regulator which has short circuit protection by means of rapidly reducing output
current once the equivalent load resistance falls below a critical level, but I feared that when overloaded, the
amp would oscillate at LF and it could be dangerous for tubes.

The Q2 base input voltage is set by the zener diode string fed by R16 25k. Some filtering of zener noise
is achieved with C10 47uF. Also C10 slows down turn on behaviour of regulator. Voltage at Q2 base is fairly
low noise.

V1 input tubes have B+ shunt regulated by the string of zeners. This eliminates any chance of LF oscillations
around the supply rails, aka "motor-boating". I have seen many amps with very LF and low level oscillations.

The lower right side of Sheet 4 shows the terminal block for entry points for the two umbilical cables from
the remote  power supply, with colour coding of pins and wires.
The previous 2006 page about the 300W amps had a SHEET 8 which showed heater circuits. These are
included in SHEET 4 above.
You are at...
300W amp power supply
Other pages....
300W amp input/driver and output stages
300W amp active protection
300W amp dynamic bias stabilization
300W amp power vs load graphs
300W amp images, tubes with blue glow, and more views of amps

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