The general characteristics of the 300W monobloc amps are described in this page.
For 99% of listeners, there is enough initial pure class A power to cover all
listening levels even with insensitive speakers rated for only 81dB/W/M.
There is individual cathode biasing, aka 'auto biasing ' for each of 12 output tubes.
There are no bias adjust pots to set, but there are test points where cathode voltage
of each tube can be measured at idle to monitor tube condition over a long time.
There is also an internal fixed bias voltage applied inside the amp which owners need
With no signal current, and for most of the time, each 6550 tube generates 22Watts
of heat from anode and screen electrodes, and 11.4Watts from filament heaters.
Filament heater power remains constant, but idle power of anodes is variable and
manufacturers always disagree about what is the best idle anode current in tubes,
often making grandiose claims that the higher the anode idle current, the better
is the music, because more pure class A is available. But when people follow that
idea, the hotter the tube, the shorter its life becomes, and in fact nobody can usually
tell any difference in music quality if the idle Pda is changed from say 10Watts to
38Watts. The happy middle path is the best, and for two 6550 having Pda at 22Watts
each means adequate class A power is available for first 10Watts, and a high ceiling
of level in AB power is available up to 60Watts, with THD being below 0.05% for 90%
of music signals.
The Ia idle current is always related to the Pda, ie, the Power dissipated at anode.
Pda at idle = Ea x Ia where Ea = Vdc between anode and cathode and Ia = idle
anode dc current. In a single ended class A 6550, one may have Pda at 27Watts
with screen Pdg2 = 3 Watts for a total of 30Watts which is 71% of maximum allowed
Pda+Pdg2. With anode efficiency at 45% max in pure class A we might get 12Watts
of class A power. Typical pure class A amps with 2 x 6550 might have Ea = +400V,
and idle Ia= 67mAdc. Two 6550 in PP could yield 24Watts, but no more than this,
providing the speaker load of say 6 ohms is transformed by OPT to appear to
2 tubes as 11,000 ohms.
Well, there are no manufacturers able to sell 24Watt pure class A amps. That's
because everyone wants more power even if they never need it. So most
manufacturers of PP amps will reduce the Pda at idle from 27Watts to say 25Watts
and raise the Ea to say 450Vdc, reduce Ia to 55mA, and reduce OPT turn ratio to
make 6 ohms appear as 5,000 ohms to the 2 tubes. Then the 2 x 6550 can produce
60Watts of class AB power, but initial class A power = 8Watts. This is still enough
class A for most people, and shops sell more 60Watt AB amps than 24Watt class A
amps because nobody can tell the difference, and cost of manufacturing an AB
amp is slightly less than one making all class A.
But 12 x 6550 as I have it will always produce enough initial pure class A.
The schematics I show indicate Ea = 488V, Ia = 42mA, Pda = 20.5Watts = 50% of
max Pda at 42Watts. The load ohms experienced by each pair of 6550 = 7,300r.
The huge OPT allows 6 parallel pairs and anode load of 1,200r with speaker
secondary at 5r6.
Audio power is 324Watts AB max allowing 5% winding losses, and first 37Watts
in pure class A so THD will be under 0.02% for first 30Watts and the dynamics
for loud drum and percussion instruments is astounding.
If the speaker load is reduced to 2.8 ohms, 400Watts is possible, but I do not
recommend that 2.8 ohm speakers be used unless the OPT turn ratio is changed
As I have the 6550 set up, they should have a very long life.
But mains power draw for each 300W monobloc is 450Watts.
load settings are available for speakers, 2.5 ohms and
The figures are strange in a world
where people like to use 4 ohms, 8 ohms, or
16 ohms, ( also written at 4r0, 8r0, 16r0.) 4,8,16 are common nominal speaker
impedance ohms quoted by makers. Impedance depends on signal frequency,
but measured in ohms, so that a given speaker may be 8r0 at 30Hz, 40r at 50Hz,
6r0 at 200Hz, 8r0 at 1kHz, 12r at 5kHz, 8r0 at 20kHz, and 120r at 80kHz.
All types of loudspeakers have very variable impedance ohms often between 1/2
and 5 times the nominal value between 20Hz and 20kHz. Therefore I have always
made my amps so that they will cope well even if speakers are 1/2 the nominal
impedance stated by makers. The lower the load impedance, the worse becomes
all operation parameters of the amp.
The OPT has 12 secondary windings. There are 6 with 48turns and 6 with 24
Option 1, 6 parallel windings of 72turns being ideal for 5.6ohms or higher,
Option 2, 9 parallel windings of 48turns being ideal for 2.5ohms or higher.
Table 1 shows available power...
|OPT load match
|Initial Class A
power limit, Watts
AB1 power, Watts
I doubt anyone can prove to me that biasing output tubes close
dissipation limits will ever give better music.
The output stage has a fairly simple OPT winding arrangement with 20% of
the total primary turns devoted to between cathodes and 0V and 80% of turns
between anodes and B+. The cathode turns act to provide cathode feedback,
very similar to what was first done commercially in a Quad power amp in
about 1950. While Quad used CFB = 10%, I prefer between 12.5% and 20%.
With 20% CFB, the local NFB in output stage varies between about 8db and
16dB depending on variation of 6550 voltage gain with load changes.
The output tubes behave more linearly and with a lower effective
plate resistance than if triode connected without CFB, but with CFB the high
output power of a beam tetrode is retained, and THD without GNFB = 1%
at full AB power. The CFB winding works to apply NFB between cathode
and both grid and screen. There is typically 5 times more THD without the
Global Negative Feedback is also applied from the output
speaker secondary to the input triodes in the conventional manner to reduce
output resistance and distortions. Total CFB + GNFB is about 20dB.
tubes are not needed because the sum of the slightly
characteristics of the six tubes on each side of the PP output stage will
usually be close to each other. The self regulation of individual cathode
bias for each output tube also makes it far less critical to use matched
Bias Stabilization is a unique circuit technique used
cathode bias during high power class AB operation when cathode bias
voltages tend to vary and slightly upset the DC balance in the OPT.
The special cathode circuit with active solid state components do not
have any effect during class A operation. The result allows the amp
to have the same low distortion advantages of a 100% fixed bias amp
but eliminates any need for bias adjustments which cause many owners
nothing but trouble.
state rectifiers & rail filters.
Solid state rectifiers are used throughout.
Noise in the dc B+ power supply is well filtered out with a CLC filter with
470uF input cap + 1.8Henry choke + 470uF reservoir cap with generously
rated following RC filters for the input stages. Shunt regulation is applied
to the input stage rail. DC is applied to the input tube heaters to ensure
hum remains inaudible. The screen supply is regulated.
The Input stage is a 6CG7 twin triode with both triodes in parallel to
make a single ended triode input stage, and Ia is supplied from a solid state
Although set up in common cathode mode, it acts as a differential amp with
input to grid and GNFB to cathode. Since 2008, experiments using the
input 6CG7 as a true differential amp with constant current sink for
commoned cathodes and running the EL84 stage as a true balanced amp
with balanced input to each EL84 grid has proven to give slightly lower THD
but probably made no change to sound.
driver stage is a differential long tailed pair, LTP,
with 2 x EL84
connected as triodes with balanced CT choke to supply dc to each triode.
The driver stage is extremely important for dynamic sound quality and the
use of EL84 in triode mode gives flawless performance. Each EL84
is equivalent to 5 half sections of a 6CG7.
Constant current sources for SET V1 anode is MJE350, and for LTP with
two halves of V1 6CG7 there is a constant current sink using MJE340.
The transistors are working as slaves to the tubes involved, and have no
active voltage amplifier role, and hence no sonic signature, and they
act as a better alternative to using resistances or active tubes.
Each 300W mono amp weighing 25Kg has its own remote power supply
also weighing 25Kg. If both power and audio circuits were on one chassis,
total weight would be 50Kg which is far too much for most people to easily
Each amplifier chassis has two industrial grade 1.2 metre long umbilical cables
hard-wired into the amp chassis. Cables are 5-core type normally used for
mobile gantry cranes needing 450Vac at 20A in each of the 5 wires in each cable.
I fitted reinforced octal plugs at cable ends. One cable carries all tube heater
power, the other carries B+ and other voltage needs.
Mains power draw.
Total mains input power is approximately 450Watts per channel.
Voltage selection can be made for 100V, 110V, 120V,
and 240V at 50Hz or 60Hz. There two power transformer primaries each
for 120V nominally, each with taps for lower voltages.
Australian mains voltages can often measure up to 250Vac. The 240V
setting is fine.
The cores are GOSS E&I laminations with high permeability, and weight = 20Kg.
Tongue = 51mm, Stack = 110mm, window L = 76mm, H = 25mm.
There are 5 primary sections each 212 turns 0.6mm Cu dia wire interleaved
between 6 secondary sections each 72 turns of 0.9mm Cu dia wire.
Each secondary is divided into 24t and 48t to allow a range of series-parallel
winding arrangements to give different load matches but while always giving equal
current density, leakage inductance and winding losses.
OPT bandwidth is 13 Hz to 270 kHz at 200Watts with no CFB or GNFB.
With NFB added and with some critical damping R&C networks the bandwidth
and phase shift is restricted to a safe 84 kHz, -3dB with resistance loads.
Pure capacitance loads are well tolerated.
Power transformers have
GOSS E&I laminated cores and are rated for
1,900VA with windings rated for 600VA. Iron losses are only 4VA with winding
losses less than 5%. Temperature rise and noise are negligible.
Active protection is provided so
excessive cathode current in one or more
output tubes will automatically turn off the amps. There is also inrush current
limiting at turn on to allow the use of useful value and sensitive mains fuses.
As I edit this page in 2014,
I have two mono amps and PSU nearly completed.
I would be happy to sell these amps if I receive a sensible sales offer.
Full service information is provided at this website. A printed copy of
schematics would be provided with amps when sold.
AMPLIFIER CHASSIS generate considerable heat and require a well
ventilated position and MUST NOT be placed on a heavily carpeted floor.
terminals are standard unbalanced RCA sockets.
terminals are 2 pairs of recessed 4mm banana sockets to
two pairs of speakers to be connected or bi-wiring one pair. Best speaker
cable connectors are gold plated 4mm banana plugs of good quality.
I do not like binding posts with screw tightened connections which always
become loose over time. Protruding binding posts tend to be broken off the
chassis or bent during amp moves.
If a speaker cable is accidentally yanked, you would want it to slip out and
away from the amp terminals, and not pull the amp off a bench onto the
floor. However, banana plugs can break off leaving the plug end in the socket
hole which must then be pulled out using special pliers. So speaker cable
placement away from foot traffic is extremely important.
chassis size and weight.
Each amp chassis is 630mm long, 250mm wide, and 230mm high and
weighs 24Kg. Chassis is welded steel frame, mild steel sheet transformer
enclosures and natural anodized aluminium top plate. The steel grille over
the tubes allows removal of tubes through grille openings.
supply size and weight.
Each power supply for each amp chassis is 300mm long, 250mm wide,
and 230mm high and weighs 26kg. The power supply enclosure is mild
steel sheeting, with the mains on/off switch for the channel mounted
in the top of the power supply cover.
any additional information contact Patrick Turner at email address
displayed at index page.
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