The general characteristics of the 300W monobloc amps are described in this page.
More detailed pages including all schematics are listed at bottom of this page.
Standard Output stage has 12 x EH6550 fitted.
Optional other tubes can be 12 x KT88, KT90, KT120, KT66, EL34, 6CA7, 5881,
Highest output power with 12 x 6550 and speaker load of 1/2 recommended = 400Watts.
Tubes other than 6550 may need workshop adjustment of several items mentioned
at technical page 300W amp power supply.
Class A1 power.
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 22W
of heat from anode and screen electrodes, and 11.4W 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 10W to
38W. The happy middle path is the best, and for two 6550 having Pda at 22W
each means adequate class A power is available for first 10Watts, and a high ceiling
of level in AB power is available up to 60W, 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 27W
with screen Pdg2 = 3 W for a total of 30W which is 71% of maximum allowed
Pda+Pdg2. With anode efficiency at 45% max in pure class A we might get 12W
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 11,000r.
Well, there are no manufacturers able to sell 24W 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 27W to say 25W
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 = 8W. This is still enough
class A for most people, and shops sell more 60W AB amps than 24W 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 = 20.5W = 50% of
max Pda at 42W. 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 324W AB max allowing 5% winding losses, and first 37W
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, 400W 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.
Two load settings are available for speakers, 2.5 ohms and 5.6 ohms.
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
power limit, Watts
AB1 power, Watts
AMPLIFIER CHASSIS generate
considerable heat and require a well
ventilated position and MUST NOT be placed on a heavily carpeted floor.
Input terminals are standard unbalanced RCA sockets.
Output terminals are 2 pairs of recessed 4mm banana sockets to enable
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.
Amp 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.
Power 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.
For any additional information contact Patrick Turner at email address
displayed at index page.
Browse the other listed 300W amp pages for more information.....
300W amp input/driver and output stages
300W amp power supply
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. Back to Power amplifiers
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