300 watt
amp chassis and power supply.
Here we have a picture of a 300 watt class
AB1
amplifier using 12 x 6550 output tubes, 2
x EL84 driver tubes, and a 6201(12AT7) input
tube, pictured in 2002. The circuit
design at that time was basically
based around conventional ultralinear with 40% screen taps.
After some careful listening tests it was found
that slightly better sound can be had using 20% local cathode feedback
within the output stage instead of the 40% screen taps.
Screens are now taken to a regulated
screen supply rail.
A brief description follows below but much
more detailed technical information with full schematics and images can
be found in the other pages associated with these
amplifiers listed at the index page.
Output tubes.
Standard Output stage has 12 x EH6550 fitted.
Optional other tubes can be 12 x KT88, KT66, KT90, EL34, 5881, 6L6,
6V6.
Tubes other than 6550 may need workshop adjustment of bias and feedback
arrangements.
Class A power.
Class A power depends on the idle bias current in
the
output tubes. With Ia = 35mA per tube, the first 30 watts is class A
when 2.5 ohms
rated load is used with 2.5 ohm load match setting, or 5.6 ohms rated
load
is used with 5.6 ohm load match setting. With the anode idle current
set at at 35mA per tube the anode heat
dissipation per tube = 17 watts so for 12 tubes
it is 204 watts.
Since class A efficiency for this mode of operation cannot exceed 44%,
a maximum of 90 watts of pure class A power can be
produced,
but the load has to be about 3.5 times the rated load of 2.5 ohms or
5.6
ohms depending on the output transformer ratio match setting.
So with the amp set for its nominal 2.5ohm load output, with a real
load = 8.75 ohms the maximum power will be 90 watts and all pure class
A.
Alternatively, with the amp set for 5.6 load match, a real load of 19.6
ohms will give 90 watts of pure class A.
Load matching.
Two nominal selections of load matching are available and
are to
suit
either 2.5 ohms or 5.6 ohms for class AB1 with the tube load being 1.2k
anode to anode in both
cases. These are
not easily alterable by an owner, since there are 24 connections to
alter on the output
transformer terminal boards.
Class A proportion of output
power.
The portion of Class A power can be increased to a maximum of
150
watts at 1.3 times the rated loads when bias current in each tube is
increased to a safe maximum of 62
mA with the plate supply of +490V, which is the highest setting for B+
possible in these amps at the
higher anode current draw.
The increase in class A power can be achieved by reducing the -14V
fixed bias supply voltage, allowing
the cathode bias to increase from approx +19V to +32V, so the anode to
cathode voltage, Ea, for class A operation is reduced from about +480V
to about +455V which prevents maximum class AB power from being
produced when the amp has its idle current set at 35mA/tube. When set
up for class A, there must be an adjustment made to the bias protection
threshold circuits.
The dissipation per tube, Pda, with Ea = 455V and Ia = 62mA is 28
watts, a
safe figure for 6550, so for the 12 tubes
there is 336 watts total anode dissipation so at 44% efficiency the
maximum pure class A = 148 watts.
The load at which this pure class A power can occur is 3.27 ohms when
set up for the nominal load match of 2.5
ohms, and 7.28 ohms for when set up for 5.6 ohms nominal.
If the 157Vac tap on the HT winding of the power transformer is
chosen
for the B+ supply, B+ = +385Vdc approx, and
Pda per tube is allowed to be 35 watts per tube by raising Ia to 90mA,
then we have total Pda = 420 watts which enables 185 watts of pure
class
A at approximately the two possible rated loads, with a considerable
class AB extra portion
of power
if the load is less than the rated loads. However, I see absolutely no
reason to force the tubes to work in
excessive
amounts of class A because tube life is shortened without any huge
increase increase in
fidelity, and
I prefer to limit anode idle current
to only 70mA so that Pda per tube never exceeds more than about 30
watts for the 6550/KT88/KT90 and a lot less if people wish to use EL34
etc.
The THD at a few watts is less than 0.03% and using much higher idle
bias
currents will reduce this by only about 6 dB.
I doubt anyone can prove to me that biasing output tubes at
close to their dissipation limits will ever give better music.
Negative feedback.
The output stage has 20% of the tube signal voltage as
local
cathode feedback applied from the output transformer to the output tube
cathodes, and is equal to about 8dB of
locally applied NFB.
8dB of traditional global NFB is also added from the
output
transformer speaker secondary to the cathode of the input triode in the
conventional manner to reduce output resistance and distortions so that
there is a total amount of local CFB plus global NFB = 16 dB at rated
loads.
Supply voltages.
All 12 output tubes have a common regulated +375V screen
supply.
Anode supply voltage is +500V and partial fixed grid bias to all output
tubes = -14V.
Idle bias current in each 6550 is 35mA for long tube life, but this
can be increased to 60mA for a larger % of class A power within the
total AB power
range.
The maximum class AB power is reduced marginally when operating the
amps for
maximum class A power.
Cathode biasing and coupling caps.
Each output tube has 500 ohms (rated at 15 watts
each)
plus 1,000 uF networks at each cathode to provide automatic Cathode
Bias.
The output stage uses 120k grid bias resistors to each output tube and
individual 0.47
uF coupling caps between the EL84 driver triode anodes and each of 12
output tube grids. The
low value of grid bias resistance ensures the output tubes do not tend
to suffer from positive bias drift
due to normal tube ageing.
Matched tubes are not mandatory.
Twelve matched output tubes are not needed since with so many output
tubes the sum of the characteristics of the six tubes on each side of
the PP output stage will usually be close. The self regulation of
individual cathode bias for each output tube also makes it less
critical to use matched output tubes.
Dynamic Bias Stabilization.
Dynamic bias stabilization is a unique Turner Audio circuit
technique used to regulate cathode bias during
high power class AB operation when cathode bias voltage tend to vary
and 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 net result allows the amp to have the same low
distortion advantages of a fixed bias amp but eliminates any need for
bias adjustments which trouble many owners
greatly.
Solid 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 using
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 imperceptible.
Driver stage
The 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. This enables the stage to produce
a high voltage swing of 150Vrms+ at low THD, but only a maximum 80Vrms
swing is needed at output
tube grids.
The output impedance of the driver stage is low, its bandwidth high and
reliance on global NFB to reduce drive amp distortions is minimal.
Input stage.
The input stage is a 6CG7 twin triode with both
triodes
in parallel. Although set up in common cathode mode, its acts as a
differential amp with the signal input fed to the
main high impedance input
grid and the second low impedance port is the cathode, to which
is
applied the global NFB
from a low resistance voltage divider from the OPT secondary speaker
connection.
Constant Current Sources.
Transistor constant current sources are used for
the
dc supply for EL84 cathodes and 6CG7 input tube plate supply to ensure
low distortion and excellent
drive voltage balance. The transistors are working as slaves to the
tubes involved, and have
no active voltage amplifier role, and they act as a better alternative
to using resistances or active
tube elements.
Umbilical cables.
The power amp chassis each have two industrial grade 1.2
metre
long umbilical cables hard-wired to the amp chassis with octal plugs
which are plugged into
the colour coded reinforced sockets on the power supplies.
See 300 watt images page for umbilical cable details.
Mains power draw.
With Ia = 35mA per output tube and B+ = +500V, each
output
tube has about 20 watts of combined anode plus screen input
power,
so total for 12 tubes is about 240 watts, plus about 8 watts in the 12
cathode resistors. The LTP driver and input stages use about 27 watts
of
anode
power and other losses.
The heaters require 150 watts. Total input power is thus 425 watts,
and with transformer losses at 5% approx, total power from the mains =
446 watts per chassis.
Output transformers.
The output transformer uses a 110mm stack of 52mm tongue
GOSS
lams, and has a bandwidth of 13 Hz to 270 kHz at 200 watts even with no
negative feedback used. With NFB added, some critical damping is used
to
tailor open loop bandwidth and phase shift to ensure that the amps are
unconditionally stable, and able to drive any type of load, and have
their bandwidth restricted
to a safe 84 kHz, -3dB.
Five primary winding sections and six secondary sections are used to
achieve the flawless high frequency response.
There are 2 secondary windings per secondary section allowing for two
waste free and uniform current density arrangements to give the load
matches
to 2.5 ohms and 5.6 ohms.
Power transformers.
The power transformer core is rated for 1,
900VA
with windings rated for 600VA and the iron losses are only 4 watts,
since
the same high quality grain oriented silicon steel E&I laminations
are used in all the transformers throughout.
Bandwidth, SNR, THD.
Operating bandwidth at 250 watts = from 13 Hz to 84 kHz,
-3dB.
Input impedance = 80 kOhms.
Output impedance = 0.45 ohms, 5.6 ohm load match, 0.22 ohms for 2.5
ohm load match.
Damping factor = better than 10.
SNR. At 200 watts into rated load = -90dB unweighted, and noise
at zero signal < 1 mV.
THD at 250 watts, 1 kHz, 5.6 ohms < 0.25%.
Protection.
Active protection is provided so excessive cathode
current
in one or more output tubes will automatically turn off the main anode
supply at the power
transformer. There is also inrush current limiting at turn on to allow
the use of sensitive mains fuses.
Warranty.
Warranty on tubes is 90 days, and two years for all
amplifier
parts, but if an amp is dropped or altered or used incorrectly, it
would
void the warranty. But most parts are easily replaced, or able to be
re-made.
Output tubes are most likely to wear out first but after 4,000 hours
cathode electron emission may only fall 10% which will have an
imperceptible effect on music and little effect on
measurements.
If the amps are used on 200 days per year for 4 hours each time, it is
800 hours, so 5 years of life is not unusual.
Sometimes there is a random failure of a tube, especially since there
are so many, but my experience is that early tube failure is unusual
and not inconvenient if a few spares are kept on hand.
Service information.
Full service information is provided with each amplifier
complete
with 8 schematics, explanations and amendments if optional changes are
selected.
THE POWER
SUPPLY AND AMP CHASSIS CONTAIN POTENTIALLY DANGEROUS VOLTAGES AND ARE DESIGNED TO
BE SERVICED ONLY BY EXPERIENCED TECHNICIANS.
Power supply temperatures.
The power supplies have no tube rectifiers and run quite cool,
and
do not require any special ventilation.
Amplifier chassis temperatures.
The amplifier chassis do require
well a well
ventilated
area, and will act as additional room heating in winter.
Input terminals.
Standard input is with one unbalanced RCA socket.
Balanced input for XLR is an optional extra and with a balanced input
transformer.
Output terminals.
Standard output sockets are 2 pairs
of
recessed 4mm banana sockets to enable two pairs of speakers to be
connected or bi-wiring. Binding posts can be provided but I don't like
them because they always tend to
work loose over time, tend to be broken off the chassis or bent during
moves, are a clumsy old fashioned way of connection,
and don't contribute anything to the great sound.
Amp chassis size and weight.
Each amp chassis is 630mm long, 250mm wide, and
230mm
high and weighs 24Kg,
and made with a 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, and can be arranged to suit
100V to 240V, 50Hz or 60Hz operation.
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.
Contact.
For any additional information contact Patrick Turner at info@turneraudio.com.au
Browse the other listed 300W amp pages for more information.