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.