Class A1 power.
Class A power depends on the idle bias current in the output tubes
and load match setting of the output transformer windings.
For 99% of listeners, there is **enough** pure class A power to cover
all the power required even with speakers rated for only 81dB/W/M.
Because there is no need to ever adjust the bias, automatic bias setting
is achieved with individual cathode bias capacitor and resistor networks
for each output tube.
This means you don't have an absurd 24 bias adjustments for the two channels.
I have set the idle current in each 6550 output tube for = 35mAdc, and
the Ea = 460Vdc approx, so each 6550 operates with Pda = 16.1 Watts.
The amount of Pda for 12 tubes =  193.2W.
The maximum pure class A power possible is 45% of the idle anode power
liberated  in the output tubes = 86.94 Watts, but the speaker load value
must be a fairly high number of ohms to achieve this.

Two load settings are available for speakers, 2.5 ohms and 5.6 ohms.

This may seem strange in a world where people like to see
4 ohms and 8 ohms printed on the rear panel of an amp.

But in fact a speaker with a nominal impedance of "4ohms" may in fact
have impedance which varies between 2 ohms and 20 ohms and 4 ohms
may be the average impedance between say 100Hz and 1,000Hz.
My amplifiers will happily drive low impedances.

Nobody I know would ever need more power than offered here.

The THD at a few watts is less than 0.03%.
I doubt anyone can prove to me that biasing output tubes closer
to their dissipation limits will ever give better music.

Local 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.
This causes the output tubes to behave more linearly than triodes and
with a lower plate resistance, while retaining the high output power
of a beam tetrode.
Global Negative Feedback is also applied 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 8dB of local CFB plus 8dB of global NFB to total
16dB.

Supply voltages.

Anode supply voltage is +500V.
All 12 output tubes have a common regulated fixed +375V screen supply.

Idle bias current in each 6550 is 35mA for long tube life.

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 Biasing.
The 12 x 500 ohm resistors regulate the anode current of each output
tube with +18Vdc at the cathode.
Fixed bias of -14Vdc is also applied to all grids to allow the highest
output power ceiling. Each output tube grid is separately RC coupled
to the EL84 driver stage anodes with 0.47uF caps plus 120k
grid bias resistors.

Matched tubes are not needed because the sum of the slightly different
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 output tubes.

Dynamic Bias Stabilization is a unique circuit technique used to
regulate 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 net 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 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 inaudible.

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
300Vrms+ grid to grid at low THD. A maximum of only 160Vrms
grid to grid swing is needed. The output impedance of the driver stage
is low, its bandwidth high and reliance on global NFB to reduce drive
amp distortions is minimal. Each EL84 in triode mode is equivalent to
5 half sections of a 6CG7/6SN7.

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. 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 be slightly superior.

Constant Current Sources.
In the 2008 schematics a constant current source is used for the dc anode
supply for 6CG7 input tube and a transistor constant current sink used
for the EL84 common cathode circuit to 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 hence
no sonic signature, 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 color 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 output tubes is about 240 watts plus about 8 watts in the
12 cathode resistors. The LTP driver and input stages use about 20 watts
of anode power and other losses. The cathode heaters require 150 watts.
With 4% mains transformer losses, total mains input power is thus
approximately 440 watts per channel.

Mains Voltage selection can be made for 100V, 110V, 120V, 200V,
230V and 240V all at 50Hz or 60Hz because there are TWO power
transformer primaries each for 120V with two taps for lower voltages.

Output transformers have a 110mm stack of 51mm tongue GOSS E&I
laminations. OPT bandwidth is 13 Hz to 270 kHz at 200 watts even with
no negative feedback. With NFB added and with some critical damping
networks the open loop bandwidth and phase shift is tailored to ensure
that the amps are unconditionally stable, and able to drive any type of load
including the most difficult ESL.
With the NFB the bandwidth is restricted to a safe 84 kHz, -3dB, with
resistance loads. 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 ideal load matches to
2.5 ohms and 5.6 ohms, thus allowing two ranges of *nominal* speaker
loads, one between 1 and 5 ohms, and the other anything above 3 ohms.
Using a speaker with nominal impedance of 8 ohms connected to the 2.5
ohm outlet setting results with nearly all output power being class A1.

Power transformers have GOSS E&I laminated cores and are rated for
1, 900VA with windings rated for 600VA. Iron losses are only 4 watts,
with winding losses less than 5%. Temperature rise and noise are negligible.

Note that if you only had 2 x 6550 to make 42 Watts max at 0.25%,
then at 25 Watts the THD = 0.20%, if operating conditions were identical.

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 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.
All parts are able to be replaced after re-making them if needed.
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.
5 years of tube life is not unusual if the amps are used on 200 days
per year for 4 hours each day or evening, which is 800 hours per annum.
Sometimes there is a random failure of a tube, more likely because there
are 24 output tubes. My experience is that early tube failure is unusual
when Pda at idle is less than 1/2 the maximum rated Pda and it is not
inconvenient if a few spares are kept on hand. Even with two faulty
tubes removed, the remaining 10 will work quite OK until you fetch a
replacement.

Service information.
Full service information is provided with each amplifier complete
with 8 schematics, explanations and amendments if optional
changes are selected.

WARNING  :-
THE POWER SUPPLY AND AMP CHASSIS CONTAIN
POTENTIALLY DANGEROUS VOLTAGES.
THEY MAY BE SERVICED ONLY BY EXPERIENCED
TECHNICIANS.

However, there would never be any need for an owner to remove
a cover over any circuitry because there is nothing he ever would
need to adjust. 

Power supply temperatures.
The power supplies have no tube rectifiers and run quite cool, and do
not require any special ventilation.

AMPLIFIER CHASSIS WILL BE WARM AND REQUIRE A WELL
VENTILATED POSITION.

Input terminals are standard unbalanced RCA sockets.
Balanced input for XLR is an optional extra and with a balanced
10k:10k input transformer.

Output terminals are 2 pairs of recessed 4mm banana sockets to enable
two pairs of speakers to be connected or bi-wiring one pair.
I do not like binding posts for spade or other screw tight methods
because the screw tight posts inevitably become loose.
Protruding binding posts tend to be broken off the chassis or bent during
moves and are a clumsy old fashioned way of connection and do not
contribute anything to sound quality.
If a speaker cable is accidently yanked, you would want it to slip out
and away from the amp, and not pull the amp off a bench onto the
floor so I much prefer banana plugs.

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. 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
info@turneraudio.com.au

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