Notes on 300W monobloc amps in the picture above :-

The amps are nominally 300 watt class AB1 using 12 x 6550/KT88/KT90 output tubes, 2 x EL84 driver tubes, and a 6201(12AT7) input tube, pictured in 2002. The original 2001 schematic had basic 40% Ultralinear screen taps in the output stage and with about 15dB of global NFB.

Experience with other amplifiers and listening tests revealed better 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, and global NFB reduced to a maximum of 8dB.
The more linear and warmer sounding 6CG7 is used at the inut instead of the 12AT7.   

I have a brief description below but much more detailed technical information with full schematics and images can be found in the other pages at :-

http://www.turneraudio.com.au/300monobloc.html

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, more blue glow, 2 amps on bench, umbilical cables, amp end view.

I have one pair near completion Dec 2008.

Output tubes.
Standard Output stage has 12 x EH6550 fitted.
Optional other tubes can be 12 x KT88, KT90, KT66, EL34, 6CA7, 5881, 6L6, 6V6.
Tubes other than 6550 may need workshop adjustment of bias and feedback arrangements.

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 power liberated  in the output tubes. Therefore the maximum pure class A1 possible = 0.45 x 193.2 = 86.94 watts, but the speaker load value must be a fairly high number of ohms to achieve this.

With the amp set to match 2.5 ohms, and with 2.5 ohms connected, the first 30 watts is class A1, with a balance of over 200 more watts of class AB1.
With the amp set to match 5.6 ohms, and with 5.6 ohms connected, the first 30 watts is class A1, with a balance of over 200 more watts of class AB1.

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 virtually all pure class A.
Alternatively, with the amp set for 5.6 load match, a real load of 19.6 ohms will also give 87 Watts of pure class A1.

While it would be possible to have much more class A1 power if the output tubes had more idle dissipation, the tube life and tube reliability will be much less and  for goodness sake, who needs more than 87 watts of class A1?
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 closer to their dissipation limits will ever give better music.

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 output tube anode 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. 

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.

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 with the dc tube current.
The output tube grids are coupled to the EL84 driver stage anodes with 0.47uF caps plus 120k grid bias resistors to each output tube.

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 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.
One EL84 operating in triode mode is equivalent to 5 half sections of a 6CG7/6SN7.

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.
A transistor 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 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.

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 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 6 ohms connected to the 2.5 ohm outlet setting results with all output power being class A1.

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%.
THD at 25 Watts, 1kHz, 5.6ohms, < 0.04%
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.

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.
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.
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.
THEY MAY 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 will be warm.
The amplifier chassis do require well a well ventilated area, and will act as desirable 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 10k:10k 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.
I don't like binding posts because cables screwed down by the post nuts always tend to work loose over time. 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 want it to slip out and away from the amp, and not pull the amp off a bench onto the floor.

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.

For any enquiries email Patrick Turner at  info@turneraudio.com.au