Integrated 5050

This is an integrated amp which was custom built in 2000 for a client in Victoria. While I built the amp,  my customer's
father who lives near me here in Canberra made some very solid speaker enclosures using NSW north coast blue gum planks
using the 'Sublime' design seen in my speaker pages. When the amp was finished I installed the drivers into the enclosures
and carefully levelled the response with a week long testing process.
Finally on one afternoon the old man and I had a listening test over a cup of tea.
We agreed the system had something really good!
My client's father later delivered the whole system to his son during a drive down to Geelong in Victoria.
After a fortnight, my client phoned to say that he had never heard a real system ever like this and that the sound appears to come from heaven. I mentioned that his dad is a retired Reverend, and so his system was at least helped by someone well connected to "Upstairs".

The amplifier has quite special output 5Kg transformers using excellent C cores that were available in Australia at that time.

The circuit configuration has 5 selectable inputs, a line level pre-amplifier, and two Ultralinear output stages with famous
NOS 6CG7  made in Australia and Russian Sovtek KT88. There is a power output of 53 watts per channel class AB1 into 4 ohms, or 36 watts per channel class A1 into 8 ohms.
Finish is brass and aluminium chassis, aluminium painted box over the transformers.
Not shown is the perforated steel cover screw fixed over the tubes.
Size is 470 mm wide, 350 mm deep, and 220 mm high.
Weight is about 30 Kgs, and the performance is quite blameless using Sovtek KT88,  but could use many other octal tubes
such as Electro Harmonix EH6550/KT88/KT90, or 6L6GC or KT66 with slightly reduced bias.

Bandwidth at 40 watts into 6 ohms, 14 Hz to 68 kHz.
Distortion at 40 watts, less than 0.2%,
Distortion at 3 watts, less than 0.05%,
Output impedance less than 0.3 Ohms.
17 Db of global NFB.
Stability is unconditional, and the amp can be used with any type of speaker load.
Fixed bias is used with full active protection and bias balance led indicators are mounted for easy viewing on the front panel. When both are extinguished, bias current balance in each channel  is set correctly.
No external preamp is required, unless a separate phono preamp is desired.

Schematic of  50 watt UL AB1 power amp....

Those of you who recall the 50 watt amp schematic I posted in the last edition of the website will see some differences
between the 2000 version and the 2006 version shown here.
The main improvement is in the driver LTP stage, V4, V5, so that each half of the LTP is a complete paralleled twin triode.
The bias resistors R17, R18 have been reduced from 220k to 68k to give better dc stability since less dcV is generated across the 68k as tube age. The use of the extra twin triode to each channel improves the dynamics of the sound and reduces distortion.
 
Elsewhere the schematic remains unchanged.

The circuit works as follows :-

The input signal is selected by the rotary wafer input switch and is attenuated by about 4 dB by the C1, R2, R3, and balance control network.
V1 is 1/2 a 6CG7 working as a very simple SET preamp stage which amplifies the signal about 10 times, 20dB, and the
anode output powers the 50k log Alps Black volume control pot. Preamps are rarely as simple as this but it works and sounds very well and since no long cables are used between the preamp and power amp the bandwidth remains wide.

V2 is the first triode of the power amp and is a SET stage acting as a differential amplifier to accept
the signal input at its grid and the feedback signal at its cathode, so the difference between the feedback signal and input signal make up the drive voltage between grid and cathode. The anode output voltage of V2 is fed to the network formed by
C3, C6, R12, and R13.
This network reduces gain and phase shift at below 20Hz to allow excellent LF stability when FB is used.
C7 and R11 also act at high frequencies above 10kHz to reduce gain and phase shift so that stability is excellent
when NFB is applied. The "gain shelving" networks in this group of components are known as critical damping
components and are chosen to suit the characteristics of the output transformer with regard to its shunt capacitances,
leakage inductances and primary inductance. Also involved in the effort to make the amp unconditionally stable
with NFB applied is the zobel network R32 and C11 across the output, and the phase advance capacitor C12,
tied across the feedback resistance, R33.

Warning! Anyone else trying to build this circuit with different quality output transformers to mine will need  choose their own values of critical damping components very carefully lest their amp may oscillate.

The reason for critical damping is to reduce the amount of NFB applied at frequencies where it is not needed to be applied,
ie, below 20Hz and above 20 kHz, because with heavy NFB applied outside the audio band there is a great tendency
for oscillations due to the phase reversal character of the amplifier without NFB.

The output from V2 is applied through the network to the single active grid input of the differential amplifier, V3, and V4.
The differential amp is also called a "long tailed pair", or LTP.
V3 and V4 have exactly equal loads at their anodes and have a transistor constant current sink to pass the tube current from the commoned cathodes to the -87V bias supply. The ac impedance at the collector of the MJE340 is many megohms,
so the transistor has no active signal controlling effect. I could have used a tube for this CCS but there is no need since the tube would have no other function than providing a high ac impedance at the common cathodes.

The grid of  both V3 and V4 are held at 0V potential via R13 at V3, and because V4grid is taken directly to 0V.
Incoming signals to V3 cause a current change  which is mirrored in V4, since the sum of the +ve and -ve moving ac currents must always remain constant and equal to the constant current floe into the collector of the MJE340.

Therefore the outputs at V3 and V4 anodes are exactly the same in amplitude but are 180 degrees out of phased.
so while one anode travels +50V, the other travels -50V.
There is no dependence on matched triode halves for this type of LTP, and nothing to adjust to get the ac signal output voltages to balance.

The LTP output voltages are applied to the V5, V6 output tube grids.  These are biased through R17, R18 , 68k,
which are fed a negative voltage from the adjustable network R19, R20, R22 plus a balancing potentiometer
to adjust the balance of dc current in each output tube.

The biasing system has a fixed grid voltage bias supply which adjustable only by balancing the applied grid bias so that
if the 10k pot is turned, V5 grid will rise in dc voltage while dc voltage in V6 grid will fall, and the effect on the idle anode currents of each tube is monitored by the two LEDs at the front of the amplifier which normally stay extinguished.
If one LED starts glowing, it means more current is in that tube than the other, so the balance can be correctly
adjusted when both LEDs remain extinguished. Anode idle dc current in each tube are within 3mA of each other when
both LEDs are extinguished.
No special tools, voltmeters or technical expertise is need for adjusting the bias of this amp. The LEDs will also
indicate a fault with an output tube. If an owner cannot balance the bias because there appears to not be enough
turn available on either of the adjust pots to extinguish the pair of LEDs for each channel then there is a problem with bias.
Thus an owner knows if he has a tube problem or other problem well before any serious damage occurs.

The amp is fully actively protected against excessive cathode current in one or more of the output tubes.

     The power supply for 5050,
 

Silicon rectifiers are used to give the best efficiency, and regulation and trouble free operation. C-L-C filtering of the +500v anode supply is used to keep noise to extremely low levels. 12V regulators 7812 are used with an 1N4004 in the adjust leg
to get close to 12.6V for dc heater supplies. The power supply uses a large conservatively rated power transformer with a low temperature rise, and excellent self regulation.

The active protection schematic and bias balance indicator amps :-

I hope everyone will forgive me for  posting such an untidy rough and scribbled sketch of what
is a very effective protection and bias balance amp for a stereo amp using a pair of KT88/6550 output tubes
in each channel. ( I will try to get around to re-drawing this schematic better....)
It is an old schematic but it works well and provides 3 things, bias balance indication,
a 3 second delay to shunt out a resistor to avoid inrush current, and active protection against
bias failure of one or more of any output tubes.
People are mad if they build a wonderful 5050 amp without a board with a circuit like this to manage the behaviour of the amp.

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