No.50
Hi-Fi Audio
SV6L6, EL34, SV6550C

This five-stage audio amplifier delivers 150 watts continuous output power with less than 0.5% distortion. Full-power bandwidth of the Six Pack goes from well below 20 Hz to over 15 kHz. This amplifier was designed ONLY with currently manufactured, readily available components, such as Svetlana tubes and Hammond transformers.

Its superior performance is due to its use of direct-coupled cathode-follower drivers and multiple feedback loops. Additional features include individual cathode resistors which facilitate tube balance checking, two individual bias controls, and an AC balance control.

This amplifier may run Svetlana SV6550Cs, EL34s or SV6L6GCs. Choice of output tube is up to the end-user and would depend on personal sonic preference.

Due to the extremely high power output at low frequencies, it is important that the power supply for the front end be highly isolated from the output power supplies. This is why a separate choke-input supply is provided for the front end. To further enhance stability, an electronic regulator is used to supply the screen grids. (see fig 3)

Sensitivity of the amplifier is such that an input of about 2.0 volts peak will drive the amplifier to full output. High-speed silicon diodes provide rectification for the three unregulated power supplies. An electronic regulator is used to derive the fourth voltage level for the output tube screen grids. All of this power is provided by two Hammond transformers; a 278CX power transformer and a 167U6 (16 amp at 6.3 volt) filament transformer.

Due to the relatively small power transformer used, the amp is 'only' capable of outputting about 150 watts continuous. However, the peak power is much higher, on the order of 250 watts. This gives the amp an excellent power reserve when amplifying music.

The first and second stages of the Six Pack Amp are provided by one 6BM8 tube. The high-mu triode section is used for the voltage amp and the pentode section is triode-connected to form a split-load phase inverter. The phase inverter feeds a push-pull amplifier stage to complete a 'Williamson' type front end circuit. See Fig. 1 for illustration of the front end schematic. A 1k potentiometer in the cathode circuit of the push-pull stage provides AC balance control adjustment.

To handle the drive requirement of the Six Pack output stage, a pair of cathode followers were built around the husky pentode sections of 6BM8s. They are direct-coupled to the grids of the output beam-power tubes. Two potentiometers are coupled to the grids of the cathode followers to provide independent bias adjustment for the two halves of the output stage.

Grid stopping resistors are used to control oscillations. 100-Ohm 2-watt resistors are used on all of the screen grids, to limit current and act as stoppers. 2.2k-3K resistors handle the control grid stopping duties.

Each output tube's cathode attaches to ground via a 30-ohm two-watt resistor. This resistor helps to stabilize the amplifier by providing some degenerative feedback on the output stage; it is also used to check tube balance between tubes on the same side. Bias is set so that the cathode resistor of the tube with the highest current is 50 mA, for a reading of 1.5 volts across each 30-ohm resistor.

The output transformer used is the hefty Hammond 1650W output, which weighs nearly 30 pounds. Local feedback is provided from the plates of the output tubes to the cathodes of the push-pull voltage amplifiers. This feedback improves the transient response and overload characteristics of the Six Pack amplifier. A snubber circuit across the primaries of the big Hammond controls ringing, which is caused by leakage inductance.

Two Hammond 11" by 17" chassis were used to prototype this monoblock amplifier. One of the chassis was used to mount the power supply components. An interconnect cable with a Molex connector connects the two chassis together.

A custom chassis was designed and prototyped for this amp; it is 11 by 18 inches deep and constructed of welded 1/4 inch aluminum stock, prepunched, drilled, tapped and nicely painted with scratch-resistant powdercoat. Only one chassis per channel is required with this option. It is set up to accept the three Hammond transformers; a sub-chassis mounts all of the capacitors and circuit boards required. Contact the author (dwolze@aimnet.com) regarding the chassis.

Figure 1

Figure 2 and 3

**The information provided in this application note is intended for general design guidance only. The user assumes all responsibility for correct and safe usage of this information. Svetlana Electron Devices does not guarantee the usefulness or marketability of products based on this material.

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