Amateur electroacoustic amplifiers - 15W Hi-Fi amplifier

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Written by: Aleksander Witort
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Aleksander Witort, Amateur Electroacoustic Amplifiers
Third Edition, Warsaw 1972

Appendix 3 of the book on amateur electroacoustic amplifiers includes a brief description of an interesting tube amplifier design. This is a rare topology that uses an autotransformer as the output transformer. Yes, that's not a slip of the tongue. Such "tricks" are rarely used today, but several decades ago, it was not unusual. You can familiarize yourself with the full table of contents of the aforementioned book here.

Appendix 3

15W Hi-Fi Amplifier

      A characteristic feature of the amplifier described below is a simple, easy-to-make output autotransformer. The entire amplifier circuit was designed with this assumption in mind (Fig. D3-1).

   The autotransformer is connected to the electron tubes in such a way that, while maintaining the symmetry of the system, two conditions are met:

  • the constant component of the anode current does not flow through the autotransformer,
  • electron tubes are connected in parallel for the variable component of the signal and therefore the working resistance of the stage is small (800 Ω), which facilitates the design and construction of the transformer (autotransformer).


Fig. D3-1. 15 W Hi-Fi amplifier (diagram according to "Radio and Television" Bulgaria)

   A certain inconvenience is the need to power the system from two separate anode voltage sources.

   The driver stage must deliver a significant voltage, as the output stage cathode has a significant AC potential. This is possible because the tubes in this stage are supplied with an anode voltage consisting of a DC component and an AC component. As a result, the ECC83 tube's amplification factor is close to maximum, and the AC voltage amplitude at the anode can be higher. This stage also serves as a phase inverter.

   The first stage acts as a voltage preamplifier and its only interesting feature is its direct coupling to the control grid of the next stage.

   With the correct resistor values ​​and good capacitors, the operating points of the tubes in the first two stages will automatically adjust. The anode current of the first tube (EF86) will flow, resulting in a positive voltage of around 70 V across the driver grids of the ECC83 tube. Anode current will flow through both tube systems, causing a voltage drop across the cathode resistor (100 kΩ) of such magnitude that the cathodes will have a voltage several volts higher than the driver grids. It is advisable to select the output tubes so that their characteristics do not differ significantly. The DC components of the anode current of both tubes at idle and at full drive should be the same.

The amplifier specifications are as follows:

  • output power 12 ÷ 15 W,
  • harmonic distortion factor 1 ÷ 2%,
  • passband 30 ÷ 18000 Hz,
  • own noise 60 dB,
  • input sensitivity 0,25 V.

   The output autotransformer should be made on a core with a cross-section of 8–10 cm² and have a winding of 1300 turns. The winding arrangement is not critical. If we use a non-split coil former, we wind the following sections: AD, DF, FB, BG, GE, and EC. If we use a split coil former, we wind the FB and BG sections at the bottom, the DF and GE sections at the top, and so on.

   In this case, the winding halves (AB and BC) should be wound in opposite directions.

   The section details are as follows:

  • FB and BG – 60 turns each; wire ɸ 0.9 ÷ 1,2 mm,
  • DF and GE – 20 turns each; wire ɸ 0.8 ÷ 1,0 mm,
  • AD and EC – 570 turns each; wire ɸ 0.3 ÷ 0,4 mm.

   The power supply schematic is shown in Fig. D3-2. One of the mains transformers should have two filament windings (6.3 V), since the first two tubes and each output tube should be filament-fed from a separate winding. A separate small filament transformer can also be used. Power supplies should be designed to deliver 60 mA at a voltage of 250 ÷ 280 V.


Fig. D3-2. 15W Hi-Fi Amplifier Power Supply

   The amplifier's design can be customized as long as general principles of proper component layout and assembly are followed. With the exception of the output autotransformer, all components are standard and readily available.