Hi-Fi Devices - Hi-Fi Amplifier with 36W Output Power (Ultra-Linear)

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Written by: Jan Różycki
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Eng. Jan Różycki, 1st Edition,
Państwowe Wydawnictwa Techniczne, Warsaw 1959

4.4. Hi-Fi Power Amplifier Circuits

4.4.4. Hi-Fi amplifier with 12 W output power (Ultra-Linear)

The amplifier in the circuit shown in Fig. 84 is primarily suitable for Hi-Fi devices intended for larger rooms and community centers.

Technical parameters:

  • output power 36 W,
  • frequency response from 20 to 20,000 Hz within ±1 dB,
  • non-linear distortion (400 Hz) 0.05% at 20 W, 0.2% at 36 W,
  • intermodulation distortion 0.8% at 36 W,
  • noise level -86 dB,
  • sensitivity 0.3 V.

Output stage

The output stage includes two EL34 tubes operating in a push-pull, class AB circuit with screen grid feedback ("Ultra-Linear" circuit). The operating conditions for these tubes in the above circuit are as follows::

  • anode voltage 400 V,
  • voltage on the screen grid Us2 400V,
  • cathode resistor 470 Ω (value with separate resistors),
  • operating resistance Zaa 6600 Ω,
  • resistor in the screen circuit Rs2 1000 Ω.


Fig. 84. Schematic diagram of an amplifier with an output power of 36 W (Mullards model 520)
R6=100KΩ; R7=400KΩ; R11=180KΩ-5%; R12=180KΩ+5%; R15=8200Ω; C5=220pF; C10 = C11 = 50μF/50V

The voltage distribution and percentage division of the primary winding of the output transformer are shown in Fig. 85.


Fig. 85. Percentage division of the output transformer windings for powering EL34 tubes in the "Ultra-Linear" configuration

The cathode resistors, separate for each tube, are wire-wound and rated for 5 W and have a tolerance of ±5%. All other resistors in the output stage can be rated for 0.5 W, also with a tolerance of ±5%. Since the voltage drop across each cathode resistor is 32 V, bypass capacitors C10 and C11 are rated for an operating voltage of 50 V.

Preamplifier for push-pull circuit

The preamplifier utilizes an ECC83 tube, whose triodes are directly coupled via a cathode resistor. The value of resistor R12 should be 5% greater than that of resistor R11. This requirement is necessary to achieve uniform voltage amplitudes controlling the output stage.

Amplifier input stage

The EF86 input stage is coupled directly to the preamplifier circuit and is feedback-coupled by applying a voltage from the amplifier output to the EF86 cathode. The specified values ​​for resistor R15 and capacitor C8 are only valid for a 16-ohm load on the amplifier. Resistors R6 and R7 should be high-quality borocarbon resistors.

Electrical parameters of the power supply:

Mains transformer

  • anode alternating voltage 2 × 400 V, 200 mA,
  • filament voltage of electron tubes 6.3 V, 5 A,
  • rectifier tube filament voltage 5 V, 3 A.

Choke DŁ1 – inductance 8 H, resistance 145 Ω, current load 180 mA.
GZ32 type rectifier tube or two 5Y3 tubes.

Jan Różycki