35 Watts Vacuum Tube Power Amplifier by E80CC and EL34

35 Watts Vacuum Tube Power Amplifier by E80CC and EL34


The circuit was designed to create a power amplifier circuit that will utilize E80CC and EL34 vacuum tubes for better results while providing 35 Watts of power.

Terminology E80CC – a special quality double triode vacuum tube that is a direct or indirect substitute of 12AU7/12AT7/12AX7 due to its identical heater voltage and pin out but draws twice as much as the heater current which can be used in a buffer stageEL34 – a glass envelope high performance audio power pentode having a plate dissipation rating of 25 Watts with convection cooling intended for audio frequency power amplification service in either pentode, ultralinear, or triode connection and single or push-pull/parallel applicationsZ2C – a medium power zener and avalanche diode sealed in D01 package with both unipolar and bipolar configurations with features such as fast response time, high temperature soldering guaranteed, excellent clamping capability, surge capability of 400 Watts, and less than 10uA above 10V of typical resistor current Circuit Explanation

The circuit was based on the Siemens and Halske classic design of 1953 where the final amplifier consisting of two EL34 vacuum tubes to produce an output power of 35 Watts. From the year 1954 up to 1989, the amplifier where this circuit was installed was being utilized. The normal operating time that has been consumed was 15 hours in one day, whenever the amplifier starts to be operated. During the time that it was being used, the only problem that has been encountered is the deterioration of the equipment itself and not to the extent of replacing the vacuum tubes, the resistors, or the capacitors because of no particular damage. This signifies that even that time, vacuum tubes are already considered as highly reliable components in an electrical or electronic circuit.

With the changing of specifications of vacuum tubes, a crucial transformation occurred as the tubes were replaced by diodes. At this point in time, a transformer was required for every manufacture which is the biggest problem encountered. Different transformers were made and can only be distinguished through the single code of every manufacturer. The use of vacuum tube EL34 can be subsidized by the use of a transformer which functions similarly as a push-pull.  Though this is the outcome, such care should be observed especially in handling high voltages to prevent the dangerous effect of an electric shock.

Part List

R2-5= 2K2 0.5W
R3=150K 0.5W
R4= 220K 0.5W
R6-10= 56K 0.5W
R7=3.9K 0.5W
R8= 220R  0.5W
R9= 1M  0.5W
R11= 39K 1W
R12-23= 180K 0.5W
R13-21=820K 0.5W
R14-22=5K6  0.5W
R15-20= 680K 0.5W
R16-19=100K 0.5W
R17-18=3K3 1W
R24=470R  2W
TR1-2=470R  1W Variable (adj. 270Ω)
C1-3-6-7=0.1uf 630V
C2=220pF 600v
C4-5=16uF 550V
C8-9=0.1uF 630V
C10-14=0.47uF 630V
C11-13=25uF 40V
Rectifier tube= Z2C
T1=Audio Transformer for 2x EL34 Push Pull


Together with solid state component, power amplifiers utilizing vacuum tubes are being designed in a preamplifier to be used in front panel input, microphone input, and line level input for a variety of functions. They are mainly used as an audio power amplifier to drive loudspeaker by amplifying the output of musical instruments.  The properties of vacuum tube amplification are combined with the modern technology of solid state electronics so it could be used in sound and general audio reinforcement. The power and reliability solid state could never match the superb efficiency, low cost, and reliability of vacuum tube electronics. But using both circuits will provide the best characteristics at a reasonable cost for an amplifier.

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