2 tube stereo amplifier with 2 x the ECL 80 definitive schematic, demo & how to make it (pt 4)

Please read the description/textbox first. Part 4 of the 2 tube amplifier videos. Final video where I demonstrate the 2 tube (ECL 80, triode-pentode, max 1.5 Watt) stereo amplifier.

Music = copyright free from You Tube. Artists: “fade away” by NEFFEX, and “pray” by Anno Domino beats. Links to earlier videos about this tube amplifier: at the end of this textbox.

Please note: I had to split up the 3 basic schematics (negative grid voltage 0-12 V DC, plate voltage, 150 V DC positive, filament voltage 6 V DC) to give a good idea about how to build such a circuit.

The circuit can be made with any normal triode-pentode, e.g. the ECL 82, or any other normal triode-pentode, that could give more audio output out (2-4 Watt).

The circuit is, in this way, in a kind of universal/adaptable and with a high success ratio because the negative grid voltage can be aligned for other (more powerful) triode-pentode tubes, that are/were made in Europe/America/Russia/China in the past.

When a power tube needs a higher negative grid voltage to work properly, you can adapt the circuit, using a higher voltage out transformer.

Always important: the audio transformer must match (in DC terms, in reality: in impedance terms) with the internal resistance of the pentode part of such a tube, so study the datasheet when you want to use more powerful tubes.

Read/study the internal resistance of the pentode, it is always in the datasheet.

A slight mismatch is not a problem.

Please note: in high quality tube amplifiers audio transformers are used that can reproduce the complete audio spectrum (40 Hz-18 KHZ) with a very low audio distortion, so with no peaks in certain parts of the audio spectrum, peaks due to internal resonance(s) of the transformer.

Bridged tube amplifiers are another chapter (not here). The “pentode way” of amplifying an audio signal (showed here) has certain properties, that differs from “bridged” tube amplifiers. That is not discussed here, the circuit is showed “as it is”, it is simple and everyone can make this with some insight in electronics/how a radio tube works.


1. The filament transformer is 12 V at 1 Ampère, supplying the 2 x 6.3 V/300 mA filaments
2. Pin 9 of the tube (triode-pentode) must be connected via a nonpolar capacitor of 470 N (=0.47 uF) to pin 1 of the tube. It couples the amplified audio signal from the triode anode to the sensitive input grid (pin 9) of the pentode
3. PIN 3 is the cathode that goes to ground (zero=0) for both tubes (!) (forgotten in the schematic to show that, but that will of course be clear when you work with radio tubes
4. The input grid (pin 9) of the pentode part must be connected to the negative grid voltage potentiometer wiper, so that you can bias the pentode. This has to be done for both tubes, they both need a proper biasing to avoid distortion of the audio signal
5. The loudspeaker must (ideally) be a broadband loudspeaker in the range of 60 Hertz to say 15 KHz (KC), their efficiency is in general high, because they are broadband types. Reason: we only have here 1-1.5 Watt audio out. But feel free to experiment with any loudspeaker box that you like and have (!).
6. Not showed: the value of both input (audio control) potentiometers is 500 K, so 500000 Ohm. Feel free to use 100 K or even 50 K if necessary to drop the audio input signal down to the level that you want/need
7. FORGOTTEN to tell: the 18 K resistor in the input circuit must be bridged with a nonpolar capacitor of 4N7 (4700 pF) minimal, or even 10 N (10000 pF). Reason: stability of the circuit, prevention of parasitic oscillations. Not soldering this 4N7 capacitor into the circuit gives problems!!

My You Tube channel trailer is here: https://youtu.be/xbgQ8T3oqh4 When you search, search always “NEWEST FIRST” to get the right overview. You can also search via the “looking glass” on my Channel trailer via keywords like ”the term that you want" (radio, audio or so).

My books about electronics & analog radio technology are available via the website of "LULU”, search for author “Ko Tilman” there.

I keep all my YT videos constant actual, so the original video’s with the most recent information are always on YouTube. Search there, and avoid my circuits that are republished, re-arranged, re-edited on other websites, giving not probable re-wiring, etc. Some persons try to find gold via my circuits. I take distance from all these fake claims. I cannot help that these things happen. Upload 7 July 2022.

Stereo tube amplifier with ECL 80 tubes part 1 (20 June 2022)
Stereo tube amplifier with ECL 80 tubes part 2 (25 June 2022)
Stereo tube amplifier with ECL 80 tubes part 3 (28 June 2022)
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