Part 2 of the project to make a simple stereo amplifier (say 2 x 1-1.5 Watt) with the ECL 80 triode-pentode.
Please read the description/textbox first. Important: the underside of the wood plate (say 2 cm thickness) where all is mounted on (with brass nails) is covered with tin plate out of softdrink cans or beer cans. The tin plate is glued to that under side. Plates of aluminum of soft drink cans do not work: they have almost no shielding effect (read: hum prevention) & you cannot solder to them. Thus use steel tin plate and solder the "earth" wire on the front of the electronic circuit to that tin plate on the back, it prevents hum & gives good shielding.
On 4.18 in the video I say "without minimal distortion" but of course I mean (driving the tube): "with minimal distortion", because of its proper biasing, set via the 100 K potentiometer.
First video about this tube amplifier is here https://youtu.be/Kw8hp9mJBVE
In this next video I show the following step of making this stereo tube amplifier.
Making a 2 x 6.3 Volt filament current/voltage supply with the lowest hum as possible (with a simple setup) with a 12 Volt/10 Watt transformer. Both tubes need 6.3 Volt at 300 mA, so I switched the filaments (watch the video) in series and made approx. 12.5 Volt with a very low ripple (10 Mv = 10 Millivolt at 12 Volt) at 300 mA, via this 10 Watt/12 Volt transformer. Open AC voltage of that transformer is/was 15.6 Volt AC.
Filtering the hum/ripple away was done by the 3 pole filter, that presses down the mains hum (in my case 50 Hertz) to the filaments to an extremely low level. So that the filament supply voltage/current looks battery (completely DC) like.
The hum suppression filter was made with 3 electrolytic capacitors with 3.3 Ohm (5 Watt) resistors in-between, schematic is in the video. I am sure this also will work in a (mains supply) 60 Hertz system/110 V AC system, given the setup (suppression by an R-C filter, not a L-C filter).
Second important part was to create the negative grid voltage circuit. Made with a mini transformer, because we only need (a little bit) Voltage (zero to max 15 Volt negative) here, almost no current, say a very very tiny current. Idea: in the 1920's that negative grid voltage was realized via a carbon-zinc battery that could last for 2 years.
The transformer for the negative grid voltage (not very clearly showed in the schematic, sorry for that ) is a mini transformer of 230 V/50 Hertz to 12 volt AC, it can give out 12 Volt at 0,75 Watt.
Open voltage (after rectifying via a bridge rectifier, = AC x 1.4) of that transformer is approx. 24 Volt.
That output voltage is given to the 2 pole filter, made with the electrolytic capacitor of 100 uF (35 V), a resistor of 47 K, and a second electrolytic capacitor of 1000 uf (35 V). Schematic is visible in the video.
Also here: this voltage must be completely free of (50 Hertz or 60 Hertz) ripple/hum. Such a hum/ripple will modulate the pentode grid, causing hum on the background when an audio signal is amplified by the tube.
I hope this R-C filter be be good enough, if not the 1000 uF capacitor can be bridged with a capacitor of 4700 uF. Though, on the oscilloscope, this does not add very much to the hum suppression, wait for the final video (video 3 in the series, I hope to upload it within 2 weeks).
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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 25 June 2022.