17 January 2022

Abraham-Bloch valve-LED multivibrator

Mr. Abraham and Mr. Bloch invented the astable multivibrator during World War I. It used two triodes to produce a square wave, actually two waves, with 180° phase shift.

LEDs have been mass-produced since 1962 according to wikipedia. I am not sure when the high-brightness LEDs hit the market.

Regardless, I have built a circuit that spans a century of electronics: a LED blinker whose active elements are vacuum tubes. And it was built with an improvised freeform architecture.

The blinker is based on the Abraham-Bloch configuration and lights up two high brightness green LEDs

I challenged myself to use whatever tube I could find in my box, it just had to contain a triode. Since I have few PCF80 (and pin-compatible PCF802) triode-pentode, I built the first circuit around their triode. 9V to the filaments (in parallel) and 18V to the anodes, and it worked at the second attempt, as I forgot to ground cathodes at the first smoke test.

Abraham-Bloch astable multivibrator.
Abraham-Bloch astable multivibrator.

Component values for an HT of about 20V are: R=10 or 15 kohm, C=0.47uF/50V , R1=R2=1 Mohm. Filaments in series (18V, 300mA) or parallel (9V, 600mA) as you prefer.

First discovery. I decreased plate voltages down to 9V and it kept blinking. Now I can power the whole circuit with a single 9Vdc source (capable of at least 700mA). Component values untouched. Yes, filament and anode are powered from the same source. Vacuum tube and safe (not lethal) voltages, what a mix!

Second discovery. Since the circuit is symmetrical, if LEDs blink uneven the difference must be in the tube. Blinking frequency is dependent on the plate voltage and at 9V I am pushing the tube limit. I managed to balance the ON times by replacing one tube. I wonder if this visual effect can help to identify tube efficiency.

First question. PCF80/ECF80/PCF802/ECF802 also contain a pentode that can be configured to work like a triode: can I halve the power consumption and use just one of these tubes?

"Poor tubes!" "Don't misuse them!" I think that this kind of misuse is better than "no use to the eternity". Unless someone comes up with a use-case where vacuum tubes of unknown source and efficiency must be used.