# Converting old relaxation oscillator vacuum tube circuit to a more modern circuit?

I am trying to figure out how to convert an old relaxation oscillator circuit from the late 30's. The history of this circuit, is this is the buzz generator for the Bell Labs Voder, which is a speech synthesizer that is played using a keyboard. I am planning on making a clone of it.

This circuit combines the relaxation oscillator, and also a way to change what the voice sounds like using 2 hand adjustable potentiometers, with a pitch pedal (which is also a potentiometer,) which changes the pitch of the output of the signal. The confusing part of the circuit, there aren't any values to the circuit, and that it has 3 different voltage inputs. I'm not sure if they are the same voltage to each or if they could be 3 different voltages in the circuit.

• Thats a tube circuit so the voltages really are three different DC supplies. The output is probably 100-200VDC or so. The inputs would be negative polarity (whats shown is not a typo) of magnitude maybe -20V each (not the same supply) All values would be dependent on the tube selected, obviously that circuit is not expected to actually be built. You could build it but you'd need some background on how to design with tubes. I know thats not what you're asking... You can substitute an N channel depletion mode MOSFET for the tube, thats the closest equivalent to a tubes characteristics. Commented Sep 9, 2020 at 2:57
• Sorry... The output from the TUBE is 100-200V DC. The output from the TRANSFORMER is something else and has no DC.... Meaning that 3rd voltage supply is in the 200VDC range. Commented Sep 9, 2020 at 4:56
• You can still buy a suitable electron tube. This is a thyratron relaxation oscillator. The thyratron is not directly and simply replaceable, but the closest semiconductor with a similar function is the thyristor. An important difference is that the thyratron is voltage controlled (high input impedance) and the thyristor is current controlled (low impedance). Commented Sep 9, 2020 at 16:59

It's also a "gas tube" or "soft valve" which has quite different characteristics from any "modern" (post-WW2) tube. Even in the 1930s they were a rarity, like unijunction transistors today.

Your best bet is to forget it altogether and concentrate on how you would re-create the output waveform today.

Start by generating narrow pulses (555?) with fixed (short) high time and variable low time.

Then add an R-C filter with separate RC time constants for attack and decay, like this: (adjusting component values to suit...)

simulate this circuit – Schematic created using CircuitLab

The left hand pot adjusts the range of the Pitch pedal, the right hand one adjusts the grid bias, which will mainly affect the amplitude of the output (though it may also affect the "attack" which is controlled by R1 below. Suitably high values of R1 (giving RC >= high time) will also control the volume by not allowing the output pulse to rise to the full input.

• What I don't understand about the relaxation oscillator, is that with a modern version of the relaxation oscillator , it seems like it has a single frequency that you can set, as it looks like the frequency doesn't change on the output on various relaxation oscillator circuits I have seen online. The relaxation oscillator above , the Voder uses infinite frequency ranges due to how multiple frequencies are required to be able to do different vocal sounds. So how do you get multiple frequencies to be outputted with a modern relaxation oscillator circuit? @Brian Commented Sep 9, 2020 at 16:11

Unijunction transistors took over the job of relaxation oscillators more than 50 years ago. The original types may be obsolete but programmable ones like the 2N6027 are available.

https://www.electronicscomp.com/datasheet/2n6027-transistor-datasheet.pdf