Converting old relaxation oscillator vacuum tube circuit to a more modern circuit?
It's also a &quot;gas tube&quot; or &quot;soft valve&quot; which has quite different characteristics from any &quot;modern&quot; (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 &ndash; 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 &quot;attack&quot; which is controlled by R1 below. Suitably high values of R1 (giving RC &gt;= high time) will also control the volume by not allowing the output pulse to rise to the full input.