3-Knob Tube Driver Build.

[DrNomis_44]

If it were me I'd be increasing the size of the cap on the oscillator circuit to lower the frequency to just out of earshot range. The charge caps in the multiplier will work and hold charge better at 20kHz than at 60kHz.
There won't be much power coming out of the IC, so if there is a voltage sag issue on the tube's plate resistor then I'd consider using the output of the IC to drive a transistor switcher so that more power (wattage) can get put into the multiplier so that the multiplied voltage is "stronger"....

So maybe increase the 220pF cap to maybe something like 1nF, or 2n2?

[Marcel]

So maybe increase the 220pF cap to maybe something like 1nF, or 2n2?

Doubling the value should halve the frequency so 1nF might be a bit big, try a 680pF first and measure the frequency if you can.

[DrNomis_44]

Doubling the value should halve the frequency so 1nF might be a bit big, try a 680pF first and measure the frequency if you can.

Can do, I think I have at least one solderless breadboard lying around in the studio room in my new flat, I also have my old DSE 20Mhz Dual Trace Oscilloscope so I'll be able to make ballpark frequency measurements, definitely going to invest in a new Digital Scope later this year.