3-Knob Tube Driver Build.

[Marcel]

Cheers for that Marcel, I downloaded a copy of that pdf, I've experimented with the Valvecaster circuit in the past but wasn't able to get it working at it's best, maybe the valves I was using were getting a bit worn out, the circuit tended to be a bit hummy, I'm thinking of building a circuit that uses two 12AX7 valves, but with a higher HT supply that's produced by a square-wave oscillator made from a CD4049 IC that drives a Voltage Multiplier circuit consisting of some 1N4007 (or similar) diodes and some 1uF/100V, or 2u2/100V (can even be some 47uF/100V caps too) electrolytic caps, I found that using the circuit I can easily generate up to something like +90V from a 15V DC supply.


Here's the Voltage Multiplier circuit:




The 10k resistor on the output of the Voltage Multiplier is there just to limit the output current a bit, the square-wave oscillator consisting of the CD4049 IC, 33k resistors, and 220pF cap runs at about 60kHz.

The CD4049 IC has six Inverters in it, three of them are used to form the 60kHz square-wave oscillator and the other three are used as inverting-buffers so that there is enough current drive to drive the Voltage Multiplier section.

I definitely need to get to it and do some breadboarding soon.

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"....

[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.