After finding a couple of triodes lying around I decided to build a valve preamp. I've built a simple boost converter based on a 555 timer to generate anode voltage.
The circuit is powered from a 9V mains adapter. VR1 is adjusted so that I get approximately 200V output.
'HV' output goes directly to the anode of the valve through an RC-filter (R=33 Ohm and C=220uF).
When I probe the output of the amplifier I get a lot of noise from the boost converter (here is example of the noise superimposed on a 4.4kHz sine wave):
When I disconnect the power from the boost converter, the noise disappears and the signal is perfectly clean (while there is some charge left in the output capacitor).
How do I filter out this noise? I thought that the RC-filter would be sufficient, but apparently it's not.
edit: I removed a 33 Ohm resistor from the RC-filter and added an LC-filter. Here is the schematic of the output:
The amplitude of the spikes now decreased, but the noise is still present.