I am thinking about several applications of magic eye tuning tubes as display pieces and had a question about their operation. I have attached a sample circuit from a datasheet for an EM84 tube. I know the shadow is controlled by the voltage on pin 1, (the triode control grid), where the shadow shape changes as the voltage goes from 0, to some negative value, usually ~ -20V.
My question is how to build a simple circuit to output a variable voltage of 0 to -20V. Currently I have the tube powered off a power transformer, so I have 6.3 VAC for the heater, rectified 300VAC for the target, and the transformer also has a 28VCT. Other than using something like LM137 after rectifying the center 28VCT outputs, is there an easier way?
Thanks in advance and sorry if my question had any errors, I am by no means an expert in this field.