Did you read the information in this link? Scroll down to the
discussion on the regulator tube.

http://www.dvq.com/geiger/heath_rc1.pdf

The 900 volt regulator tube operates in three regions; corona,
glow, and arc. It is sensitive to the high voltage supply conditions,
i.e. ripple can cause it to oscillate between the modes. Also, it is
sensitive to the filter capacitance.