Heaters for cathodes with a large swing

[lndm]

I want to contain a mu stage within +/- 300V, for direct coupling purposes. My problem is the (upper) heater to cathode insulation. This stage is not meant to swing the entire 600V but it would try to, given half a chance and I don't want any surprises.

Is there anything wrong with just tying the heater to the cathode? Alternatively, what can I expect if I just float it at 40V or so?

[Brian Beck]

I have done that and it can work fine. The heater transformer must have low capacitance between the secondary and the primary windings to avoid loading the cathode. I used split bobbin types which measure mere tens of picofarads between windings. The heater supply can be a fully regulated DC supply so long as there is no connection to ground except for small stray capacitance. In that case the cathode is connected to the negative terminal of the heater supply so that the heater is always slightly positive relative to the cathode. I also added a 10K safety resistor from cathode to the supply, just for some additional isolation.