I had several 3TF7 ballast tubes fail in my R390's and they're becoming more expensive every day. So I designed this
solid-state replacement. 3TF7.PDF I've used it for about 20 years now with no problems.

The circuit is a single transistor current regulator connected across the DC legs of a full wave bridge rectifier. It's low tech
as it gets but regulates much better than a piece of iron wire in a hydrogen atmosphere

Here is how it works, in case you want to modify it to replace a different ballast tube. The resistor R2 and zener diode D6
clamps the base of X1 at 5.1V, with D5 roughly compensating for the base-emitter voltage. The regulation current is then
5.1V/R1. The four diodes D1-D4 are a full wave bridge that steers the current to the appropriate terminals of the regulator
circuit. If the filiment supply is DC you can leave them out. But as it is there's no right, or more importantly, wrong way to
connect the circuit once you have identified the correct two pins of the ballast. Being somewhat of a klutz I like that feature.
The capacitors are for noise suppression and their values are not critical. In fact none of the component values are critical.
If you have only a 3.3V or 6.8V zener feel free to use it and adjust R1 accordingly.