[morzh]

   

I've been meaning to do that for awhile. I added a zener across the cap that filters the filaments.
The cap is 25V and the voltage is 10.5V when run from batteries. Should any tube go open, the voltage will rise to the previous cap's voltage. Too high.
In goes a small 600W 13V surge suppressor, a high power TVS zener in its nature. It clamps at about 15V. Which is enough to keep the 25V cap safe and the filaments too if inserted hot (never a good idea but....).

That inconspicuous black rectangle close to the chassis edge is it.