[morzh]

I was trying to remember why I suggested that Zener. Then I remembered, it was charging the capacitor if a tube is blown or extracted while the radio is hot, and then inserting a fresh tube while it is still hot. The cap charged to the full B+ voltage (no divider present) will discharge through the filaments chain, potentially blowing all or some of the tubes' filaments.

Well, I have always been and will be a vehement opponent of plugging/unplugging things hot, unless I specifically know they are designed for it (or when I designed the device with hot-plug option).
But even when designed for it: for instance, Guppy (I think it is Guppy) FireWire camera.
FireWire is a hot-plug interface, but if done to this camera, it fries. Literally. And it is over $2K. Very painful realization. We lost many a cam this way, until everyone who deals with them learned the power-down sequence. And even then....did not warn a tech in the compliance lab, and he killed it the same way. By simply unplugging and then plugging back into the computer board.