[morzh]

We discussed this here before, some years ago. SS generators used with tube equipment should be protected either with a zener or a double diode to limit the voltage it can see. BTW the transformer might not help: it will reflect the voltage spike if it is 1:1.

Depending on the amplitude of the signal I would either use two diodes (in reverse-parallel) in parallel to the output after the capacitor, or a symmetrical Zener, also in parallel.