[morzh]

Try 25L6. 25Z6, now that I looked at the hookup again, won't affect anything much.
And if you are up to it, try that idea with 100 Ohm and extra electrolytic cap.
You could possibly leave the existing filter in place, disconnect only the wire that goes to the output xfmr primary and connect R=100 Ohm (1-2W for the experiment) and C=30uF to 100uF (with proper WV rating) between them, the cap being on the xfmr's side and the resistor going between that and where the wire used to go.