not S/R loop problem
Just in case it was an odd batch of ECC83 , I tried with others and the
same.
Keeping same input and changing frequency
4KHz , 26V pk-pk on V7a grid , pin 7, and ok normal soft roll top "clipping"
2KHz, 48V pk-pk , bad trace hard clipping
1 K , 35V , bad
200, 15V bad
100 Hz, 5V, normal trace shape

so to find an errant cap that is involved in OD1 and OD2 channels