It might be C-5 The only .05 in that area. Pins 7,8,and 1 (blank) all go to ground, and so does one end of C-5 from R-11. This would be a tie point.
Your pick of 10MFD & 22MFD should be just fine.
Pin 7 of the 6a8 is the ground side tie point of that heater... so basically it's just a ground.
There are only 3 caps in that area one is a .1uf c4 from ground to pin 4 of the 6A8...and the other two are both .05uf.
Those two .05 caps are both listed under a single part ID = C5
So the two C5 caps each have one end going to ground and are each separately located one on each end of R11 ( a 1000 ohm resistor)
There is another .05 cap called C2 it connects one end to ground and the other end to R4 ( a 1 meg) resistor at the top end on the vol pot.
Let me know how that works out for you.
good luck,
Peter