[morzh]

Since it is in series, it could be anywhere. As long as it is only in series with the filaments and not therest of the circuitry.

I can't makse sense of what this sch shows without seeing it myself and matching it to the sch, but I think "Brown" is defunct as this probably was the cord's resistor.
So it could be in series with the "White (before the 25Z5 filament)".
Or it could be before the 6A7 filament (as long as the Common wire going to cathodes' feedback resistors still goes to the same pin it was going to before). However, the fact it goes to Common might help pwith hum.

Sp, I suggest the "White" and 25Z5 filament as the point of the 8uF cap insertion.

The value of the cap is approximate: 8.2uF is very common and is being sold at Tubesandmore (Amplified parts today).

This is Solen, @$4.95, 8.2uF 630V
https://www.amplifiedparts.com/products/...ylene-fast

This is Audiofiler, $3.75, 8uF 400V
https://www.amplifiedparts.com/products/...ypropylene

For your purposes, I'd go with the latter. It is also smaller.


PS. When you need an AC rating for a cap that gives only DC rating, divide your DC voltage rating by 1.414.
In your case you need to odrop 98V AC, which is 139VDC, so for good measure your cap should start from 200VDC rating. 250V is a good value to not go below.