(03-03-2016, 01:01 PM)Roman Ships Wrote: Thanks ! So is the value not super critical? That is, 8uF might ok? 8 uF electrolytics seem easier to find than 6.8.
If you use the ultra-low, the value is semi-critical for the intended frequency response.
Consider putting caps in parallel if you use the ultra-low.
I could run some plots via spice etc to see the difference with 8uF, which might be minimal, but with 8uF you are going to get more of a low end boost than intended (those 8uF rated caps may be 9uF in realtime anyhow) - the ultra low is kind of like an overall boost that shaves off some ultra lows and highs, which ends up being like a low-MID boost. So a larger cap would end up boosting a wider band extended more in the lows than with the 6.8uF cap.
It's significance may be more about taste. But this is low voltage - for a preamp tube's cathode, 25v caps should be plenty. I have 6.8uF caps at that voltage myself I think, just because. I'd parallel a 4.7uF with a 2.2uF if not.