All tube operating points below are by no means definitive good sound recipe for your particulate tube project. Lowest THD point is not necessary best sounding one and there is always some variance even in a NOS tube batch from the same manufacturer! Below information is only useful as a general starting point and is ment to get you in a ballpark when no measurement equipment is available.
Now that’s one ugly duckling. On one hand it’s another very linear soviet Pentode, on the other – it’s a disaster. Just look at that histogram and appreciate that parameter spread. That’s a data set from 50 tubes I got in my stash! Furthermore, this tube knows one trick that only soviet triode-wired-pentodes know how to do – oscillate at audio frequencies!
I tried all the tricks in my book to tame it – all for nothing. It’s not HF oscillation! I even made EMI probe to look at whats going on without applying scope probe capacitance to the anode/grid2. Nothing. It’s all at 5-50 kHz range.
As can be seen from spectrum above, some tubes are worse, some are better, but still – not usable. So from all my stash of 50 I ended up with 9 tubes that don’t oscillate and only two pairs with close current. That’s a really miserable yield.