But the "A" is important on the 7189A. Check the plate voltage rating. I would not stick a 6BQ5 in a circuit built for 7189As. I have used the Russian tube though, in my brothers Fisher. As far as I know, they have not melted - yet.
I am not a Fisher Afficiando but I can understand your enjoyment of them. I was recently at my Friend Tom's house he was working on a Fisher Tuner for a member of our club. I think you met Tom at Ktown.
The chassis was inverted as he was working on it and the build quality was obvious even to me!
It will make some one happy.
Now go get the Jackie Gleason , Ron!
Paul