Hi Lee, Al,
Not wishing to disagree with Al's reading of the specs but I think you'll be fine with 5R4 running that lowish voltage. I have a pair I used in 5-20 monoblocs and swapped with GZ34 and 5U4 without issue. Further, these monos had first caps of 150uF(!), but they did have 100R on each anode, the mains TX impedance sometimes needs consideration.
That was a good few years ago so I had a quick google and this thread throws more info on 5R4,
https://www.thegearpage.net/board/in...r-tube.694885/