Odd, if the tube data here is correct, these tubes have different heater
voltages, and different current ratings, so I would guess they are not
used as a series string in your amp. So first check out to see if they are in
series of not.... Then it looks like the plate voltage is no more than 90v.
http://www.radiomuseum.org/tubes/tube_3s4.html
http://www.radiomuseum.org/tubes/tube_1s5.html