Since your output tubes are similar in type and rating to the 6BQ5 and EL84 - my guess might be right! For instance, the 7189 is a commercial/military grade version of the 6BQ5/EL84 and can handle higher screen grid voltages - but amps that use it cannot then use the 6BQ5/EL84 as a substitute! Russian tubes are notorious for being unable to handle their published rating specs!
Power tube temperature
My Sophia Electric Baby started distorting real badly from its left channel when I turned it on yesterday. It plays but at a little lower volume as compared to the right channel with the distortion superimposed on the music. It was completely fine two days ago. Its a push-pull model putting out 10 wpc using four unusual 6P1T power tubes. They look much like EL84 but not the same. It always ran very hot but I attributed it to being a class A design so didn't pay much attention. Anyway, I measured the tube temperature and was shocked to see numbers like 260-300 F across the four tubes depending where I pointed the temperature gun. After a few minutes the temperature dropped a bit and settled on around 210 F. I have isolated the issue to something in the left channel by switching driver and power tubes and tubes seem fine since the right channel sounds just fine. The question I have is what is the ballpark temperature for power tubes in a class A tube amplifier, especially ones with the smaller tubes like EL84 or EL34. Or better yet, input from anyone has one of these amps. And is it normal for them to start high at turn on and drop after warm up? I'm thinking something got damaged by the excessive temperature and causing the distortion on the left channel. I opened the back and see no obvious damage. Appreciate input from anyone with experience with these amps. Thanks.