Hello
I had the same thing with a vintage HK power amp. First check the tube, my Ktt88 was bad then check the bias. I replaced the resistor with metal oxide 1% and I matched and replaced all of them on four tubes. Later after it was running for a few weeks I replace all caps with poly SCR`s and diodes also. But watch your Bias!! Good luck.
I had the same thing with a vintage HK power amp. First check the tube, my Ktt88 was bad then check the bias. I replaced the resistor with metal oxide 1% and I matched and replaced all of them on four tubes. Later after it was running for a few weeks I replace all caps with poly SCR`s and diodes also. But watch your Bias!! Good luck.