Today, I've wired together my first tube power amp. a bit nerve wrecking I might add . Two KT88's PP (for now, will be four later on) driven by a ECC99 PI into a Hammond 1650T in ultra linear mode. Attached image is the result into a 4 ohm resistive load. There's no global feedback at the moment and bias is set to -60V. B+ is 410 volts. I'm still a noob, but can't imagine the crossover distortion measured to be normal! The signal going into the KT88s is fine (blue line). What's the reason for this distortion? No feedback? Bias to low? It's a learning experience so any help is welcomed. Thanks.