Music Man 100 RD 2100 RD power section

[xtian]

I am refurbing this Music Man 100RD head, and I want to understand the power section better. Plates are 715v, screens 350v, control grid fixed at -22v by Zener D14, and cathode is wiggled via NPN transisors.

What am I to make of the legend "25mV min" at the Emitter of the transistors? Is adjusting TR1 the way to adjust that voltage, and why only spec a minimum voltage?

Asked another way, how to you set the bias?

Owner wants to replace power tubes with KT88. Ra-a looks similar, and heater requirements are OK...any other concerns?

Screenshot 2024-06-05 at 1.39.06 PM.png

[martin manning]

https://ampgarage.com/forum/viewtopic.p ... 64#p461064

Same one? There is a link in the third post to a thread with some discussion on the power amp and biasing.

[xtian]

Thanks! I was feeling deja vu. But Google search didn't find my last post.