What is the bias of Forte 1a?

I believe the 1a was purported to be pure class A, but I have some doubts about that claim. You might want to contact Threshold or post this question on diyaudio.com. I don't believe this was a Nelson Pass design, but he generously offers advise quite often there. Others there may have already traveld the same path as well.

Get ahold of Jon Soderbreg at Vintage Amp. He worked there when they were built.

http://www.vintageamprepair.net/