My Mouser parts arrived today, and the ST-70 board went together in about 45 minutes. The amp is working and sounding great. One strange thing I'm trying to figure out is the bias. I put 10 ohm resistors from the cathodes of the 7591's to ground, but I only get 1/10 the voltage I should at the 4 test points. 50 mv instead of 500 mv - verified on 2 meters. The balance pots on the driver board work as expected, and the takeoff points on the board read -35 volts. The 7591's are running very cool to the touch.
What am I missing here ?
By the way, I went with a 40 mfd 500v off the 5AR4, then a Hammond 156R choke, 100 mfd 450v (Transformers connected here) then a 1.5 K resistor and 40 mfd (Screens here), 5.6K resistor, 40 mfd (driver 350 v), 22k resistor and 40 mfd to the driver 300 v... No hum at all, with no filament balancing resistors. The B+ is pretty high - 500 volts either side of the choke. Nothing running hot or exploding yet, but I need to do something about it. I tried one of those big Chatham 5R4WGY's which are supposed to have like a 60 volt drop, but I still got 500 volts.
Sorry for the long winded post, I'm just at a loss here - the amp sounds good and is nice and loud. I guess the next step is to take some measurements. Yes, they are 10 ohm resistors, not 1 ohm ! haha
Thanks for all of the help along the way.
New JJ 7591's NOS Raytheon 12AU7's NOS RCA 12AX7 Used Heathkit/Mullard GZ-34/5AR4 Allman Brothers - Eat at Peach CD