Originally posted by Jean
<snip>I am thinking maybe tip2955/tip3055 keeps failing on turn on every time? My rail voltage is 36 volts, I remember reading with over 35Volts tip2955/tip3055 is not recommended.
Hello, I thought the output transistors were something different? Well, that is definitely a possibility. Remember your rail voltage is actually 72 volts, not 36. AFAIK
TIP 3055/2955 have a Vcc max of 70 volts. Bad news, could be the cause.
I suggest before putting in the output transistors check the biasing voltages again. I would still look for Q9 and Q4 to be defective under loaded conditions.
Edit: Also check your input BC546 transistors, all three of 'em. If R6 has no current through it (therefore, 0 volts drop), maybe one of the input transistors isn't conducting properly. A little bit of DC in the input can cause one of these to implode...