Thanks to all that commented.
Plan of action will be:
Replace the current pass transistor with an NTE181 (not cheap, about $6 at
Jameco),
but has the best combination of power handling and break down voltage (30A,
100V, 200W).
It appears, based on a quick search of the archives, that TenTec actually
used these as well.
The 2N5302 ($2 at Surplus Sales of Nebraska) is 30A, 60V, 200W
Googling "TO3, NPN, 50A" found a 2N5686 (50A, 80V, 300W) and 2N6277
(50A, 150V, 250W) with similar gain (hfe 15-60) but
I'm not enough of a circuit guy to tell what they might do as a replacement
from the data sheet. And as we discussed, if the heat
sink isn't cutting it, there is no sense in moving up to a "bigger" device.
Thanks for the comments on the thermal grease -- I do tend to go to heavy on
this stuff. Also, the
last time I replaced one, mica was the TO3 insulator of choice. Should I
move to the new "gray pad"
material, versus the mica/grease combo?
I also plan to add a quiet 12v fan to the heatsink, and we'll see how it
goes.
Bill
W1PA
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