Don't know enough about the minipa 70 to comment.? Is there a circuit out there to look at?
The Wa2EBY I have has run at 65W (40M) brick on key mode for nearly 20M before I called it
good.? Only slightly larger heatsink but the real factor was stability. and avoiding overdrive.
I used a 4DB pad on the amp as almost of the radios I have are about 4W to both protect
the amp from overdrive, provide a stable load for the radio and input to the amp.? Also
everyone that reported killing that amp was running more than 28V.? FYI that used IRF510s.
I built it back in 2006 its still running the second set of transistors and has been run into shorted
coax, opens (no antenna or no coax!), wrong lowpass filter (on 10M with 80m filter...oops).
What happened to the fist set of IRF510s?? Testing to see what they could take, they failed
at 85W out on 40M (190W DC input) and 36V and the likely failure mode was overvoltage
on the gates (needed 5W drive for that power) with bias set to 200ma per device.? Previous
test at 80W ran for 5 minutes without issues?though the heat sink did hit 55C and the case
of the device nearly 80C.? Backing off to 28V and keeping drive under 1.2W has proven?
blast proof.
I have encountered enough IRF5xx mosfet amp designs and most a at best conditionally
stable and if they go unstable they will oscillate and, overheat and die. Some do not like the
driver (what ever radio used) and many are even less tolerant of bad SWR usually due to high?
reactance of the antenna not tuned out.? At least one case it was built so open
(no screening at all) that RF from the antenna was getting to the input causing
oscillation.? ?Another case the conclusion was the amp was ok but the QRP rig was?
unstable with a reactive load and would go into oscillation, over drive the amp and
kill the IRF520s used .? I helped him build in a TX only input attenuator(3DB) into
the amp and problems went away.
I've seen this sort of failure and misbehaviour in bipolar and LDmosfet?and even
vacuum tube amps as well.??
Allison
