On Jun 2, 5:49*pm, "Joel Koltner" wrote:
Something I've wondered for awhile now is this: Since you get the same VSWR
with a load both higher than or lower than your "expected" load (we'll call
this Z0, and the actual load Zl), either Zl/Z0 or Z0/Zl respectively, is one
siutation more likely to kill an amp than the other?

When ZlZ0, you tend to (ignoring phase effects here) end up with a higher
voltage sitting on the finals than when Zl=Z0... but less current than the
matched case. *With ZlZ0, it's typically lower voltage but higher current.
Based on what I know from, e.g., power supply design, there's usually more of
a margain from overvoltage than there is from overcurrent, in that you can
often find parts that inherently withstand a 2:1 or better overvoltage margain
with just a bit of extra cost, whereas overcurrent will tend to overheat the
power devices and getting rid of, e.g., 2-4x as much heat (vs. matched case)
can require significantly greater heatsinking which often isn't cheap.

In the extreme case, I'd expect that far more amps will blow when shorted than
when open-circuited. *But I don't really have much experience in this area...
so... could anyone comment on what you tend to see in real designs? *Perhaps
also compare and contrast tube-based amp with solid-state ones?

Many simplest solid-state amps "SWR protection" circuitry is in fact
"overvoltage protection" circuitry (e.g. ZlZ0).

The output transformer and low-pass network can also be seriously
overstressed when ZlZ0.

For a while, thermal runaway was a big problem with bipolar continuous
duty (especially FM) amps when ZlZ0. Today most solid-state amps
(even if not bipolar) have overtemp and overcurrent protection as a
belt-and-suspenders on top of temperature-dependent bias. Overtemp by
itself won't stop the worst transformed loads but with overcurrent
protection the past couple generations of HF finals aren't so easy to
kill.

BTW... my Ten Tec Triton IV (1976 vintage) still has the original
final transistors.

For a lot of medium-power homebrew gear the cost of any final
protection is more than the cost of the finals. Witness the many
IRF510 amps. For a while on my bench I was burning through IRF510's
like they were goin out of style :-)

Tim N3QE