"Jack Twilley" wrote in message
...

I am hoping to preserve the lifetime of my tubes and maximize the
power available to me.

My question is: what is an acceptable SWR range? I know that 1:1 is a
desirable SWR, but when does SWR become undesirable to the point that
it shortens the lives of my tubes?

Impedances of 25ohms, 50+j39ohms, and 100 ohms all represent loads that
could present a 2:1 SWR to the transmitter. Which is worse depends on
how the matching and filtering networks reflect the load back to the
transistor.
With a networks that look like a half wavelength of transmission line, the
25ohm
load would cause more transistor stress than the 100 ohm load. Adding
another
quarter wavelength of transmission line wouldn't change the SWR, but would
make
the 100ohm load more of a problem than the 25ohm load.

I am using a Kenwood TS-520 and an MFJ Versa Tuner II.

You may be in luck. You can calculate the plate power (plate current*plate
voltage).
Subtract the power output and you have a very good idea on how much the tube
is
stressed. You can then measure the SWR and see if how much of a correlation
there
is between SWR and tube stress. Another useful measurement may be to
measure
the temperature of the tube with one of those non-contact IR thermometers.

73--Zack Lau W1VT