D. Huizinga wrote:
My 40 mtr doublet is fed with a balanced line of 300 Ohms. Nicely tunable
to 1:1 . But after twenty minutes or so the coil gets rather hot! It is a quality
coil of 3 mm and shiny silvered or chromed on a ceramic body of 1 inch.

I once had a similar problem using a 130 foot dipole
fed with 450 ohm ladder-line on 40m. The SWR on the
ladder-line was about 15:1 and the 4:1 balun was
looking into 30 ohms. The balun presented 7.5 ohms
to the tuner and the coil got hot. That's when I
decided to come up with my no-tuner approach at:

http://www.w5dxp.com/notuner.htm

Is your 40m dipole 66 feet long fed with 60 feet
of 300 ohm twinlead through a 6:1 balun? If so,
your tuner is installed in a lossy configuration
and heating is to be expected. The simulator at:

http://fermi.la.asu.edu/w9cf/tuner/tuner.html

says that 16% of the power is lost in the tuner.
16 watts is enough to heat up a coil. Ever touch
a 15w appliance bulb when it is on?
--
73, Cecil http://www.w5dxp.com