Be aware that the ohmic resistance
of a Tungsten light bulb has a very strong positive temperature
coefficient! The resistance of the filament drastically increases as
it heats up when power is applied.
Take your ohm meter and measure the
filament resistance at room temperature, with no power applied. Now,
assuming you are using a standard incandescent light bulb (say
somewhere around 50 to 100 watts), look what happens to the power
consumed at the moment the bulb is first powered up and the power
that is consumed after the filament heats up. P=E*E/R The wattage
rating of a light bulb is measured at full rated voltage in steady
state conditions. That means a 100 W bulb pulls quite a bit more than
100W at the instant it's turned on. You know how it seems that an
incandescent light bulb always burns out when you flip the switch to
turn it on.
So, the result is that as you are
keying your light bulb filament (dummy load), the actual load seen by
your transmitter rapidly varies. It starts low and goes to a higher
value with each dit and dah.
The old tube rigs didn't much care –
they were tough. But your output transistors are a bit more finicky
in their diet. Oops! There goes that magic smoke again.
But for QRPer's - not to worry too
much. At 5 watts (figure your voltage into 50 ohms), a 50 to 100
watt light bulb probably won't undergo too much heating. If you
have enough power to make the filament glow (even dimly), that load
resistance is already on the way up.
What it might do, though is contribute
to a “yooupy” signal, if your rig is sensitive to a changing
load. Remember how the changing load presented by a blowing wind on
a wire antenna could affect the old single tube MOPA transmitters and
regin receivers back in the days of yore?
Oh, well... Enjoy! That's what ham
radio is all about, anyway.
Bruce – KK0S