I still have a couple of neon lamps from RS. I will try using them instead of the 40s just for fun.
As long as you don't expect them to work in the same circuit.
The #40 is a low voltage bulb (6 - 8V @ 0.15A), basically #47 with a screw base. My first
transmitter used a #49 ( 2.0V @ 0.06mA) link coupled to the output stage because it took
less power. However, I was always burning them out. Low voltage bulbs are a good current
Neon bulbs require high voltage (typically 70 - 90V) at very low current, so are used to
indicate the presence of RF voltage
. The typical AC power indicator uses a NE-2
in series with about 100K to limit the current: neon bulbs are similar to VR tubes or
zener diodes in that, once the breakdown voltage is reached, there is no inherent
I remember a friend testing for 220VAC by holding one end of a neon indicator and
touching the other end to the wires.
A series low-voltage bulb will be a better indicator for low impedance loads, and a shunt
neon for high impedances. For a given antenna, this may vary from one band to the
next, of course. I've heard of hams adding both in their antenna, and using whichever
one gives the best indication on each band. Required voltage and current will depend
on impedance and power: sometimes it may be useful to provide an way to adjust the
sensitivity. But if the antenna is fixed, either a series or shunt bulb will increase in
brightness as more power is delivered to the wire.
A low voltage bulb can often be clipped across a few inches of antenna wire to give an
indication of current: then you can change the spacing between the clips to adjust
the sensitivity. LEDs can also be used this way, though the require a series resistor.
(The "Ultra-Bright" LEDs can show an indication at very low levels of current.)
A fluorescent tube will light all on its own from an electromagnetic field, so you can
hold it in your hand and wave it around your antenna to find where the field is
strongest (if you have enough output power.)
And all of these are distinct from using a light bulb as a dummy load, or the old
"twin-lamp" SWR meter. There are lots of ways that lamps can be used as indicators,
and they are much cheaper than a meter when you only need a relative reading.