Innards of Ballast Tube L-49-B

Had a short between pins 7 & 8, so I cracked it open to take a looksee.

Resistant coil wire wrapped around a mica board. white stuff seem like
plaster for glue.

I suppose I could stuff it with an 180 ohm wireround 25 watt ( 3 & 8 ),
45 ohm 5 watt ( 7 & 8 ). Any tips?
Tho, I did place an order for 3 NOS tubes.
_JD_

"_J D_" wrote in message
...
Had a short between pins 7 & 8, so I cracked it open to take a looksee.

Resistant coil wire wrapped around a mica board. white stuff seem like
plaster for glue.

I suppose I could stuff it with an 180 ohm wireround 25 watt ( 3 & 8 ),
45 ohm 5 watt ( 7 & 8 ). Any tips?
Tho, I did place an order for 3 NOS tubes.
_JD_

The resistance wire is iron which was chosen for its relatively high
temperature coefficient of resistance (alpha) which is 0.005671 per
degree Celsius at 20 C. Compare this to nichrome or constantan alloys
which are 0.00017 and -0.000074 per degree C respectively. Note that for
iron and nichrome, these values are positive meaning that as these
materials heat up, their resistance increases.

In a ballast tube, the iron wire is normally placed in an inert gas. The
pressure is chosen for the proper heat transfer coefficient allowing the
tube to act as a current regulator or ballast.

Since modern line voltages are quite constant compared to the days when
radios using ballast tubes were designed, it is usually possible to use
fixed power resistors to replace them. I did find a little data on the
L49B. You might find the table below to be useful. It is from the
Valve Radio and Audio Repair Handbook by Charles Edward Miller.

73, Barry WA4VZQ