Motorola Ballast tube

[jshorva65]

Originally Posted by John Folsom

Amperite made a glass bulb replacement for the Motorola balast tubes, and gave them the same part number as Motorola:

17A485459 - For TS18
17A470303 - For VT71

The glass bulb was filled with an inert gas to aid in heat transfer, and should be more reliable than the open air balast tube.
Check around with the variosu tube dealers out there... these show up from time to time.

Thanks, John! There's an excellent video on YouTube in which Bob Andersen (bandersentv) demonstrates the inrush-absorbing properties of the Ballast (using the Amperite glass replacement). We're using one of the Amperite units in a client's Motorola 7" set (TS-4H chassis). We just received the part from vacuumtubesinc. At $25 each plus $7 shipping, it's somewhat pricey unless you order multiple items (the $7 is a "minimum" shipping charge which covers shipping for the first 6 items on an order). Yes, they are pricey, but the cost is justified considering the difficulty of attempting to replicate the part with ordinary resistors and the fact that the Amperite unit provides both the voltage-dropping and inrush-limiting functions without the complex calculations required to incorporate thermistors or their equivalent into a substitute resistor array. My guess is that the Amperite ballast is probably filled with nitrogen (typical of standard incandescent light bulbs rated over 25 watts, incidentally). The use of nitrogen assists heat transfer to the glass (unlike a vacuum) while eliminating oxygen (which was the main cause of ballast failure in open-air units due to oxygen's corrosive effects on metals at high temperatures) from the resistance elements' environment. Nitrogen has been used for many years in standard light bulbs to better utilize the glass envelope for cooling, and to counteract atmospheric pressure (improve structural strength and prevent bulb implosion in case of breakage).