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 Post subject: Ballast tubes question
PostPosted: Jan Thu 25, 2018 10:39 pm 
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Joined: Jan Wed 11, 2012 5:12 pm
Posts: 522
Location: Colorado Springs, CO
I have a radio with a whopping 10 tubes, three of which are ballasts. Talk about a tube count. Anyway, all three ballasts (200R, 250R, 320R), show continuity, the 320R through pins 1, 3, & 4. The radio is untouched, so I can't try them in circuit at this time. Oddly, the 250R has only two wires leaving the tube, as can be seen in the photo, but the schematic shows three. Ohmmeter shows continuity only between pins 1 &4.

My question is concerning the appearance of the interior of the tubes. The 250R has a rusty red coating with a dull green edge. The 320R has a large quantity of black dust or powder sliding around inside. Has the 250R lost its vacuum, or is there any vacuum (or gas fill) at all? Is the black powder part of the function of the 320R?

It looks like a pretty crappy radio, but I think it has historical interest.


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200R.jpg
200R.jpg [ 44.5 KiB | Viewed 354 times ]
250R.jpg
250R.jpg [ 42.86 KiB | Viewed 354 times ]

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 Post subject: Re: Ballast tubes question
PostPosted: Jan Fri 26, 2018 12:51 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
International Kadette was famous for making ten-tube radios which were really five tube radios with lots of ballasts and other tubes which inflated the count without contributing to the radio's performance in any meaningful way. People who didn't understand would see all those tubes and figure it had to be a better radio. It finally got the company into trouble when the Federal Trade Commission held hearings and cracked down on such practices in 1937. After the new rules went into effect, ballasts could no longer be included in radio tube counts, and every tube that was included had to perform some beneficial function besides being part of a heater string. The FTC test was that the radio had to stop working or be noticeably degraded if the tube was pulled (keeping the heater string intact if it was a series string set). International Kadette went belly-up shortly thereafter. So your radio does have a certain historical notoriety.

Anyway, in answer to your questions, those ballast tubes were normally filled with hydrogen or other gases intended to conduct heat from the filaments to the bulbs, providing a cooling effect. This was so the temperature would rapidly reach equilibrium with the surroundings and help the ballast regulate the current passing through it. The wire elements inside are made of iron or nichrome. The stuff on the inside of the bulbs is material from the wire elements that has boiled off the wire and condensed on the colder glass. It indicates that the tubes were used for lots of hours. The fact that they still check out okay with an ohmmeter means that they will probably still work. For how much longer is anybody's guess, however, and I would recommend not using the set as a "daily driver," or perhaps connecting it to a bucking transformer to lower the line voltage going in. Ballasts can be replaced by various other components such as diodes, resistors, and capacitors that were not available years ago, so preemptively changing them is another possibility.

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 Post subject: Re: Ballast tubes question
PostPosted: Jan Fri 26, 2018 4:10 am 
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Joined: Jan Wed 11, 2012 5:12 pm
Posts: 522
Location: Colorado Springs, CO
Thanks for the explanation. It all makes sense. I knew that the 50A1 in my T.O. was hydrogen filled, didn't know about these. I didn't want to get into the make and model in case I need to start a thread in Restoration when I get into it.

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