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 Post subject: 100-70 Ballast tube replacement
PostPosted: Jun Thu 13, 2019 10:41 pm 
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Joined: Apr Tue 17, 2007 6:14 am
Posts: 38
Location: Orinda, CA
Hello-

I'm working on a Zenith 6D311 for the California Historical Radio Society. Unfortunately, the section to drop ~52 volts in the 100-70 ballast tube is open and we have none in stock.

So I thought, what about using a 50L6's heater as a ballast/volt dropper? I'll need to do the math, but it should work fairly well. Worn out 50L6's would work just fine! The other section of the ballast, a shunt for the pilot lamp, can be handled with a WW resistor.

Certainly easier/safer than to cobble some big WW resistor into a tube socket!

Anyone ever tried that?

Regards, Kent


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 Post subject: Re: 100-70 Ballast tube replacement
PostPosted: Jun Thu 13, 2019 11:03 pm 
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Joined: Nov Tue 14, 2017 5:09 am
Posts: 1877
Location: Austin, Texas
kstlfido wrote:
Hello-

I'm working on a Zenith 6D311 for the California Historical Radio Society. Unfortunately, the section to drop ~52 volts in the 100-70 ballast tube is open and we have none in stock.

So I thought, what about using a 50L6's heater as a ballast/volt dropper? I'll need to do the math, but it should work fairly well. Worn out 50L6's would work just fine! The other section of the ballast, a shunt for the pilot lamp, can be handled with a WW resistor.

Certainly easier/safer than to cobble some big WW resistor into a tube socket!

Anyone ever tried that?

Regards, Kent

Your plan will not work. The 50L6 is a 150mA heater and the radio has a 300mA filament string.
An 8uF, 250VAC, rated capacitor is the best substitute for the high drop resistor in the ballast tube. If the pilot light section is still good, leave the tube in place and put the cap across the socket terminals for the high drop section. Since the cap has no power dissipation, you don't have to worry about heat. Just keep it away from the audio circuits so it doesn't cause hum.
The cap will give better tube and pilot lamp life since it provides almost a constant current to warm up the tubes.

Jay


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 Post subject: Re: 100-70 Ballast tube replacement
PostPosted: Jun Thu 13, 2019 11:23 pm 
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Joined: Apr Tue 17, 2007 6:14 am
Posts: 38
Location: Orinda, CA
Hi Jay-

Hadn't thought it through, thanks for the clarification. Excellent tip! I'll install the film cap this evening. Might have to cobble together values for 8uf.

Thank you! -Kent


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 Post subject: Re: 100-70 Ballast tube replacement
PostPosted: Jun Thu 13, 2019 11:32 pm 
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Joined: Feb Sat 06, 2016 1:47 am
Posts: 2941
Location: La Mesa Califonia
The correct AC capacitor is here.

https://www.digikey.com/product-detail/ ... ND/4694508


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 Post subject: Re: 100-70 Ballast tube replacement
PostPosted: Jun Fri 14, 2019 2:37 pm 
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Joined: Jan Mon 18, 2010 2:13 am
Posts: 19075
Location: Dayton Ohio
..... or use two 25L6s in series. 300 mA and 50 volts. Oh wait, that requires another socket... Oh well.

-Steve

_________________
Radio Interests
-Zenith
-Sparton
-Pre-War FM
Consoles and floor models, the bigger, the better!


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