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 Post subject: Filament-dropping resistor in NC-183
PostPosted: Jan Sun 06, 2013 10:11 pm 
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Location: Potomac, MD 20854
A puzzler (maybe not to Carl): Checking out a National NC-183 after a friend's recap, I see a 4.3 ohm resistor in the filament line supplying V10, the 6H6 limiter. Looks like an original, probably 2 watts with characteristic slightly rough exterior. No such value in the parts list. Measures 4.5 ohms, not bad after 50 years!

But why is it there? It's the only tube with a dropping resistor in the filament line.

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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Sun 06, 2013 10:13 pm 
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I've seen that done in many radios using a dual diode detector. I think it reduces shot noise.

Pete

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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Sun 06, 2013 10:17 pm 
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I've heard of that, but this is the noise limiter, not detector. It also isn't in the Sams schematic or the schematic in the manual I downloaded, though I know that doesn't necessarily mean anything.

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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Sun 06, 2013 10:43 pm 
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Lowering the cathode temperature in a vacuum tube diode reduces the contact bias that occurs when electrons emitted from the cathode contact the plate. That bias must be overcome before rectification can take place. The effect of contact bias in a vacuum tube diode is similar to the forward drop in a silicon diode.

Both National and Hallicrafters used filament dropping resistors in the 4 to 7 ohm range for the 6H6 and 6AL5 tubes used for detectors and noise limiters in many of their receivers.

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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Sun 06, 2013 11:09 pm 
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Ed nailed it 99.9%.
With such low signals and no contact bias the tubes acted as if just filaments only were present with the resultant early failures. A filament reduction helped as noted with the detector. In the NL it was a case of copy cats without a firm reason as I was told by those older than I and who I trusted :roll: :lol:

Carl


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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Mon 07, 2013 1:23 am 
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Burnt Fingers wrote:
...In the NL it was a case of copy cats without a firm reason as I was told by those older than I and who I trusted.


Wonder whether it was a later addition. So should I leave it alone or take it out? Leave it, I guess, if it doesn't impair NL operation.

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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Mon 07, 2013 5:48 pm 
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I think these were common in NC183Ds; perhaps they were original as from the factory. There was a thread about a year ago that had photos of these resistors, which were odd-looking things if I recall correctly.
-DS


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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Mon 07, 2013 6:13 pm 
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Doc, I can't find that thread. The resistor doesn't look that unusual, just obviously oversized by today's standards for a 2-watt unit, which I'm reasonably sure is the rating since it's dissapating about .6 watt. And old, with that subtly pebbly surface that resistors had in the '40s.

I'll check tonight to see if the detector stage also has a dropping resistor for the filament of that 6H6.

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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Mon 07, 2013 11:20 pm 
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My NC-183D schematic in the manual shows a 4.3 ohm resistor (R-30) in the V10 filament circuit. The parts list indicates that is a 1 watt.

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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Tue 08, 2013 2:55 am 
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Just checked: no dropping resistor for the second detector. Just the NL.

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 Post subject: Re: Filament-dropping resistor in NC-183
PostPosted: Jan Sat 12, 2013 7:00 pm 
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Its also in the NC-300/303, 400, HRO50-1, HRO-60, etc, etc.

Dead center below in a 300

Carl


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