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 Post subject: Globar Thermistors- Questions
PostPosted: Jun Wed 16, 2021 9:03 am 
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Location: MD-DC 20855
Hi, All-

I posted a question about these non-inductive resistors over in the
Tubes, Parts, and Services forums. So far no replies, though there
have been a few dozen reads. In the past I've had really good luck
getting help and suggestions in the Receivers and Transmitters
forum, so I thought I'd try here too. The original post is:

viewtopic.php?f=19&t=398358&p=3394200#p3394200

Any and all comments and suggestions will be much appreciated!

Thanks, David

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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Wed 16, 2021 5:06 pm 
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I seem to recall they might be used in CRT power supplies to protect from surges. It's more likely a solution to a problem that doesn't really exist.


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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Wed 16, 2021 5:24 pm 
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Joined: Oct Thu 01, 2020 10:48 am
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Even though "thermistors" is in the title, I'm going to address glowbar kanthal resistors. They can certainly be used in plate parasitic chokes, but they are difficult because you can't solder directly to them, so you have to have mechanical hardware or fabricate some mechanical way of using them. For that reason I prefer simply employing 2 watt carbon or metal oxide axial lead resistors.

Glowbars are most often encountered in dummy and reject loads. But basically any situation requiring a non-inductive resistor with relatively high dissipation will involve them, or wire wound Clarostats that have the winding on the ceramic form spiraled on in two directions to collapse the magnetic field and make them non-inductive.


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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Wed 16, 2021 9:12 pm 
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bob91343 wrote:
I seem to recall they might be used in CRT power supplies to protect from surges. It's more likely a solution to a problem that doesn't really exist.


Quote:
a solution to a problem that doesn't really exist

Does sound likely. Thanks!

David

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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Wed 16, 2021 9:32 pm 
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K5UJ wrote:
Even though "thermistors" is in the title, I'm going to address glowbar kanthal resistors. They can certainly be used in plate parasitic chokes, but they are difficult because you can't solder directly to them, so you have to have mechanical hardware or fabricate some mechanical way of using them. For that reason I prefer simply employing 2 watt carbon or metal oxide axial lead resistors.

Glowbars are most often encountered in dummy and reject loads. But basically any situation requiring a non-inductive resistor with relatively high dissipation will involve them, or wire wound Clarostats that have the winding on the ceramic form spiraled on in two directions to collapse the magnetic field and make them non-inductive.


Thanks! Well, it so happens I found a couple of 100 ohm Kanthal resistors on the auction site; two in parallel
gets me close to 50 ohms and I use it for a 200W class load, convection cooled. Indeed these types, about
1" diameter, do need clips for terminations.

I also have two old Heathkit KW dummy loads and am in the process of rebuilding one of them. The resistor
element was still good and close to the 50 ohm nominal value. They too have the hollow ceramic core
and require clips on the ends.

The globar types I'm asking about here have solid bodies with metallized ends and soldered wires attached.
I'll post some photos later today. A few of these appear perfect for plate parasitic suppressors and are in the
range of 20-30-70 ohms (hot). Most of what I have are high value units, however- in the 1-11M range. I am
hard pressed to see how I can find any uses for this type.

Thanks, David

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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Wed 16, 2021 10:19 pm 
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Image

Some examples of the globar resistors I've been collecting. The FRT-1 looks perfect
for use in a plate parasitic suppressor in a 1KW type RF power amp. The purple unit
measures just under 1 megohm. Color code dots (lower left) appear to say 7.5M
nominal (hot). These three black units fall in the 8 to 11M range. All are solid body/
metallized ends with attached leads.


Image

The two Kanthal globars here fell on the low side of 100 ohms. I've added a 20W
TO-220 type non inductive resistior, 5 ohms, to bring it up to 50 ohms. The globars
were rated 93W each, as I recall.

David

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Last edited by Clutter on Jun Sat 19, 2021 4:38 am, edited 2 times in total.

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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Wed 16, 2021 10:30 pm 
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K5UJ wrote:
or wire wound Clarostats that have the winding on the ceramic form spiraled on in two directions to collapse the magnetic field and make them non-inductive.


Years ago I acquired 4ea. Dale NHG-200 (35W). These are non-inductive wirewounds and
in parallel they made a reasonably good 50 ohm load through about 15 MHz or so.

David

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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Thu 17, 2021 1:40 am 
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I have some of those also. The main reason I haven't thrown them out is that it's psychologically impossible for me to throw away anything electronic. So they end up taking valuable space until I die and my family throws them all out.


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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Thu 17, 2021 3:04 am 
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bob91343 wrote:
I have some of those also. The main reason I haven't thrown them out is that it's psychologically impossible for me to throw away anything electronic. So they end up taking valuable space until I die and my family throws them all out.


Funny! You are describing me exactly. 57 years collection all neatly boxed and up
on bunches of steel shelves. I can actually find things (usually). :)

David

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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Thu 17, 2021 10:17 am 
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I never throw away parts. If I have anything radio related I don't think I'll ever use, I take it to a hamfest and put it out on a table and either sell it or give it away for free. Someone gets it and uses it, or sells it...I don't care as long as my parts are out there in circulation instead of in the ground. At a minimum my payment is in new space at home and less clutter. I recently hauled all my old QSTs to a hamfest and gave them to a friend. He is enjoying them. I was too for several years but then I quit looking at them, they took up space, and old QSTs are on-line, unlike ER which I'll never get rid of.


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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Thu 17, 2021 5:27 pm 
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Fifty seven years? Wow! Well, I have stuff from about 70 years ago. What a shame that the only pleasure gotten from it is my occasional rediscovery and nostalgic throb.


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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Sat 19, 2021 7:05 pm 
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Location: Rochester, NY.
My encounter with Globars was on the Hallicrafters S-120. The Globar (R20) is in series with the two series #47 dial lamps that are also in parallel with R19 to keep them protected from surge at turn-on.
Someone had marked-up a modification on the schematic to change the 50C5 to a 35C5 and change the lamps from #47 (150mA) to #44 (250mA)?
The dial lamps still flare at turn-on and are short-lived. I doubt that LEDs are an option as the dial lamp circuit resistance is part of the heater string load.

https://bama.edebris.com/download/halli ... 0/s120.pdf


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 Post subject: Re: Globar Thermistors- Questions
PostPosted: Jun Sat 19, 2021 7:10 pm 
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Joined: Jan Tue 10, 2012 8:39 am
Posts: 2896
I don't think it would be difficult to put LEDs in. They could be protected with a zener diode or two and might help the tube heaters last longer.


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