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 Post subject: Question About Globar Resistors- How Used
PostPosted: Jun Tue 15, 2021 11:40 pm 
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Hi, All-

I've been gradually acquiring these types over the years and I have
a fairly good assortment. My main use for the globar type resistor
would be for plate parasitic suppressors in high power RF power
amps (1 KW class). They can be used in place of paralleled carbon
comp types. I was first exposed to this type of use in that 3-1000Z
GG amplifier that appeared in the '63 and '64 ARRL Handbooks.
Low values are used, of course- on the order of 50 ohms +/-.

Here is my question- how were the higher value globars used in
radio/electronics? The one hint I have is "high voltage". I have a
fair number of these types in the 1 to 11M ohm range, sizes from
1/4" to 3/8" dia. and 1-1/2" to 2" long. I'm having a hard time seeing
any advantage to a resistor with a pretty high negative TC. They are
of course non-inductive, but what has me stumped is the high TC.

For things like HV DC meters one would want stable, high value
multiplier resistors. Yes, I have some of those, too. (grin)

I have no trouble visualizing how medium value globars could be
very useful in tube amplifiers (~1k to 100K ohm range) as non-
inductive loads, such as across the grid L/C of a grid-driven triode etc.
Also in passive grid setups (50 ohms in, 1:4 balun, 200 ohm res.
across grid).

I'd also appreciate any comments regarding carbon comps vs.
globars- in general- used in plate parasitic suppressors at HF.

Thanks, David

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 Post subject: Re: Question About Globar Resistors- How Used
PostPosted: Jun Fri 18, 2021 12:28 pm 
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Joined: Jun Fri 19, 2009 6:34 pm
Posts: 10885
Location: Long Island NY
"Globar" is kind of a generic term for just about any bulk ceramic resistor. The resistance comes from silicon carbide and other materials in the ceramic. They are made in a variety of styles and with different temperature coefficients. Thus a globar resistor for a parasitic suppressor may have different temperature characteristics than a globar for a color TV degaussing coil, and both of them may be different animals than a globar heating element. The chief advantages of this class of resistor are being non-inductive, generally more reliable than wirewound resistors since they have no wires to break, and being able to absorb huge spikes of power with ease.

Being non-inductive and able to absorb high energy spikes would make a globar a nice resistor to use for a transmitter parasitic suppressor, but one would probably not use one with a high temperature coefficient, or at least not allow it to get hot enough to change its resistance a great deal. That could possibly result in a situation where the grid drive or power output changes as the transmitter heats up.

As for the very high value ones, the Kanthal general catalogs do not show values higher than about 1 megohm, but custom values are probably available if one is willing to order a large enough quantity, and it's always possible that some values once available are no longer being made today. There are a lot of places they could have been used, for example to dissipate static charges on communications and power lines, high voltage power supply bleeders, focus voltage power supplies for CRTs, etc.

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 Post subject: Re: Question About Globar Resistors- How Used
PostPosted: Jun Sat 19, 2021 3:57 am 
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Thanks, Chris-

Well it looks like at one time they did make globars up around 8 to 10 megohms.
The ones in my photo in the lower left corner appear to be marked "755" with the
color dots (violet green green?). They measure anywhere from around ~8M on up
to around ~11M. I have a dozen of those. I am hard pressed to imagine how these
high values were used, that's why I wrote this post in the forum.

Thinking out loud, I'm thinking of antenna spark gap setups where a regular
~1m resistor was placed across the gap for a static drain to constantly DC
discharge an antenna. In the event of a hit, a globar might actually survive?
(inasmuch as a lot of the energy would be in the gap discharge) Depends of course
on just how close and how energetic the lightning stroke might be...

As for plate parasitic suppressors, the ones I have are not too wicked a TC. A graph
on the package shows them starting out around 175 ohms cold, 100 ohms at 100mA,
and 20 ohms at the rated 600mA. Of course designing these in is a bit of an art,
and generated a good bit of heat and controversy a couple decades back as two
hams squared off and the 'parasitic wars' ensued. The controversy even involved
QST Magazine at one point. Typical unit I would try would be the FRT-1 shown.

(Oops- forgot to mention, the globars photo is over in this forum,
scroll down to 6th post:)
viewtopic.php?f=5&t=398374&p=3394650#p3394650
I was getting reads but not any responses in this forum until Chris chimed in.

I have seen so many approaches with suppressors. One 4-1000A amp I worked
on had one of the hollow core NI 50 ohm units like today's Kanthals. About 3" long.
And various nichrome wire inductors... I have seen plenty that just use two or more
2W carbon comps in parallel, plus a small copper coil paralleled. Or copper
strap. Et cetera. In a VHF parasitic susceptible circuit, if that suppressor is
doing its job, the parasitic never gets a chance to start up- so the dissipation in
the resistors is usually small except up closer to 30MHz. AG6K (SK) wrote a very
interesting article on suppressors in QST. He also used low value, inductive,
metal oxide types at times. And bits of nichrome wire or ribbon...

The globar I described above is going to get a chance in a triode KW amplifier
I'm working on (single 5868) I am guessing it would sit with the resistance
somewhere around 100 ohms most of the time, but if it heats a bit that value
would be dropping. From it's size it is clear the little guys can absorb quite a
wallop of transient energy if needed. Anyway it's worth playing with. Sometimes
I just use a few low value 2W carbon comps.

David

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