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Author Topic: Cantenna dummy load resistor  (Read 6624 times)
NQ4T
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« on: September 14, 2017, 11:08:38 AM »

Hi,


I recall reading a while ago that all that were inside the cantenna dummy loads were a carbon tube resistor and (toxic) oil.

I've seen several DIY dummy load instructions, but never using these types of resistors. I have one from a 300 watt one from a MFJ Versa Tuner.

Are these suitable for dunking in oil? What kind of power handling can I expect? I have an Ameriton AL-80B I need to not only test....but probably run long enough to regetter the 3-500Z tube.

Thanks & 73,
-Jay/NQ4T ex-KM4JOJ
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KC4ZGP
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« Reply #1 on: September 14, 2017, 12:53:19 PM »


I've been thinking of putting the 50 ohm resistor from my Versa Tuner in a paint can filled with motor oil.

All the oil does is dissipate the heat. Right?

You go first. OK.

Kraus

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K6AER
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« Reply #2 on: September 14, 2017, 02:11:24 PM »

Kraus,

Most non vegetable oils are toxic.

Oil dissipates the heat.

Lubricates the electrons for faster VHF performance..................

Keeps the Joules from rusting...........

























The last two I made up.
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K0UA
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Posts: 1355




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« Reply #3 on: September 14, 2017, 02:48:44 PM »

Kraus,

Most non vegetable oils are toxic.

Oil dissipates the heat.

Lubricates the electrons for faster VHF performance..................

Keeps the Joules from rusting...........

























The last two I made up.

That one got a laugh out of me..
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WB6BYU
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Posts: 17051




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« Reply #4 on: September 14, 2017, 03:47:45 PM »

Mineral oil is non-toxic.  A gallon is cheaper from a vet or a farm supply store.
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K4HB
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Posts: 233




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« Reply #5 on: September 14, 2017, 05:15:44 PM »

Definitely not motor oil, it's flammable, toxic, and has additives that's corrosive to the resistor. The recommended replacement is PCB-free transformer oil. I've heard that mineral oil will work, but won't dissipate quite as much heat as transformer oil. Never tried it, so don't really know.
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K8AXW
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« Reply #6 on: September 14, 2017, 05:21:22 PM »

At one time (I have one) the Cantenna was filled with transformer oil that contained PCBs.  PCBs is considered now to be carcinogenic.

However, it is worth researching if present day power pigs (transformers) use this same type of oil.  If it doesn't I recommend trying to scarf a gallon from the local power company.

Motor oil is out.

Mineral oil will work but has less ability to dissipate heat than transformer oil.

The resistor used in the Heath Cantenna is a hollow, non-inductive 50 ohm resistor.The oil flows around and through the resistor to cool it.

You won't be able to run it continuously long enough to getter a 3-500Z.  The Cantenna has a curve painted on the side telling you how long you can expect to operate it at "X" amount of watts.  I can provide you with this information if you want it.

I can't answer your question about using the 300 watt resistor from the MFJ being submersed in oil.  Oil just might break down this type of resistor.  No doubt you will get that answer shortly.

  
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WB6BYU
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« Reply #7 on: September 14, 2017, 06:06:08 PM »

I think MFJ sells an equivalent to the CanTenna.  You could check the parts list and see if it is the
same resistor as in their dry dummy loads and/or tuners.
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W8JX
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« Reply #8 on: September 14, 2017, 06:12:25 PM »

Definitely not motor oil, it's flammable, toxic, and has additives that's corrosive to the resistor.

Non detergent SA motor oil can still be found 10, 20 and 30 weight and is no more flammable than mineral oil.
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--------------------------------------
Ham since 1969....  Old School 20wpm REAL Extra Class..
KC4ZGP
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Posts: 1637




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« Reply #9 on: September 15, 2017, 05:54:04 AM »


Yup JX.

One gallon of Pennzoil 10W-30 I think will fit the bill. But the best dummy load is a tuned antenna.

I'm a mono-band antenna man. A regular chick magnet.

If one loads and tunes on the antenna, then they're on their way to where they aim to be.

Say K6AER, you what blows fuses don't you. Answer queer electrons.

Kraus

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NQ4T
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« Reply #10 on: September 15, 2017, 06:49:15 AM »

Sounds like I may need to get some HV transformers and build something that will regetter them out of the amp...I don't know how long that will take. I don't even know if it needs to be done...all I know is this amp has sat idle for a couple of years.

I may be better off just buying a new replacement tube.
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WA3SKN
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« Reply #11 on: September 15, 2017, 06:58:00 AM »

OK, the resistor from the 300 watt tuner is rated 20 watts continuous. You do 100 watts for 10 seconds max, then let it cool awhile before keying again... you are testing, right?   Yes, you can put it in oil to help dissipate the heat.  It does not have much reactance through about 60 MHz. (PCB is out, vegetable oils are in).
There are other non-inductive resistors out now, but they also need heat sinks for their wattage ratings.  You shield either of them to keep from radiating much power.
And having a known good dummy load is the first step in trouble shooting either transmit or receive troubles, so have one available.
73s.

-Mike.
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N3DT
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Posts: 1252




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« Reply #12 on: September 15, 2017, 07:28:52 AM »

If I were you and want to get that tube red hot, I'd feed it into an 80M antenna in the middle of the day when the band is dead, FM or CW mode. Or any other band that is closed a lot, like 10M lately. I doubt anyone will complain. Just ID yourself. I keep a mirror on the output vent of my 80B and it doesn't take maybe 5-10 seconds in FM mode at 750W to get that thing red hot. That cantanna will take to long to cool off. It's good enough for a tune up though.
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KD0REQ
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Posts: 2017




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« Reply #13 on: September 15, 2017, 01:59:46 PM »

uh, "antenna as dummy load" is also called jamming, and it's against the law. that's reserved for 14.313.

the Cantenna is a test device, not a killer load for a full gallon (as they said back in the day.) the MFJ equivalent is functionally the same thing.

if you are getting color on the plate, you're heating it enough to getter. doesn't have to be bright orange.  on some amps like the AL-80 B, you can change the transformer tap for gettering to cut the plate voltage, and the dissipation.  see the W8JI website for details.
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W8JX
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« Reply #14 on: September 16, 2017, 07:11:01 AM »


One gallon of Pennzoil 10W-30 I think will fit the bill.


You would want to go to a farm store on get old style SA rated non detergent straight weight oil (non detergent oil was never multi-viscosity) not modern motor oil which has a lot of additives. 
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Ham since 1969....  Old School 20wpm REAL Extra Class..
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