[AMPS] dummy loads

[Rich Measures]

>.......i am designing and building a 1.5kw linear amplifier, and i'm to the 
>stage where i've roasted an MFJ264 air-cooled 1.5kw dummy load.  it 
>is now a 150 ohm unit. 

Increased R indicates that dissipation capability has been exceeded.  If 
you check the actual rating of the resistor, I'm guessing that the 
resistor probably has about a 150w rating.  This may have something to do 
with why it's called a 1500w Dummy Load. 
-  If the "3kW" MFJ tuner has problems at substantially less than the 
advertised power, I suppose that it might be advisable to subtract 6 - 8 
db from MFJ power ratings.  If MFJ comes out with a "10kW" tuner and 
dummy load, maybe it will be able to handle the amateur radio power 
limit.   

> i know they only dissipate 1.5 kw for 10 
>seconds, but i guess i exceeded that.
>
>i've e-mailed mfj to see if they will supply replacement resistors, 
>but in the meantime, i bought an MFJ250 which seems to be like the 
>old heath "cantenna".  
>it is oil filled, and i'm using the xfmr oil 
>they supplied. .....

The Heath Cantenna appeared to contain roughly a 75w-rated resistor. 

> ....... it is rated at 1kw for 10 minutes, but they 
>don't say anything about 1.5kw at all...

... and they don't say anything about what this kind of overload does to 
the resistance of the 50 ohm  R.  

To minimize damage to Dummy Loads, use a tuning pulser.  I tune and test 
with a 30% duty-cycle pulse.  It tends to make things last longer.  

Bird sells terminating resistors that carry honest ratings.  
Rich---

R. L. Measures, 805-386-3734, AG6K   


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