[Amps] Derating Dry Loads

[Bill Fuqua]

    The best way to determine the rating of a dry load resistor is  to 
apply power and remove it about
30 seconds before the SWR goes up.
    Just like tightening head bolts on a V8. You torque the bolts until 
just before you fill them give and they snap.
73
Bill wa4lav


At 02:10 PM 12/7/2010 -0500, Sam Carpenter wrote:
>Be careful, sometimes the heat dissipation may not be the only factor. I can
>tell you that an internal arc at legal limit can do a lot of destruction. If
>the load takes it and runs away from 50 ohms with rapid heat, it could go
>boom. I once thought that I was switching from the south pacific leg of 75
>array to the northern Europe antenna but instead went to a 20 meter beam.
>(result of 807s) It blew the smoking coax completely out of the pl259 and
>smoked one of the three 3-500z tubes running. Just a thought that the
>conductors inside may be close enough to arc or burn with a shift in voltage
>point and it may be the amp that takes the hit too.
>
>Sam N9FUT
>
>-----Original Message-----
>From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
>Behalf Of Randall, Randy
>Sent: Tuesday, December 07, 2010 10:25 AM
>To: 'Mike & Becca Krzystyniak'; amps at contesting.com
>Subject: Re: [Amps] Derating Dry Loads
>
>I have used a 15 watt dry load @ 100 watts for 5 seconds without incident.
>It is the model that Radio Shack sold approx.  10 years ago.  It was also
>sold under the Welz (sp) brand.  In the specs it was rated for 100 watts for
>10 seconds.
>
>Good Luck,
>Randy
>
>Randy E. Randall AB9GO
>Network Engineer
>UC Health
>Randy.Randall at UCHealth.com
>Phone 513-585-7146
>Fax 513-585-7159
>
>-----Original Message-----
>From: amps-bounces at contesting.com [mailto:amps-bounces at contesting.com] On
>Behalf Of Mike & Becca Krzystyniak
>Sent: Monday, December 06, 2010 9:20 PM
>To: amps at contesting.com
>Subject: [Amps] Derating Dry Loads
>
>    I have Bird 8173 Coaxial load rated at 300 watts continuous.
>
>    I'm not that familiar with "dry RF loads" and I was wondering if these
>loads can be derated for brief periods of higher power like oil filled loads
>can?
>
>    I'm thinking of 1500 watts of CW dots for 5-10 seconds?  Just curious if
>I should even try this, or maybe go buy a few hundred feet of RG-58 to put
>in front of it...
>
>Thanks in advance.
>
>Mike K9MK
>
>
>
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