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[AMPS] Re: resistance

To: <amps@contesting.com>
Subject: [AMPS] Re: resistance
From: w8jitom@postoffice.worldnet.att.net (w8jitom@postoffice.worldnet.att.net)
Date: Wed, 26 Nov 1997 10:34:18 +0000
> To:            amps@contesting.com
> From:          km1h@juno.com (km1h @ juno.com)

> I have brought this point up many times here...I'll try AGAIN.
> 
> A 2W carbon resistor will show a gradual and steady R increase when run
> above 1W or so....this is at DC. 


Amen Carl. That's why carbon resistors should NEVER be used in power 
supplies across electrolytics.
 
> At RF, the heating usually occurs in the 10-15M range, depending upon the
> tubes used and the suppressor design. 
> At RF the 2W carbon can double, triple, etc.  in value in a few minutes
> or over several years...again it all depends on the design. 

Once again VERY true. That is a natural characteristic of the 
resistor. You have to NEVER run the resistor close to its rating, or 
in a very hot environment unless you use a specified quality of 
carbon resistor.

> My own rule of thumb is that 300-400W is the maximum RF on 10M per 2W
> carbon for reasonable long term suppressor reliability. If you have more
> RF then double or triple up the wattage or switch over to a modern metal
> oxide. 

Most metal oxides are inductive, be very careful using them.

> IMO, the 2W carbon will quickly become obsolete.

It's on the way, and so are tubes. 

73, Tom W8JI

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