[AMPS] parasitic suppressors

[Rich Measures]

>Rich has said on a number of occasions, and, I believe, without
>contradiction, that it is well worthwhile measuring the value of
>parasitic suppressor resistors after a flashover. My question is:-
>
>Do the values change because of long term over heating, which would
>possibly show up in surface colour change?

When a carbon-composition resistor is overheated, visable signs tend to 
appear in the phenolic covering -- and the resistance slowly increases.  
When a MOF resistor is overheated, the color in the coating tends to fade 
-- and the resistance slowly decreases.   
>
>OR
>
>Is there an enormous pulse overload causing the value change? If so, why
>is there no apparent visual colour change - or is there? Does it depend
>on the resistor type?

There is apparently no colour change during a vhf parasitic osc.  
Typically, the event appears to be so brief that, short of an explosion, 
the heat does not have enough time to reach the surface.  
>
>If we assume that Rich is correct with the parasitic theory, then it
>suggests that the suppressors do get a large power input during the
>parasitic oscillation duration. If the change is due to long term
>heating merely because of the RF power dissipated from the plate current
>flow dividing between the resistor and the shunt inductor, then the
>change should be similar to that which would appear from heating the
>resistors with DC.

The footprints are different enough to enable one to tell them apart.  
cheers
Rich...

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


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