[Amps] Parasitic Resistors

[R L Measures]

On Jun 28, 2006, at 8:53 AM, Tom W8JI wrote:

>> I had used some metal film that RF Parts sells as
>> non-inductive 'flame proof'.
>> When you get to talking to them, the 'engineer' will tell
>> you that they are
>> slightly inductive.  I am curious if 'slightly inductive'
>> is too inductive for
>> this application.
>
> It depends on the resistor, but I would not use them.  In
> many cases they are too inductive.

Some are, some are not.  To see what the construction is like, scrape  
of the paint and see if the MOF spirals or is mostly straight.

A Matsushita (Panasonic) 100-ohm 3W-rated MOF resistor has c. 11nH of  
L. and will dissipate 12w for 1 hour in still air with no problems  
other than a decrease in R to c. 90-ohms.
>
> Most parasitics in tubes like the 3-500Z actually occur up
> around 180-200MHz. The exact frequency varies with grid lead
> to chassis length.

Some examples:
SB-220 --  110MHz
TL-922 --  120MHz
2K-4, L4-B -- 95MHz
Al-80, SB-1000 -- 150MHz.
SB-230 -- 155MHz
all figures are +/– a few MHz.
>
> ...
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