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Re: [Amps] Grid fuses (was: Life and gain of 3-500Z)

To: <Gudguyham@aol.com>, <noway@nohow.com>
Subject: Re: [Amps] Grid fuses (was: Life and gain of 3-500Z)
From: "Tom W8JI" <w8ji@w8ji.com>
Date: Thu, 20 Jul 2006 06:08:44 -0400
List-post: <mailto:amps@contesting.com>
> In a message dated 7/19/2006 3:17:48 PM Eastern Daylight 
> Time,
> dezrat@copper.net writes:
>
> That's  news to me.
>
> In my experience, a fuse is one of the most reliable of 
> all electronic
> components, especially when compared to the active devices 
> in an
> electronic protection circuit.
>
> To each his own.
>
> Bill,  W6WRT

Bill,

What is the typical reliable opening current and opening 
TIME of a fuse compared to the safe holding current? Have 
you ever looked at it?

What is the reliable opening point (where it goes 
near-infinite resistance) of a 1/4 watt resistor compared to 
safe working current? Have you ever looked at that?

How many designs do you see that use resistors as fuses to 
protect components?

Under what conditions would the resistor open?

Under what condition would an electronic circuit open? How 
long would it take?

If we had a fault from anode to grid, which is the ONLY case 
where a resistor would reliably open, would we want to 
remove anode voltage from the tube or would we want to let 
the grids float up to full high voltage, making the grid 
several thousand volts positive with respect to the cathode? 
Where would that several thousand voltage go, if not to 
ground, when the grid is closely spaced to the cathode?

I think if you stand back and look at the claims fuses or 
(much worse yet) resistors are a good idea in the grid by 
looking at the entire SYSTEM and what you actually want to 
protect, you will see the answer. Of course the decision of 
what we want to believe can be based on measurable and 
documented fact or on blind faith in someone's word or 
guess. I prefer fact, others may not.

73 Tom 


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