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

[Bill Turner]

ORIGINAL MESSAGE:

On Thu, 20 Jul 2006 13:37:57 -0400, you wrote:


>>>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?
>
>>I have no idea. I would not use a 1/4 watt resistor as a 
>>fuse.
>
>>>Under what conditions would the resistor open?
>
>>Properly sized, it will do the job.
>
>How do you know that if you said you have no idea what the 
>opening time is?

------------ REPLY SEPARATOR ------------

You misquoted me. 

My comment about "properly sized, they will do the job" was about B+
glitch resistors in the anode supply, not about a 1/4 watt in the grid
circuit.

I'll say it again: I would not use a 1/4 watt resistor in place of a
fuse. They are not designed for that service and there is too much
variation between resistor types to consider them reliable. Even if
you tested a particular type and found it suitable, there is no
guarantee the manufacturer might not change the manufacturing process
sometime down the road and make it unsuitable. It's just not good
engineering.

Likewise, a B+ glitch resistor is not necessarily a fuse either, but a
current limiter in case of a major malfunction. It may or may not open
in such a case, depending on the energy available from the power
supply capacitors, the nature of the short and the characteristics of
the resistor.

Bill, W6WRT