[Amps] grid fuses

[Paul Christensen]

>I joined this list to learn about amp design, not repair.

It's a tough neighborhood here -- no doubt about it.  However, I have 
probably learnd as much about amp design from the hostile discussions as 
much as the friendly ones.  Make no mistake: I am not advocating hostility. 
I'm simply stating a peronal observation.

I can see the
> point about not designing a fuse or limit resister in the grid circuit
> because it seems to be a band-aid for poor fault design.  How do you 
> justify
> designing it in?

I don't know whether or not grid fuses are appropriate in some amp designs, 
but I will offer this answer to your question:  Sometimes, the least complex 
solution is the best compromise.  While electronic grid protection may offer 
the quickest response to a grid fault condition, the designer must design 
the circuit in such a way that its failure causes no additional harm to the 
amplifier above and beyond that caused by the underlying fault condition. 
For example, if a switching transistor is used for grid protection, do we 
have assurance from the designer that a collector-emitter short will not 
cause additional harm to the amp?  Is there even the remotest chance that 
the circuit can fail and not provide 100% assurance of grid protection?

Some good electronic grid protection circuits do exist.  But in terms of the 
complexity/effectiveness ratio, I still ask this question: are most grid 
failures the result of instant, excessive Ig, or sustained Ig?  If a fused 
grid protection circuit can be made effective in the design, is it necessary 
to add additional cost, complexity, and additional potential points of 
failure in the system?

By analogy, we could employ electronic current sensing on all our home 
appliances.  When AC current rises above a safe level, the AC circuit could 
be made to immediately open rather than depend on a fuse or circuit breaker 
to blow or pop.   Notwithstanding UL and code issues, if a cruder method 
(the fuse) is just as effective, less complex and less costly, is it right 
to condemn the designers of all home appliances because they chose to use 
fuses rather than electronic current-sensing switches?

Paul, W9AC