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

[Tom W8JI]

> How does tying the grid hard to ground help, apart from 
> the profits of the
> tube retailers?


Nonsense about manufacturers being in a conspiricy to make 
money really is just a last ditch attempt to appeal to 
emotions rather than common sense and good engineering.

Common sense and good engineering dictate any fault 
protection for flash-overs be in the HV supply line, not 
downstream in a grid.

Protection for excessive drive or RF induced grid current 
belongs in the cahode path of a GG amplifier.

The grid is a barrier that protects and isolates the input 
from the output, it belongs connected to ground unless there 
is a compelling reason to NOT do that and the designer is 
willing to accept an increase in input system failures as a 
price for floating the grid.

> The fact that the grid "is the first thing in line" is not 
> in dispute,
> HV-grid fault current and its duration is.

As I said, protect it at the source. And at least do 
something responsible and sensible like using the correct 
compoenets, not carbon grid resistors as fuses!

> It seems that you are inferring that a frangible component 
> in the grid
> circuit, when subjected serious overload provides no more 
> protection than a
> solid strap.

1.) Floating the grid opens a can of worms. We lose the 
barrier between HV and cathode.

2.) A carbon resistor is about the strangest thing I've 
heard of to use as a fuse to protect sensitive components. 
It is horrible engineering.

3.) Universally suggesting people remove overload systems 
based on not having even read or understood a schematic is 
not helpful to anyone.

There are right ways to do things, and wrong ways. Fusing a 
grid is bad enough, using a resistor as a fuse is worse, and 
telling people to get rid of real overload protection takes 
it all one level lower.

73 Tom