[Amps] SB220 Question... fuses

[George T. Daughters]

It has been written:

> >I am doing some mods from Harbach on the 220 and one of them is the
> >fine wire fuse between the HV feed through and the PC board.  Has
> >anyone done that and could you use a small amp fuse and fuse holder
> >rather than the wire and PC board mod?

Rich replied:

> €  A fuse will not prevent damage** to the tubes.  A fine wire will
> not prevent damage to the tubes because it is essentially a fuse..  A
> glitch resistor - that can pass the screwdriver test - will prevent
> such damage. 
>  The trouble with fuses is that they do not limit peak fault current. 
>   
> Glitch resistors do. 
> 
> ** (filament-grid short)

I've told this story before, but maybe it's time to repeat it.  When 
I worked at Fairchild in the early 60's, we tested many of the common 
fuse types to see which would be best at protecting (then) expensive 
silicon transistors.  To sum up a very extensive spread sheet (we 
called them "data tables" back then) even the lowest rated fuse would 
carry a HUGE current for a while.  A 1/8 amp fuse would carry 20 
amperes for milliseconds... long enough to zap the transistor.  I 
just made up those figures, (I don't have the data with me) but you 
get the idea.

Our summary statement was "An expensive silicon transistor is an 
effective way of protecting a cheap fuse."  

The same might be said for the grid of certain expensive tubes in 
their ability to protect fuses.

73,

George T. Daughters, K6GT