[Amps] SB220 Question... fuses

[Richard]

>
>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."  
>
€  chortle.  
-  Furthermore, when a 250v fuse is used to interrupt current flow from a 
tyoical 2500v anode supply, before the fuse can interrupt the flow of 
current, a metal-vapour arc takes place inside the fuse as the metal 
element melts.  The voltage drop across the arc is only c. 20v until the 
fuse explodes.  Hence, current limiting does not begin until after the 
explosion.  This is why fuses carry a current rating and a Voltage 
rating.  

>The same might be said for the grid of certain expensive tubes in 
>their ability to protect fuses.
>
€   1/4w - 1/2w  carbon-film (frangible) grid-fusing resistors, plus a B+ 
glitch resistor, have proven to be fairly successful at protecting 
3-500Zs from filament-to-grid shorts. 
,
cheers, George

-  R. L. Measures, a.k.a. Rich..., 805.386.3734,AG6K, 
www.vcnet.com/measures.  
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