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

[R L Measures]

On Jul 19, 2006, at 8:29 AM, Bill Turner wrote:

> ORIGINAL MESSAGE:
>
> On Tue, 18 Jul 2006 18:57:55 -0700, you wrote:
>
>
>>> What is the fusing current rating of the resistor? Why would
>>> you fuse a grid?
>>
>> Because 0.25w carbon-film resistors cost less than 3-500s.
>
> ------------ REPLY SEPARATOR ------------
>
> I'm a bit late getting into this discussion, but my curiosity has been
> aroused.
>
> Two questions:
>
> 1. Why not use a real fuse?

Real fuses work okay provided they aren't slow-acting.  Real fuses  
also have resistance.  I like to use a frangible resistor since one  
does not need to install a fuse holder, resistors are less expensive,  
and a carbon film R tends to limit current better than a fuse before  
it blows.
  - note - We currently use 0.25w, 30-ohm carbon-film resistors for  
grid-fusing 3-500Zs, 811s, and 572Bs.

> There are some micro-miniature fast blow
> fuses available which would not add much lead inductance, if that is a
> consideration.
>
Lead L is not a factor since the grid is grounded for RF through caps.
As I see it, smaller is a disadvantage if there's room for the  
ordinary stuff.
> 2. If the fuse blows, you wouldn't want the grid to be floating I'd
> think.

Nor would I at first glance, however, nothing awful seems to happen  
when the grid floats.  Examples:  When a SB-220 blows a grid RFC,  
nothing happens -- or when a 3-500Z is high-pot tested at 9000volts,  
it makes zero difference if the grid floats.

> Do you parallel the fuse with a high value resistor to maintain
> DC continuity?

No, Bill, but the TL-922 has a 5k R in parallel with the grid RFC,  
presumably for this purpose.  I can see the logic of using one, but I  
know it does not prevent filament-grid shorts or other damage to a  
3-500Z.
>

cheers
> Bill, W6WRT
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R L MEASURES, AG6K. 805-386-3734
r at somis.org