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

[R L Measures]

On Jul 19, 2006, at 10:53 AM, Tom W8JI wrote:

>> 1. Why not use a real fuse? There are some micro-miniature
>> fast blow
>> fuses available which would not add much lead inductance,
>> if that is a
>> consideration.
>>
>> 2. If the fuse blows, you wouldn't want the grid to be
>> floating I'd
>> think. Do you parallel the fuse with a high value resistor
>> to maintain
>> DC continuity?
>
> Why would a person put a fuse between a control grid and
> ground?
>
> First, if we want to protect for grid current from drive
> power it should be an electronic protection.

Agreed, Tom.  Over-drive goes with over-currents of perhaps 500%,  
gauging by the use of 1A to 2A transistors in such circuits.   
However, the 8877 is easily capable of >50A of emission.   What is  
50A likely to do to even a 4A transistor?

> Any fuse is
> well-known to be too unreliable in a system like a control
> grid.

Mr. Rauch. both you and I have tested 3-500Z grids at red/orange heat  
sans-damage.  Fuses melt first.

> Fuse failure is always a time issue related to ambient
> temperature and overload amount. Resistors are totally
> useless in this application.

IME, specific resistors can be used to guard against grid damage.
>
> Second, if  the issue is protecting from a HV fault the fuse
> should NEVER be in the grid circuit. It should be in the HV
> path to the tube.

Please explain

> The very last thing in the world we would
> ever want to see happen is a fault open the grid to chassis
> path and allow the grid to go positive to the anode supply
> rail.

If the tube is not gassy, and the grid is floating, the electron  
stream from the cathode probably raises the grid-cathode (fil-CT)  
potential to a few volts negative.

> If the grid goes up to the supply rail, and it will
> during an anode-to-grid fault,

only in a gassy tube.

> a shunting resistor won't
> help a bit. All that HV will dump into the cathode circuit,
> and perhaps right back to the exciter.

There's a new one.
>
> Lifting a grid in a GG amp is a very last resort. It was
> only done in the 811H and 572B to equalize tube drive power,
> and intentional protection was added to minimize damage to
> exciters during a flash over. That was a calculated tradeoff
> between tube life (from unequal current in four tubes) and
> risk of exciter damage, and clamps were added to protect the
> exciter.
>
> Common sense should tell us using a resistor as a grid fuse
> is one of the worse ideas possible unless there is a
> compelling reason to add intentional grid path resistance.
> It is the worse kind of fuse,

Worse than #18  buswire?

> it does nothing for normal
> grid overload, and it is a disaster to have there if the
> tube arcs.

Normal grid overload is not the problem here, Tom, it's intermittent  
parasites.

Cheers
>
> 73 Tom
>
>
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R L MEASURES, AG6K. 805-386-3734
r at somis.org