[Amps] grid fuses

[Will Matney]

See Below;

*********** REPLY SEPARATOR  ***********

On 7/20/06 at 11:46 AM Tom W8JI wrote:

><But for an amateur transmitter, you can replace a resistor 
>(yes, a little
><more work involved) and get on the air again. If the fault 
>is severe, both
><systems will keep you busy until the problem is resolved.
>
>The issue is if the resistor adds any protection at all, and 
>if adding it causes other problems.
>
>The answers are:
>
>No, it is not a good protection method for grids. That can 
>be easily proven. The primary source of grid damage is 
>excessive current from mistuning or over driving the tube. A 
>resistor is much too slow to open and much too unreliable in 
>opening point to protect any sensitive grid.
>
>Yes, it creates a new level of problems. Now when the tube 
>faults and if the resitor opens, the grid can rise to full 
>anode voltage. Clamping the grid to several thousand volts 
>positive is not good protection.

I'm having a hard time seeing this. I'm not sure if any others do. If the grid is not connected to anything after
the resistor or fuse blows, how can any damage be done where the grid can't make a circuit to anything else?
There cant be any grid current from the cathode to the grid since it's dis-connected from ground. There can't be
any circuit between the grid and the anode either unless for an arc. However again, there's no circuit as the grid 
is not connected. Operating current also can not go backwards in a tube from anode to cathode. It can only flow 
from cathode to anode. About the only thing I could see is the triode act as a diode with the grid open. If the tube
can stand the current operating as a diode momentarily until the amp is shut down, I don't see a problem here. Am
I not seeing something here?



>
><An electronic protection board is welcome, I don't see 
>anybody disqualify
><this option, but it is not so simple and more costly, 
>speaking in terms of
><amateur amplifiers
>
>Directly from http://www.somis.org/perfect_amp.html
><<Grid Protection
>I have performed autopsies on too many kaput amplifier tubes 
>that died in HF amplifiers. Some of these tubes had damaged 
>grids - but the damage was the unique type that is caused by 
>VHF or UHF current. Strangely, I have never found a grid 
>that was damaged by excessive HF grid current. Perhaps this 
>isn't so strange. I'm sure it's possible to roast a grid. 
>Tuning up key-down for a couple of minutes on 10m with the 
>load capacitor set for 40m comes to mind. This would result 
>in very high grid current and almost no RF output. However, 
>since most people - myself included - tune a grounded-grid 
>amplifier for maximum RF - and maximum RF virtually 
>coincides with normal grid current - very few people are 
>likely to overheat a grid. Thus, complex electronic 
>grid-protection circuits are unnecessary. A major 
>disadvantage of electronic grid-protection circuits is they 
>are not effective against the major source of grid damage - 
>sudden, large bursts of VHF or UHF grid current. A more 
>foolproof method of protecting the grid is a fuse or fuse 
>resistor. Carbon film resistors make good grid fuses.>>
>
>73 Tom
>
>
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Best,

Will