Please see below:
Tom W8JI escribió:
> <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.
How fast can evolve this level of problems?. I know of an industrial HF
oscillator (350 KW) in which a system protects the triode by opening tube's
grid DC return (done by means of a reed relay), the difference is that after
the protection triggers also cut mains isolators to kill plate voltage.
>
> <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
. 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
Tom, I was referring to the fact that complex cicuits may not be
necessary,
(I wrote disqualify as to be disregarded) as a solution; well, maybe my
interpretation of the messages written in english is not 100% accurate.
> 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
>
>
Best regards
Guillermo - LU8EYW.
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