On Jul 20, 2006, at 8:46 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.
3-500Zs fail from the filament helices bending laterally at their
unsupported mid-point and touching the grid - causing a filament-grid
short. Are you implying that mistuning or overdriving can cause this
problem?
> A resistor is much too slow to open and much too unreliable in
> opening point to protect any sensitive grid.
Sensitive? Did you not tell me you experimentally ran a 3-500Z grid
red hot without causing any damage?
>
> 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.
How does the grid become positive when it is being continually
bombarded by negative-charges (electrons) coming at from the cathode?
> Clamping the grid to several thousand volts
> positive is not good protection.
>
> <An electronic protection board is welcome, I don't see
> anybody disqualify
> <this option,
Can a 2A transistor be depended on to interrupt a >50A surge of grid
current?
> 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.>>
And .... ?
>
> 73 Tom
>
>
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>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
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