<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.
<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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