To: <amps@contesting.com>
> Date: Wed, 20 May 1998 08:57:17 -0400
> From: Bill Cotter <bcotter@pop.uky.edu>
Hi Bill,
> How can the application of a grid resistor (as a fuse) be harmful?
The suggestion is that a resistor be used to provide current limiting
for faults, and to provide overcurrent protection in operation. The
most harmful part of that suggestion is advice given that suggests
an amp owner doesn't need fast electronic overload systems, and that
they should be removed!
1.) The proper place, and by far the best place, to install a current
limiting impedance is in the anode supply line. If you depend on a
grid resitor for fault protection, it allows the grid to reach anode
voltage during a fault. This directly couples the anode supply to the
catode, because the grid is heavily forward biased. It also
encourages a direct arc from anode to cathode, since the shielding
effect of the grid is reduced or eliminated.
2.) Fuses are bad enough for protecting grids. Fuses have thermal
lag, and unreliable time and ambient temperature dependent opening
currents. If you select a fuse close to the maximum desired current,
it generally will fail from fatigue at some point in time. If you
select one with enough headroom to prevent fatigue, it will not fail
faster than many grids.
3.) Resistors are worse. They not even designed to be fuses.
Many small resistors take hundreds of amperes for milliseconds to
fail, and some even fail in a low resistance mode!
> Is there any harm in replacing the normal choke with a resistor?
A resistor is better than a choke, but the best solution is to ground
the grids unless you are using the grid impedance for some sound
reason that outweighs loss of cathode circuit arc protection.
The stuff about negative feedback or better stability is *mostly*
nonsense. That can be proven with measurements and circuit analysis.
I can't think of many reasons to float a grid in a triode GG amp,
unless you want to decrease stability.
73, Tom W8JI
w8ji.tom@MCIONE.com
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