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!
If the electronic protection circuit shorts during a major fault, which
hardly seems to be uncommon, there is no grid current protection. It is
my opinion that a foolproof, frangible reasistor is a more reliable.
>
>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.
That why I use a current limiting resistor in the anode current path And
a current limiting resistor in the grid current path. However, in 8877s
and the like, the grid should be directly grounded, and a.
In this case, the grid fuse R goes in series with the grid-I meter shunt.
A 50-120 V MOV should be connected in parallel with the grid fuse-R.
> This directly couples the anode supply to the
>catode, because the grid is heavily forward biased.
However, when a grid fuse-R opens, grid current stops.
>It also
>encourages a direct arc from anode to cathode, since the shielding
>effect of the grid is reduced or eliminated.
Not if the grid is bypassed with low V disc ceramic capacitors.
>
>2.) Fuses are bad enough for protecting grids. Fuses have thermal lag,
How much lag does a Collector-Emitter shorted transistor have?
>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.
>
My guess is that he is guessing.
>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!
That's why we use 30 ohm, 1/2w carbon film resistors instead of metal
film resistors.
>
>> 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.
Why? In a TL-922 the coax that connects the tuned inputs to the cathodes
is resonant near the anode-resonance freq of 130 or so MHz. In a SB-220,
a similar situation exists.
>That can be proven with measurements and circuit analysis.
>
When I used standard AC circuit analysis to show that your assertion that
virtually no VHF current flows in a typical VHF suppressor "coil" was off
the mark, you summarily dismissed the calculations.
>I can't think of many reasons to float a grid in a triode GG amp,
>unless you want to decrease stability.
>
In a TL-922, or a SB-220, directly grounding the grids slightly decreases
the grid's resonant frequency.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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