Rich Measures wrote:
>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.
>>
Wouldn't it be more logical to ask *why* the electronic protection
shorts during a major fault, and fix that problem?
Removing the electronic protection means there is NO protection against
lesser faults, such as incorrect loading and/or persistent overdriving.
These are equally capable of destroying the grid - it only takes a
little longer.
Rich's own web pages describe how to protect the grid and anode current
meters from the current surges in "major faults", using shunt diodes.
Since the fault protection circuit is essentially connected across the
grid current metering resistor, the grid current sensing circuit ought
to be as safe as the meters.
If it doesn't work out that way, then there's a design error. The meter
protection diode clamps at abut 1 volt, so a surge limiting resistor
between that diode and the base of the sensing transistor ought to do
it. Another possibility is voltage drops in the chassis itself, if the
emitter of the sensing transistor is connected to chassis at a different
place than the diode.
Find the error, fix it, and you can have routine fault protection that
will also survive a major fault.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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