measures wrote:
>>We may be talking about different transistors. Let's start again.
>>
>>Where in the circuit is the transistor that you say burns out in a
>>glitch?
>
>In series with the grid current meter shunt. This transistor must open
>to stop the flow of excessive grid current. If the transistor has too
>much current, it C-E shorts.
I don't understand that circuit idea - who uses it? It does seem a
strange way of protecting against excessive grid current, under "normal"
conditions of excessive RF drive, incorrect loading or low B+. That's
before you even begin to think about its behavior under glitch
conditions of huge overload.
All the circuits I've seen use the transistor to sense the voltage drop
across the grid-current metering resistor. If there's enough grid
current to turn on the transistor, it switches the amp back to standby,
or something similar that will prevent RF drive from being applied.
There is a design "gotcha" in that circuit under glitch conditions
(which is what I thought you meant) namely that the overload - depending
on its precise nature - can apply either large amounts of base current
or a large negative base voltage. That's what the 10K resistor in series
with the base can prevent.
>>
>>Why do you believe that it should be removed, if that also involves
>>disabling grid current protection in normal operation?
>
>Grid current protection is disabled by the C-E shorted transistor. By
>replacing the shorted transistor with a fuse or fuse resistor, protection
>is restored.
You're talking about protection under glitch conditions - severe
overload. I'm talking about "normal" high grid current that can occur
due to excessive RF drive, incorrect loading or low B+.
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.com/g3sek
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