>
>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?
I do not know. One of my customers told me that a transistor shorted in
the grid protect circuit during a glitch.
> 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.
>
There is obviously a better way.
>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,
the better way
>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.
>
OK
>>>
>>>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+.
>
My guess is that gold-sputtering is caused by caused by UHF.
cheers
- Rich..., 805.386.3734, www.vcnet.com/measures.
end
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