Rich Measures wrote:
>>I didn't say it was "the power FET" that could be driven into reverse
>>voltage, but "the series stabilizer" as a whole. The point is that if
>>this happens, the stabilizer has no more effect and the negative screen
>>current is free to drive the screen voltage upwards. (The output
>>impedance of the screen supply was maybe a few tens of ohms when
>>actively stabilized, but then jumps to something like a few k-ohms,
>>determined mostly by the values of the dropper and shunt resistors.)
>>
>>The rising screen voltage increases the cathode current and also the
>>secondary screen emission, which drives the screen voltage up even
>>faster until ... !!
>>
>However, there should be, a screen bleeder resistor to obviate this
>scenario.
>
Sorry, I'm not making myself clear.
I *did* assume that there is a screen bleeder resistor. My point was
that it does not necessarily prevent this scenario, unless the value has
been carefully selected to cope with negative screen current.
There are three "normal" scenarios for a series regulator with a screen
bleeder resistor.
1. The tube is not sinking/sourcing any screen current. The series
regulator is supplying just enough current to maintain the correct
voltage across the load (the bleeder resistor).
2. The tube is sinking positive screen current. The series regulator has
to supply more current so that the voltage stays the same.
3. The tube is sourcing negative screen current. The series regulator
now has to supply LESS current so that the voltage stays the same.
Now here's the problem:
4. The tube is sourcing MORE negative screen current. The series
regulator is supplying ZERO current. This is the borderline of voltage
regulation.
5. The tube is sourcing EVEN MORE negative screen current. The series
regulator is already supplying ZERO current so the voltage across the
bleeder resistor will start to rise. There is NO voltage regulation.
That was the point I was trying to make. The permanent standing current
(in condition 1) through the screen bleeder must be greater than the
maximum negative screen current that can come out of the tube under any
conceivable operating conditions. If the resistance of the screen
bleeder isn't low enough to ensure this, then there will be no screen
voltage regulation on peaks of negative screen current - and that is
dangerous to the tube and the screen bypass capacitor (if used).
OK, I accept what Rich and Steve say about the possibility of using a
transistor shunt stablizer from a supply that's higher than its
breakdown voltage... but I still don't feel comfortable about it. To be
reliable, it certainly needs more careful design than most amateurs
would be prepared to put into a screen supply.
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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