>
>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.
>
If one assumes that the amp "designer" did Not follow the tube
manufacturer's recommendation, all bets are assuredly off.
>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).
>
ok, which includes the sampling circuit current for the comparator.
>2. The tube is sinking positive screen current. The series regulator has
>to supply more current so that the voltage stays the same.
>
ok
>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.
>
ok
>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.
>
The solution is to fire the "designer" and increase the screen bleeder
current to the tube mfg. recommendation.
>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.
Agreed.
>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.
>
So, operate a series string of name-brand 20v - 30v 5w zeners at no more
than 2w, biased from the anode supply through a suitable R, and one has a
foolproof screen regulator for smaller tetrodes. For tetrodes with
handles, the positive floating 723 pass circuit works well.
- later, Ian.
Rich...
R. L. Measures, 805-386-3734, AG6K, www.vcnet.com/measures
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