[AMPS] Screen regulator circuit needed

[Rich Measures]

>
>Rich Measures wrote:
>
>>>Note that the necessary breakdown voltage foir a shunt stabilizer device
>>>is not the output voltage, but the INPUT voltage, to handle transient
>>>situations where the device is completely cut off and the
>>>drain/collector voltage rises all the way up to the input voltage. (This
>>>is also why you can't use a transistor shunt stabilizer with a dropper
>>>resistor from a B+ supply of more than 1-1.5kV.)
>>
>>I disagree.  It seems to me that there can never be more potential across
>>a shunt regulator than the regulated output voltage.
>
>Don't forget the transient conditions - like, how do you switch it on?

Piece of cake.  One causes the shunt transistor to temporarily switch On 
as the  input potential is ramping up.  Or, for a pass transistor, one 
puts a R-C delay ahead of the Source, or Collector.  [Figure 8. on my Web 
site].  

>Before the feedback loop stabilizes, the shunt device is drawing zero
>cirrent and is exposed to the full unregulated voltage (almost as if the
>dropper resistor wasn't there), and this may well destroy the
>transistor.

...not with the aforementioned circuitry.  
>
Another workaround for small tetrodes is to employ a string of 20 - 30 
volt, "5w"*-rated zeners to shunt regulate the screen..  {*the real-world 
dissipation per unit is 2w}.  

>Active (feedback) regulators are subject to a variety of other transient
>conditions, so it seems to me like a bad risk to use devices that
>wouldn't withstand the full voltage that could be applied; but once
>again that is a matter of design style.
>
This is why the <2000v screen regulator circuit on my Web site uses a 
crowbar-SCR circuit to protect the 600vds pass FET.  When the Source 
Drain potential exceeds c. 80% of V-DS, the crowbar fires its SCR, and 
nothing is damaged.  
>>>
>>>For higher input voltages I'd be inclined to go with Rich's series
>>>stabilizer circuit. Rich does point out that it demands a shunt resistor
>>>capable of accepting the full worst-case negative screen current without
>>>driving the series stabilizer into reverse voltage, to avoid the very
>>>real problem that John describes.
>>
>>Power FETs can not be driven into reverse-voltage because they have an
>>internal diode between the Drain and the Source.
>
>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.  

>Even if the transient is very short and doesn't go into that runaway
>situation, the spike of screen voltage will have a pretty dramatic
>effect on IMD. 
>
Which is another reason to use a screen bleeder R.  
>>>
>>>If voltage is not a problem, I generally prefer the shunt circuit
>>>because the IC and associated components are usually close to chassis
>>>potential, and easier to work on than the series circuit where they're
>>>floating above screen potential.
>>
>>In large tetrode Class AB1 amplifiers, the screen is usually grounded.
>>This means that the (pos.) Drain of the FET pass-regulator is also
>>grounded.
>>
>That's true, and a point in favour of the series regulator in that case.
>
>>>But mostly it's a matter of design
>>>style and preferences. Properly designed, either circuit can work well.
>>>
>>A 1500V, 150mA shunt regulator requires a >>1500v transistor/s that
>>dissipates 225w during standby.   A 1500v, 150mA series-pass regulator
>>requires a 500v FET that dissipates perhaps 10w during standby.  
>
>I agree about the voltage ratings, but you're only considering the
>positive current. If you're rating the screen supply to either deliver
>+150mA or sink say -100mA, the series regulator needs a shunt resistor
>of 15k from screen to cathode, and is always passing 100mA more than you
>assumed. The best solution depends on the tube in question, and how
>likely it is to go into negative screen current.
>
The higher the screen potential, the lower the negative screen current.  
For example:  With 1500v on the screen of an 8171, 25mA of screen bleeder 
current is more than enough.  As I recall, the max.  neg. screen current 
during tune-up was under 10mA.  

-  later, Ian.

Rich. Measures.,www.vcnet.com/measures, 805.386.3734


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