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Old 17th January 2007, 10:36 AM   #21
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Could an LM317 be used effectively as a current source, to develop a fixed voltage across a resistor to apply to the gate of a MOSFET series regulator, do you think?
Yes, it could. There are a lot of CCS options; in the regs I was using in my tube amps some years back, we used a simple jFET current source to pull a reference voltage across a resistor. But whichever method is used, I'd strongly recommend temperature compensation. And the use of wirewound resistors- high voltages mean lots of excess noise.
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Old 17th January 2007, 11:20 AM   #22
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I guess the trick is to to strap a zener across the LM317, so its maximum voltage can't be exceeded when the amp is under low signal conditions and the raw B+ is high. The LM317/zener combination would have to be in series with another resistor, I suppose, to minimize the risk of that happening.
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Old 17th January 2007, 11:25 AM   #23
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But that, of course, means you have to pay close attention to what happens at the reg output during startup.

The more I look at this, the more I think that a bypassed Maida is still your best bet. The parts count will be about the same as your CCS reference scheme plus pass device and the performance will be better because of the feedback.
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Old 17th January 2007, 03:26 PM   #24
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SY -- how about 10 or 15 mA ? -- I would compare the Maida, bypassed Maida and LastPAS regulators.

I am a bit leeeeerrrry of connecting my HP3577 to an HV power supply to run the impedance, but at least I can get an RMS estimate of the noise.

Should this be split to another thread?
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Old 17th January 2007, 05:30 PM   #25
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Quote:
Originally posted by ray_moth
Hi Wavebourn,

Thanks for putting up the schematic and the explanation. I must be a bit slow, I can't see a positive FB loop nor what would cause a soft start nor how it senses the amp starting to draw current. Could you please give me a clue?
The reference voltage B+ regulator takes from bias shunt regulator. Bias shunt regulator takes current from B+ regulator through the load. When no load the current is small and it very slowly charges the capacitor in the bias regulator; but if it is a refererence voltage for the B+ regulator it's increase causes increase of it's voltage that increases current that charges the cap in the bias regulator and so on... As soon as toobs become hot they start draw current and more quickly charge the cap in bias regulator up to the zener + VBE value, now full reference voltage is applied to B+ regulator and it increases the output voltage up to the nominal value.

However, if you draw a current from the bias regulator that is higher than the current that initially flows from B+ regulator, it will never start. Use it for bias purposes only.
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Old 17th January 2007, 10:39 PM   #26
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Hi RAY MOTH ,

I am a big fan of LM 317T ( I have hundreds of them on my
stock and I use it everytime its possible ) .

I agree entirely with SY , that Michael Maida regulator is
the simplest and the best design of HT regulators .
But if you dont want to use it , the decision belongs to you .

About your idea to use the LM 317T as a CCS , feeding a
resistor and strapped with a zener diode acroos it , regar-
ding to protect it against H.V. , I think that it will not work
any way , because with HIGH VOLTAGE like that , the zener
will take control , and the LM317 will lost the control about the
current , making the resistor (s) on the circuit , the only thing
to limit the circulating current . I do not recommend to do that .

If you really want to do that , make it with a semiconductorized
CCS , and in this case , the best choice would be The Ring of
Two , that uses two HV transistors , and has good stability .
Follow the SYs advice about wirewound-resistors , to lower the
noise at all .

If you want the best of two worlds , I suggest that you take a
look in this circuit , because it has a series MosFet regulator ,
an error amp and zener diodes , all of them , being fed by single
transistor CCSs , to sustain the stability . I think that you will
like the design .

http://www.duncanamps.com/images/dds...cct_mosfet.gif

I hope that all above , helps your final choice .

Best Regards ,

Carlos
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Old 17th January 2007, 10:46 PM   #27
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Quote:
Originally posted by jackinnj
SY -- how about 10 or 15 mA ? -- I would compare the Maida, bypassed Maida and LastPAS regulators.

I am a bit leeeeerrrry of connecting my HP3577 to an HV power supply to run the impedance, but at least I can get an RMS estimate of the noise.

Should this be split to another thread?
When we get to the test setup and results point, we can start a new thread.

Yes, I'd LOVE to see those results. Impedance is possible to measure with a scope- shunt the reg output with a resistor that draws, say, 10mA, then parallel that with a common emitter transistor with a collector load sized to draw, say, 5mA when the transistor is saturated. Then drive the base with a square wave. In theory, you could capacitively couple the supply to the 3577 with an f3 well below the square wave and see how much voltage bounce you get.
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Old 20th January 2007, 12:26 AM   #28
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Default MOS FET H.V. Power Supply

Hi Ray Moth ,

I really want to hear from you , about the comments and
suggestions that I did on my Post # 26 .

Did you see the schematics ??? What do you think about it ??

I am asking you , because I would like to build a similar
power supply for my work bench .

Any comment from you , will be wellcome !!

Carlos
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Old 20th January 2007, 04:23 AM   #29
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Hi Carlos,

I fully agree with you and SY about the use of wirewound resistors in a power supply. I also like your idea of using a ring-of-two CCS to set up a constant voltage across a (highly stable) resistor. Using the right transistors means one doesn't need to worry too much about destruction by high voltages. It seems like an attractive alternative to using a stack of zeners or VR tubes.

Duncan's voltage reg schema seems very complicated for what I want to do. It may be very accurate but I think that's probably not so important for sreen stabilization.
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Old 20th January 2007, 05:35 AM   #30
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Quote:
Originally posted by ray_moth

Duncan's voltage reg schema seems very complicated for what I want to do. It may be very accurate but I think that's probably not so important for sreen stabilization.
I've used couple of 120V zeners and one 0A3 with IRF730 source follower for this amp (0A3 mostly as an aestetical indicator of HV presence):


Click the image to open in full size.
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