HV benchtop power supply, help on opamp setup - diyAudio
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Old 27th November 2007, 01:13 PM   #1
panos29 is offline panos29  Greece
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Default HV benchtop power supply, help on opamp setup

designed and tested the power supply shown below. Its a 100-300V regulated power supply capable of providing about 700mA according to Vout. The hum component is about 2mV at 700mA for rated output (100-300V). Good thing about it is that it is virtualy indestructable. I mean even if you accidentaly shortcircuit the output the pass element tube shuts down and no harm is done.
What I would like to ask is if it is possible to use instead of 6AU6 pentode and reference a better comparison device as an OP-AMP where we can benefit from extra gain available. As I am not much into solid state I would like to ask how is it possible to be done. Note that in order to achive the needed voltage adgustment, voltage between cathode and grid of the tube should be about -40V to -90V with respect to the cathode.

Any Ideas, help?
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Old 27th November 2007, 01:14 PM   #2
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Obviously something like that is needed.
I also noticed that there are not many high voltage opamps available accepting 80-100V
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Old 27th November 2007, 01:37 PM   #3
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No need for a high voltage op-amp per se, but then you need a driver stage for the tube pass element. The tricky thing with pass tubes is low mu, so quite a lot of drive is needed to turn them off. One thing which is useful, is a floating low voltage supply for the op-amp, makes things much easyer.
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Old 27th November 2007, 04:21 PM   #4
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floating allright, that is what I thought, but since I am not into SS i got no idea of where to reference the ground potential of the floating supply for the opamp. Moreover do I need a symmetyric suply for the opamp or a sigle one?
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Old 27th November 2007, 05:30 PM   #5
ilimzn is offline ilimzn  Croatia
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A symetrical supply makes it easyer for the Opamp, it's ground is referenced to the output of the regulator. To be honest, it's a 'bit' complicated to do with a ube as the pass element (many supplies needed, heater, HV drive, op-amp). You could use a single supply for the OPamp as well, but lay out the 'error amp' you use the op-amp for so that a single supply is enough, in whicjh case it will probably be best if it's positive side is referenced to the output of the regulator. Since you are using the cathode of the pass tube as the output, you could use it's heater supply to derive the supply for the OPamp.
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Old 27th November 2007, 05:35 PM   #6
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If HV opamps will help, check out the PA series from Apex. Not terribly cheap, but for a one-off, so what.
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Old 27th November 2007, 06:31 PM   #7
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U1 -- LR8N3 -- is a part which costs less than $1 -- used in switching supplies to regulate the front end it can be deployed thusly for a fixed voltage. Substitute a potentiometer for R2 and it'll be adjustable:

Click the image to open in full size.

Of course you can use it with a 6L6 for the pass element if you desire!

I keep a supply of them on my webstore, Mouser carries them as well.
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Old 28th November 2007, 08:07 AM   #8
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Allright, it seems that tube pass elements are not so popular. Thus let me unfold my reasoning in using 6C33. It is ment to be used in a stereo amplifier with another two 6C33s as output tubes.
The rectifier gives a 360VDC out and my needs for the amplifier are 240V adjustable (170-260V for testing purposes) with total current demand of about 400mA. Thus if I was opting for a mosfet I would need a heatsink to dissipate about 50W continyously, which is huge. On the other hand I allready have a good transformer with lots of sepparate windings for auxiliary purposes and space to mount the 6C33 pass element. And finally if this works out it can also be replicated for a very robust bench supply. ANy ideas of a HV comparator that I may use? I found LM143 up to the task but it is obsolete. On the other hand, is it possible to setup a common opamp driving a fet of bipolar to extend its output from about 20v to be able to swing about 100 that it is needed for the grid of the tube?
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Old 28th November 2007, 09:15 PM   #9
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A while ago I was browsing either Analog's or TI's site and saw they had an opamp with inputs that auto adjust for high voltages. I tried looking them up again but never figured out what they were.
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Old 28th November 2007, 10:29 PM   #10
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You can use something similar, just replace a trimpot by a pot, and recalculate some values:

http://amfone.net/Amforum/index.php?topic=4842.0

I don't believe that you need better regulation for tubes.
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