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Old 8th May 2010, 01:37 PM   #1
alecu7 is offline alecu7  Canada
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Default HV using zener diode

I'm building a SET KT88

Anybody can point me to a HV regulator for B+ using zener diode ?
How does it compare a regular CRCLC to a cap + voltage regulator + cap?

Thanks,
Alex
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Old 8th May 2010, 03:24 PM   #2
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High-voltage zenner diodes have too small dissipation power to handle such power requirements. You need to build full-blown regulated power supply if you want stabilized B+ voltage. T-Reg from Elektor 3/2009 is just fit perfectly your purpose.
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Old 8th May 2010, 04:36 PM   #3
alecu7 is offline alecu7  Canada
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Quote:
Originally Posted by LinuksGuru View Post
High-voltage zenner diodes have too small dissipation power to handle such power requirements. You need to build full-blown regulated power supply if you want stabilized B+ voltage. T-Reg from Elektor 3/2009 is just fit perfectly your purpose.
Could you provide the schematic please?
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Old 8th May 2010, 04:38 PM   #4
Arnulf is offline Arnulf  Europe
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Originally Posted by alecu7 View Post
Anybody can point me to a HV regulator for B+ using zener diode ?
Do you know how to build lower voltage regulator with zener element at the base/gate of pass transistor ? Well, replace zener for a string of higher voltage ones, makes sure you don't exceed their maximum dissipation and add a capacitor in parallel so that you get "soft start" effect of capacitance multiplier until your regulator hits zener breakdown voltage.
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Old 8th May 2010, 04:45 PM   #5
alecu7 is offline alecu7  Canada
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Quote:
Originally Posted by Arnulf View Post
Do you know how to build lower voltage regulator with zener element at the base/gate of pass transistor ? Well, replace zener for a string of higher voltage ones, makes sure you don't exceed their maximum dissipation and add a capacitor in parallel so that you get "soft start" effect of capacitance multiplier until your regulator hits zener breakdown voltage.
Thank you.

What kind of pass transistor do you recommend? Mosfet HV?

Alex
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Old 8th May 2010, 06:52 PM   #6
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Quote:
Originally Posted by alecu7 View Post
Could you provide the schematic please?
This works OK.

Click the image to open in full size.

You can build a better reg, but this one is simple and decent.

Quote:
Originally Posted by alecu7 View Post
What kind of pass transistor do you recommend? Mosfet HV?
IRF820 perhaps?
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Old 8th May 2010, 07:26 PM   #7
alecu7 is offline alecu7  Canada
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Quote:
Originally Posted by dsavitsk View Post
This works OK.

Click the image to open in full size.

You can build a better reg, but this one is simple and decent.



IRF820 perhaps?

Thank you for the schematic. Is it really necessary to add regulator module for vacuum tubes? For this amp I'm building B+ will be ~400V 175mA?
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Old 8th May 2010, 07:29 PM   #8
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Shirley it would be better to use a regulator tube to drive a pentode as a shunt regulator?
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Old 8th May 2010, 08:00 PM   #9
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High voltage zeners have horrible tempdrift. The voltage will rise by several volts as temp goes up. I've found a simple voltage divider with just two resistors, buffered my a MOSFET, makes a better voltage regulator than zeners. Simpler as well.
If the voltage divider feeds into a large capacitor you also get the benefits of a slowly rising voltage, which give the circuit a nice comfy slow start up.

Or use a regulator tube, such as 0B2 or the likes. Reg-tubes might be noisy in high gain preamps, so may need a little filtering and buffering. U dont want to place a cap in parallel to a reg tube since it'll likely oscillate.
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Old 8th May 2010, 08:09 PM   #10
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Hmmm. I reread your original post, and realize u want regulation for the whole amp, and not just the pre-section. That case a voltage divider will drop too much voltage. Unless u've got lots to spare.
175mA is a lot, and a triode in SE has no benefits of a PP cancellation of power supply ripple.
If it were me, I'd use CLC and then experiment to a cap value that shunts some ripple to the cathode of the output tube. That trick works great. That cap should be the value of the cathode bypass cap, divided by the triode's mu.
SET amps usually have a little ripple noise, and usually it's low enough that most of us can live with it.
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