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my regulator isn't regulating

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Posted on May 20, 2020 at 16:39:19
floydandrews
Audiophile

Posts: 318
Location: Port Townsend, WA
Joined: December 29, 2003



I am trying to build a simple regulator to power the voltage and driver stages of a tube amplifier that I am building, but it doesn't seem to be working. I want 300 volts out. The front end of the amplifier draws about 50 ma. On the attached schematic, the voltages that I am getting (with all tubes in place)are shown in boxes with arrows pointing to the DMV attachment points. I have tried R values (the resistor between B+ and the zener string) from 18.2k ohm down to 500 ohm. I have swapped in a new IRF 840. I have swapped in new zeners. I have checked for shorts to the heatsink. I am still getting about 320 volts at the top of the zener string with all values of R. I am getting about 330 volts +/- 3 volts at the output. I have built this style of regulator before, with good results. (meaning that I get the voltage out that I expected) What am I doing wrong? What should I try next?

 

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What you have drawn should work, posted on May 20, 2020 at 21:21:24
Chip647
Audiophile

Posts: 2651
Location: The South
Joined: December 24, 2012
 Make sure the MOSFET is not touching any metal and is isolated from its back metal tab. Make sure there is no shorting to a heat sink.

Test the Zener stack without the MOSFET, Just a 10K resistor to the 360v supply to the Zener stack. This should be 304v. If this is ok, then when you put the MOSFET back i place you should have 304 on the Gate pin of the MOSFET and 360 on the Drain.

If this is true, you should get 300 on the output.

Put a 10uf cap across the Zener stack.

 

RE: my regulator isn't regulating, posted on May 21, 2020 at 14:52:47
Blackdog
Manufacturer

Posts: 1505
Location: Ontario
Joined: March 20, 2006
 Does it regulate at the higher voltage? Where did you get the diodes? The tolerance spec is 20%. That may account for the difference.


Dan Santoni

 

RE: my regulator isn't regulating, posted on May 22, 2020 at 17:46:01
floydandrews
Audiophile

Posts: 318
Location: Port Townsend, WA
Joined: December 29, 2003
 Thanks for responding. I suspected that I had somehow fried the MOSFET while I was troubleshooting the regulator. I rebuilt it with a new IRF840 and new resistors. I also reduced the 18200 ohm resistor feeding the zener string with a 2200 ohm resistor to draw more current through the zeners. I also added a diode between the drain and gate. This seems to have worked. I am now getting 304 volts out. We'll see if it keeps working. Wish me luck.

 

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