Hi everyone.

I finally finish the circuit I asked about some days ago. This time I have been carefull and I think there are no typos and I have labeled each component. It is part of a triode tester and this is the power supply for the anode/plate. It is supposed to give 0 to 300V.



The problem is, it only gives 0 to 181V. Since is inside a cage, I have removed the PCB to run some tests. These tests have been made with 230V instead 300V because the 300V transformer is inside the cage and It is difficult to do the tests in that way. Some results:

- It keeps giving 181V maximum, no matter if I ran it with 300V or 220V
- At low voltages, R1 gets very hot. It is rated 1 watt
- D6 have a voltage drop of 0,028V
- The original potentiometer was 200K but mine have 180k. I have made new tests with a 220k potentiometer and It goes to 211V
- I repeated the test with 250V and only goes up to 213V output


I suffer from severe ADHD and my first idea was I made a bad connection, but I have checked all the connections 3 times following then in the PCB and marking on the printed image of the circuit to be sure and everything looks fine.

I´m clueless about this. Could anyone help me?

 

This circuit is not good, but I will try to help.

Rv1 & R7 makes a voltage divider. Very likely 180K is +/-20%. So maybe test the value of Rv1. I see you found that problem. You can try reducing R7 by 10% and see if you are getting more voltage. Place a 36k across it. This might all be a voltage divider problem AND a problem that the Gate to Source turn on voltage of your MOSFET is not well known.

 

hi Rufus,
This is what LTSpice shows, i have made the 180K fixed for the test.
Check my circuit for accuracy.
Added the LTS asc file.
E

 

Rufinus: The schematic has many fatal errors:

-it has no voltage reference.
-to obtain the full output voltage without a floating auxiliary supply, you require a depletion, not an enhancement, series pass device.
-a Mosfet designed for switching operations, like the IRF840, does not have the required SOA to operate continuously in the linear mode. More so, when the output voltage is low.

You want to see what a proper vacuum tube power supply circuit looks like? Go to;
Tubecad dot com slash august99
And read from page 8 onwards.

 

Please note that I have re-read your original post and it appears that this circuit comes from a commercial product? Am I understanding this correctly?
That doesn’t preclude that it is poorly designed.

 

Additional comments;
The website that I linked likes to keep all their circuits using vacuum state devices. Everything can be replaced with modern semiconductors, but I would still urge you to keep the series pass device as a vacuum tube.
Why? Mostly because of the SOA indicated earlier. Tubes take abuse that would destroy even a rugged semiconductor in milli seconds. If the plate starts glowing an intense red, you have time to turn it off.

 

Hi Rufus,
Using LTSpice, with the 180K changed to 300K and a 230Vac [ 325Vdc] the Vout is approx 300V with 10mAload current.
E

 

Q2 data sheet: Under this test condition it might take 2V to 4V to turn on the transistor. Most likely it will take about 3V but could be 2V or 4V. This adds errors. If the next transistor came from a different batch, it would act different. Probably your SPICE is assuming a Typical value for gate-source threshold voltage. The Gate threshold voltage is not temperature stable.


Here is a more typical error amplifier in a power supply. V2 is a 1% or 0.5% voltage. We know it is 2.5V.

Please don't give up on this supply. It will probably work for you just fine. It does not follow "good engineering practice."