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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Voltage Regulator Tube with MOSFET Follower for SEP Screen Supply

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drteming

drteming

Member
Joined 2016
#1
I am working on a SEP amp with the 6BG6GA tube. To regulate the screen supply, I have a pair of 0D3 VR tubes in series to provide a voltage reference to a MOSFET source follower:

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It seems to work fine on the breadboard, but I have this little nagging voice in the back of my head wanting to stick a resistor between the source and ground. I know a MOSFET does not care where the ground is, as long as the drain is higher than the source, and the gate is somewhere in between. Should I stick a, say, 300k resistor from source to ground to ensure that the MOSFET is always on, or should I drown out the little voice with a bunch of fermented beverages?
 

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#2
According to the specs the K5A50D MOSFET has already a built-in protective diode between source and drain. You may add a zener to protect the grid from source to grid for more safety.

So far I haven’t seen any problems with my MOSFET PSU’s (various boards from 400 to 600 VDC) without any default load. However a minimal load will not hurt much as long as you keep the MOSFET’s dissipation low and the MOSFET cooled.

Regards, Gerrit
 
#4
Isn't 6BG6 a different 6L6?

6L6 can have negative G2 current in some part of its operating range. I'd want a bleeder for about 10% of nominal G2 current.

And seeing the minimum current of the gas tube is about the same value, I dunno what the MOSFET is really doing here.
 
#5
PRR said:
Isn't 6BG6 a different 6L6?

6L6 can have negative G2 current in some part of its operating range. I'd want a bleeder for about 10% of nominal G2 current.

And seeing the minimum current of the gas tube is about the same value, I dunno what the MOSFET is really doing here.
Click to expand...

Gotcha. Thanks for the tip. I guess if I had VR tubes for each screen, the shunt regulation will take care of the excessive G2 voltage when there is negative current.

The VR tubes are just serving as voltage reference for the screens of the power tubes and the drivers. There will be a separate transistor for each screen and the driver tube to cut down on cross-talk and interaction between the stages. Plus, I can slap big capacitors in there and not have to worry about making a relaxation oscillator.
 
#6
drteming said:
I am working on a SEP amp with the 6BG6GA tube. To regulate the screen supply, I have a pair of 0D3 VR tubes in series to provide a voltage reference to a MOSFET source follower:



It seems to work fine on the breadboard, but I have this little nagging voice in the back of my head wanting to stick a resistor between the source and ground. I know a MOSFET does not care where the ground is, as long as the drain is higher than the source, and the gate is somewhere in between. Should I stick a, say, 300k resistor from source to ground to ensure that the MOSFET is always on, or should I drown out the little voice with a bunch of fermented beverages?
Click to expand...

The output voltage at the source does vary with the load current. One way to limit this output voltage variation is to 'pre-load' the output. Your idea to add a resistor to ground is a good idea. It adds a constant current draw to the source, decreasing the relative current variation, decreasing the source output voltage variation.

Jan
 
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