Simplistic MosFET HV Shunt Regs

[iko]

One thing to consider, maybe the zener is soldered on the G pin directly. Should not be on the G pin, but on the other side of the 220R grid resistor. I once had them make my circuit oscillate.

 

[Salas]

Its supposed to be on the other side on Merlin's as I showed him yesterday to do.

 

[iko]

Ah, ok. I agree it's best to take them out for now.

 

[merlin el mago]

without zeners
108Vout max when I want to go higher with the trimmer the voltage drops 30V less
voltage across R1 2.47V = 52mA

 

[Salas]

Ok, that is better. Is there a possibility your preamp asks more than 30-40mA?

 

[merlin el mago]

It's possible, I have 33R, 22R, 18, 16R for R1 wich one to start?

 

[Salas]

33

 

[merlin el mago]

 

[merlin el mago]

R1 33R
Voltage across 2.40V = 72mA
Vout 108 max. when I want to go higher with the trimmer still the voltage drops 30V less
dummy load 15K
zeners connected

Do you want that I try with Puresound MM instead the 15K load?

 

[Salas]

Seems you got some stickiness in the output section. Input section seems alright. Can we see a close up pic of the construction? Is it this circuit done?

 

[Salas]

OK, that is not end of the world, that RC it fixes some impedance dip in the lows. Its input section part and that works for you anyway. Are all other things exact put one by one? Is there VGS ~4V on IRF840, VBe ~0.7V on MJE350, etc? Scan a bit again. Trimmer connected OK? Zeners exactly? Are the cables to the pre very long, not twisted?

 

[iko]

Unfortunately, if you don't see voltages like I showed you in my previous post, it's not working. All of that 72mA has to go somewhere. If the shunt mosfet doesn't show you around 4V Vgs, then I question the mje350, even if it tested ok with the diode tester.

 

[merlin el mago]

Salas & Iko
Tomorrow I will test all the layout, connections (cold joints) and voltages, cables short & twisted, etc.

Thanks again & good night
Felipe

 

[merlin el mago]

OK, that is not end of the world, that RC it fixes some impedance dip in the lows. Its input section part and that works for you anyway. Are all other things exact put one by one? Is there VGS ~4V on IRF840, VBe ~0.7V on MJE350, etc? Scan a bit again. Trimmer connected OK? Zeners exactly? Are the cables to the pre very long, not twisted?

IRF840
VGS 3.91V

MJE350
VBe 0.008V instead 0.7V

It's possible that SK170BL is burned?

 

[Salas]

The fet can have a problem or the trimmer, sure. But the BJT change for sure.

 

[merlin el mago]

So I have to change the Fet & the BJT?

 

[Salas]

Better both, and check the trimmer's connections they follow the schematic.

 

[merlin el mago]

I don't have more MJE350 on hand, others PNP that I have on my stash:

MJE15029
MJE15030

D45H8

 

[Salas]

They are not enough Vmax.

 

[merlin el mago]

There is others compatibles PNP?, because last friday I bought the last MJE350 in the local shop.