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how to increase the bias on these mosfets?
how to increase the bias on these mosfets?
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Old 1st September 2020, 07:26 PM   #1
donovas is offline donovas  South Korea
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Default how to increase the bias on these mosfets?

i just built the thing called hotfet preamp found here

im trying to make the mosfets run hotter and cant seem to do so. so far i've increased r2, added 3 more leds on each side, and lowered the r9 and r10 resistors. i would like to get the bias up to 500ma if possible.

Click the image to open in full size.

also the thing sounds rather shrill on its own. im thinking of changing the jfet to something better. perhaps 2sk170?
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Old 1st September 2020, 07:41 PM   #2
Rick PA Stadel is offline Rick PA Stadel  United States
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I'm tempted to comment, but there are 12 screens of the design to sift through. Could you post the component values you built here, at least. Maybe a pic or two as well?

Thanks,
Cheers
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Old 1st September 2020, 07:47 PM   #3
donovas is offline donovas  South Korea
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Sorry i had linked the webpage under the word 'here'

HotFET Pre: J-FET audio preamplifier schematics | MyElectrons
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Old 1st September 2020, 08:34 PM   #4
llwhtt is offline llwhtt  United States
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Increasing the current thru VT1 and VT3 will cause an increase in the current thru VT2 and VT4 also. The max. on VT2 and VT4 is only 10ma, 500ma will let the magic smoke out of VT2 and VT4 me thinks.

Craig
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Old 1st September 2020, 08:41 PM   #5
Rick PA Stadel is offline Rick PA Stadel  United States
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That's OK -- I found it -- that was the 12 screens of development details mentioned -- that I'm reluctant to spend the time sifting through, if that wasn't the exact design you built .. or if there are component substitutions .. or aspects to your construction methods .. or, you get the idea.

Actual build component values on your posted schematic, if that's what you built, and maybe a picture or two. Please.

Cheers

edit: Thanks llwhtt. And forgot to ask, 'Why, exactly, does the OP want it to run that hot?' Or, 'What is so special about 500 mA?'

Last edited by Rick PA Stadel; 1st September 2020 at 08:51 PM.
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Old 1st September 2020, 10:33 PM   #6
Rick PA Stadel is offline Rick PA Stadel  United States
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+1 llwhtt -- Yup!

Even if you could get the MOSFETs happy at 500mA, the JFETs can't get anywhere near that. Measure the Idss @ Vgs=0 for your JFETs -- that's more than they can conduct in any related circuit. Plus, it blows the primary design objective of the circuit: Operating the device at the point of its maximum linearlty!(According to the developer on the website.)

Again, why do you want to conduct 500mA?

Cheers
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Old 2nd September 2020, 08:32 AM   #7
Mark Tillotson is offline Mark Tillotson
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Quote:
Originally Posted by donovas View Post
i just built the thing called hotfet preamp found here

im trying to make the mosfets run hotter and cant seem to do so
Cascodes don't have current bias. So long as the voltage bias is large enough
not to limit the input devices the current is constant, controlled by the input devices. Cascodes just pass current through.
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Old 5th September 2020, 08:26 AM   #8
donovas is offline donovas  South Korea
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So how do i increase biasing of this circuit? Im thinking of replacing the jfets to metal cans and adding heatsinks to it.
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Old 5th September 2020, 09:07 AM   #9
soekris is offline soekris  Denmark
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how to increase the bias on these mosfets?
You could parallel J2 with multiple high Idss jfets, it's not the cascodes that control the current.... The bottom current source could then be changed to something else, like a bipolar transistor, or to multiple high Idss jfets....
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