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Old 8th January 2012, 05:40 AM   #221
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Hi everyone, i've just got some fancy 15R 200W variable resistors that i'm going to try using, i'm sure they will get almost as hot as a bulb too

I hope my .8 K/W heat sinks will do it (with lower power anyway)

I was first thinking about a degeneration resistor but didn't want the local feedback so i thought about a bypass but realized it had to be pretty BIG so i'm
gonna go with (to start with) a battery for the negative bias supply.

Can i use a ~50k resistor between the gate and the negative side of the bias supply? (thinking gate leakage)

We all love pictures so i'll post some along the way too
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Old 8th January 2012, 09:30 AM   #222
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finally buiding Fig 10 , i see no earth reference in the negative power supply schematic ? i have some LM337 negative supply board with ground reference can i use it with on the gate on the sit?

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Old 8th January 2012, 10:13 AM   #223
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Quote:
Originally Posted by juanitox View Post
finally buiding Fig 10 , i see no earth reference in the negative power supply schematic ? i have some LM337 negative supply board with ground reference can i use it with on the gate on the sit?
Hi, what is that schematic you posted from? Yeah, i think you could use that just fine (if it has the right voltage about minus 2-3v)

you connect the positive (ground in the schematic you posted) to the ground of the amplifier (same as the Source of the FET) and the negative (Vout from the LM337) to one side of the 47k resistor, and the other side of the 47k to the Gate.

TIP: Set your negative bias supply to ~12v and use a 5-10k multi-turn pot to adjust the bias with.

And please feel free to enlighten us if/what difference you hear with the fixed vs auto-bias
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Old 8th January 2012, 10:31 AM   #224
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"French is not a crime"

Vive l´amitié franco-allemande......!
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Old 8th January 2012, 11:09 AM   #225
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"French is not a crime"

Vive l´amitié franco-allemande......!
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Old 8th January 2012, 11:37 AM   #226
generg is offline generg  Germany
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ha.ha, wonderfull!

THX
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Old 8th January 2012, 01:53 PM   #227
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Any thought on the 2sk60? I ask as I can source them mich easier then 2sk82.

Thanks,

D.
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Old 8th January 2012, 02:09 PM   #228
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Quote:
Originally Posted by Chris Goldring View Post
Hi everyone, i've just got some fancy 15R 200W variable resistors that i'm going to try using, i'm sure they will get almost as hot as a bulb too

I hope my .8 K/W heat sinks will do it (with lower power anyway)

I was first thinking about a degeneration resistor but didn't want the local feedback so i thought about a bypass but realized it had to be pretty BIG so i'm
gonna go with (to start with) a battery for the negative bias supply.

Can i use a ~50k resistor between the gate and the negative side of the bias supply? (thinking gate leakage)

We all love pictures so i'll post some along the way too
Hello Chris,

I'll try to answer a couple of your questions:

1. .8C/W isn't going to be enough. Running these devices close to their limits without adequate heat sinking will probably lead to dissapointment.

2. A battery for the voltage reference presents the problem that as the battery drains the Id goes up and eventually your SIT goes poof, so you'll want to factor that.

3. You can try a 50K. If the Vdrop is manageable, then go for it. My SITs so far have show around 1uA of gate leakage current, which is minor, so I'm using 47K.

Hope that helps,

Mike
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Old 8th January 2012, 02:14 PM   #229
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My thoughts are: get the 2SK82s from CircuitDIY Affortable Components for Hobbyists
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Old 8th January 2012, 02:41 PM   #230
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As a general note, I'd like to point out that these devices are not MOSFETs.

To take advantage of the load line cancellation, you must consider both the operating point and the load when dialing in these SITs. With SITs, a small change in conditions can have a big effect on distortion and the mix of second to third harmonic, whereas MOSFETs are more forgiving because of their curves. I've worked up some examples of this for a future article.

Of course, once properly tuned, the SITs are magic.

I certainly encourage experimentation, but this is important to understand when you start plugging SITs into different circuits with different current sources, loads, etc.

My suggestion, when possible, is to build them as published then make your experimental comparisons from there.

Last edited by Michael Rothacher; 8th January 2012 at 02:58 PM.
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