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Old 29th November 2009, 01:50 PM   #1
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Default 2SK170 limit

Have not worked with 2SK170 at high B+ before. For a future Zen-inspired preamp-project I intend to use it like below with B+=90-100V, Vd=35-38V, Id=8mA. This is quite close to the limits of 40V/10mA.

Will there be any immediate risk of frying it due to the high B+?
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Old 29th November 2009, 02:38 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
the high Vds of 35V combined with the medium Id current of 8mA requires a junction dissipation of 285mW. That's 71% of maximum rating.
Far too hot for continuous duty.
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Old 29th November 2009, 02:56 PM   #3
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Quote:
Far too hot for continuous duty.
Hey Andrew,

Is this something you have found out empirical as this info is nowhere else to find? Toshibas datasheet only states 400mW at 25 degrees. Found another typical JFET where dissipation derating above 25degrees was close to 3mW/degree.

Though my main concern was running B+ of two times max Vd, .

Last edited by revintage; 29th November 2009 at 03:06 PM.
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Old 29th November 2009, 03:43 PM   #4
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To clarify, my concerns are what happens at "power up" and "power down".
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Old 29th November 2009, 05:21 PM   #5
juma is offline juma  Germany
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Quote:
Originally Posted by revintage View Post
To clarify, my concerns are what happens at "power up" and "power down".
It's very easy to find out - eventual sacrifice of one JFET will not cost much and you'll know for sure.
OTOH, cascode is an easy way around the Pd problem (with additional benefit of characteristic's linearization and larger swing capability).
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Old 29th November 2009, 05:59 PM   #6
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Originally Posted by juma View Post
It's very easy to find out - eventual sacrifice of one JFET will not cost much and you'll know for sure.
OTOH, cascode is an easy way around the Pd problem (with additional benefit of characteristic's linearization and larger swing capability).
Hey Juma,

Good idea, this is cheap stuff.....

Maybe one should try a MOSFET capacitance-multiplier with long time-constant to get around the evntual start up problem and also get better PSRR.

Will go singlestage Zen-style(not BOZ), cascode is two stage.

About linearity and swing this one with a single 2SK170 will put out ca 25Vrms into 10k(Zout below 500ohm) at 11dB gain with below 0,1% THD. This as shunt feedback is a lot more effective than current feedback commonly used here.

Question though is if it will sound as good as the figures indicate?
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Old 29th November 2009, 06:15 PM   #7
iko is offline iko  Canada
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Quote:
Originally Posted by revintage View Post
To clarify, my concerns are what happens at "power up" and "power down".
Valid concern IMHO. FWIW 2sk117 gives you another 10V to max Vds. Another way to deal with this would be to go the cascode way, by using a jfet with larger Vgs(off) on top. I use a 2n3819, but many others would qualify. My 2c.
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Old 29th November 2009, 06:22 PM   #8
juma is offline juma  Germany
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Quote:
Originally Posted by revintage View Post
Hey Juma,
...Will go singlestage Zen-style(not BOZ), cascode is two stage.
No, it's still one gain stage with two active devices

Quote:
Originally Posted by revintage View Post
... with a single 2SK170 will put out ca 25Vrms into 10k(Zout below 500ohm) at 11dB gain with below 0,1% THD. ....
So, you want that poor JFET's Drain to sit at 40V and swing 35V (peak). It means that you'll have Vds=75V during the positive peak. Doesn't seem viable to me.
Zout will be Rd (6k8) || RLoad.
THD figure is unrealistic too.
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Old 29th November 2009, 06:23 PM   #9
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Originally Posted by ikoflexer View Post
FWIW 2sk117 gives you another 10V to max Vds. Another way to deal with this would be to go the cascode way, by using a jfet with larger Vgs(off) on top. I use a 2n3819, but many others would qualify. My 2c.
Hey,

Checked 2SK117 and unfortunately it had 25% lower dissipation. Otherwise it look good with much lower Ciss.
About cascode this isnīt an option as I told Juma above that I donīt want to go two-stage. Will anyway sim a cascode side-by-side.
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Old 29th November 2009, 06:28 PM   #10
EUVL is offline EUVL  Europe
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2SK223. 80V, 400mW.
Don't blame me if you fry one.

Patrick
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