N JFET characteristics

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Paul Barker
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#1 N JFET characteristics

Post by Paul Barker »

I get no help from google. I’m building a jfet casode circuit from an article hand the upper jafet suggested has max 50v. Is that a typical high figure for a n jafet? Or does anyone have access to catalogues that might find a similar high transconductance alternate that tolerates greater
Suply voltage. It’s just that I ideally want an output towards 50v peak which would get me there in a single Amplification stage as the designer predicts 34db gain. But I’m not averse to a small gain valve stage to take over.

Oh the circuit uses 2sk246 which Id like to upgrade if possible to greater meet my requirement for more amplitude.
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Nick
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#2 Re: N JFET characteristics

Post by Nick »

Thats generally the limit for a jfet. Why do you need more than 50v? seems out of jfet territory. TBH, 35 is normally the Vmax for a lot of devices. Higher than that you are looking at a mosfet. Maybe depletion to get you clossr to a jfet.
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#3 Re: N JFET characteristics

Post by ed »

I'm not in a position to check at the mo Paul, but off the top of my head have you looked at the usc offerings...

if memory serves they are something like ujn1200 series
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#4 Re: N JFET characteristics

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Thanks both.
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#5 Re: N JFET characteristics

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#6 Re: N JFET characteristics

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Thanks Nick
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#7 Re: N JFET characteristics

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Its for a 10y 801a A2 amp.It would be good to try either the 200v or the 300v one but Mouser Not in stock RS don’t list, no “shopping” result. eBay non listed.

For the mosfet source follower to drive the output valve to 35mA grid current I’ve got two types mentioned in posts & blogs to audition.

I can drive a 10y way into clipping with 50v peak But for 801a full a2 power I’d need more headroom.

It’ll be my first hybrid amp.

Sadly the only transformers I’ve got that are higher impedance are boat anchors. Hay ho. It’ll have to be mono blocked, so I can practice and fine tune in mono.
Last edited by Paul Barker on Mon May 04, 2020 9:37 am, edited 1 time in total.
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#8 Re: N JFET characteristics

Post by Nick »

Why a jfet in that use. I would have thought a mosfet would be fine in that situation.
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#9 Re: N JFET characteristics

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the driver is mosfet
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#10 Re: N JFET characteristics

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The jfet cascode is line level so chasing lower input capacitance and reported to be quieter in general than mosfet. The lower fet is 2sk170. Running the cascode five or six mA it is intended through level shifting DC to mosfet gate. But the balancing act which might be more effort and risk is dc the source follower mosfet to grid of 10y and 801a.

On the one hand dc right through is a worthy desire as long as it doesn’t just shift the reason for sound degradation Or at least colouring from coupling compromises to power supply Caps and cathode load bypass caps.

The Main b+ power supply caps I can lose by building no r no c after Stephie., which I’ve used in my home stuff and showed at the first Eggborough in a px25 amp.

I might struggle to get the negative supply capacitor less. Not because it’s negative ; because it’s yet another juggling ball in the air and with so many balls in the air at once all interlinked catastrophe isn’t far from view during prototyping.

The last one I built 100% capacitorless B+ and signal section and dc throughout took quite a few casualties until I finely balanced each element until the whole worked properly. But the resultant sound best I’ve heard.
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