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2sk79 with b1 buffer
2sk79 with b1 buffer
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Old 11th March 2021, 05:04 PM   #1
vanofmonks is offline vanofmonks  United States
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Default 2sk79 with b1 buffer

I'm currently running a 2sk79 with a mu output from a Pimm self bias ccs. Looking at some alternatives I think I can run a ccs loaded stage from as little as 40v biased with about 20v on the output but I would like to use a b1 on the output instead of a mu output. The ccs I am thinking of using has a 5v minimum so that should leave plenty of swing for a pre. I think I have three main questions.

How can I modify the b1 to work in this situation? Is the cascode ZenMod suggested the way to go? Are there any existing schematics I can refer to?

Is there a better choice for an output buffer than a modified B1?

Is there a better choice for the ccs on top of the 2sk79. The bias point for the 2sk79 should be around 6ma with about 1.2v on the "source?" and 20v on the "drain?". So at idle it will drop 20v and I would want at least 10 v peak of swing, with I guess more being better. I have used this ccs before and it may be familiar to some, but I don't want to give details as I'm not sure it is in public.
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Old 11th March 2021, 05:06 PM   #2
vanofmonks is offline vanofmonks  United States
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I just realized my title is poor. Can a mod change it to something more appropriate, like 2sk79 with b1 buffer?
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Old 11th March 2021, 08:28 PM   #3
Zen Mod is offline Zen Mod  Serbia
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2sk79 with b1 buffer
if you bias it as you said (Ub/2 at drain of 2SK79), you're good - no need to change anything

CCS is good

to keep dissipation of JFets on sane level, take care that Iq through them is not bigger than, say, 8mA

if you want to alert Mods, report your post #2, explain in report note

report is exclamation mark sign, left bottom post corner
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Old 11th March 2021, 09:22 PM   #4
vanofmonks is offline vanofmonks  United States
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Thanks ZM i will give it a go
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Old 25th November 2021, 06:29 AM   #5
Adams_Leo is offline Adams_Leo  China
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I have one crazy idea,can we change to use the 2sk79 to replace the outpu k170 as buffer?does it workable?
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Old 25th November 2021, 10:46 AM   #6
Zen Mod is offline Zen Mod  Serbia
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2sk79 with b1 buffer
of course it is

you need to think of :

- setting Iq and working voltage , taking in account dissipation and Rout
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Old 25th November 2021, 05:35 PM   #7
codyt is offline codyt  United States
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2sk79 with b1 buffer
Funny you posted this - I’m running this exact setup right now. I even made some boards up, though it’s easy enough for point to point (which I did first).

I can post my RTAs and other info later today.
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Pearl II >> B1v1 / B1v2 / B1K / B1K79 / H2v2 / BA1FE / BA3FE / WBA18 / ACP CT / Broskie-SRPP / K79-SRPP / Iron Pre / K2013 CT >> F4 / F5(K1530/J201) / F6 / AJ / AJSS / ZMJ2 / SONY VFET-SE / SONY VFET-PP / SB77B
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Old Today, 02:01 AM   #8
koja is offline koja  Canada
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2sk79 with b1 buffer
indra too had some circuits modeled with that rare device:
SERFE - a Single Ended Rush Pair Front End
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Old Today, 02:32 AM   #9
codyt is offline codyt  United States
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2sk79 with b1 buffer
Thanks so much for sharing that! Hadn’t seen before. I’ve been making boards with the same footprint for easy swapping. So far have the B1/K79 and SRPP/K79. Might be fun to add this one to the list too. Would be a fun ‘K79 experimenter’s set’ . Or, just get a stack of perf board and some bits if wire…
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Old Today, 02:45 AM   #10
koja is offline koja  Canada
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2sk79 with b1 buffer
I would love to have a few of those devices (2SK79s). Since i do not i briefly considered Indra's circuit with a tube instead but getting a transformer to run that, plus too many ready made choices here on the forum made that a dead end for me. It would be nice though to have a small Vfet FE for the lottery VFET amp since the latter only has ~10dB of gain.

edit: I see some on eBay: which Idss values are the most useful?

Last edited by koja; Today at 02:56 AM.
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