2sk79 with b1 buffer

vanofmonks

Member
2006-05-15 6:37 pm
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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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

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codyt

Member
Paid Member
2019-08-26 7:05 pm
IN, USA
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…
 

koja

Member
Paid Member
Paid Member
2009-10-04 12:10 pm
Ontario
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?
 
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codyt

Member
Paid Member
2019-08-26 7:05 pm
IN, USA
I like them in the B1 quite a bit. Unfortunately I've not heard a Luminaria. For me it comes down to what amp I'm using – these pre's plus a vfet amp can start to be just a bit too sugary. With the right amp, they're superb!

Standby – gonna post my files in just a minute.
 

codyt

Member
Paid Member
2019-08-26 7:05 pm
IN, USA
B1-K79

As mentioned elsewhere, I picked up a matched quad of 2SK79 a while back and have been playing around with them a bit. Initially, I built the ubiquitous SRPP circuit that is often referenced to the K79 in DIY circles. To me, it appears to be a fairly straightforward SRPP implementation and sounds great (I'll post that stuff next). The other logical place for me to try them (read: easy/lazy) was in the B1v1 circuit.

Attached is my implementation of the B1 with 2SK79 instead of the 2SK170. Its straight out of Nelson's article, though I discarded all of the PSU parts in favor of a Salas UltraBIB. Overall, I really like the sound of the B1 with the K79's. Just a nice dose of H2 – clean, relaxed, nice depth, etc.... I tried a number of different supply voltages, but ultimately landed at the stock 18-20v.

Regarding the board, I was trying to accommodate the parts I had on hand, and the obvious monstrosity is the large space for a 'fancy' output cap. You'll also notice a smaller electrolytic at the output. This is completely optional. I use mine to drive the primary of an AVC, so needed an optional to include additional capacitance on the cheap. As you'll see when I post the SRPP boards, I wanted something I could swap out easily, so maintained dimensions between the two boards.

Finally, note the pinout of the K79 is different from the K170's, so don't assume the parts are interchangeable. You can make it work, but it requires some fumbling around. I have a version of the board that does accommodate both parts (K170/K79) – if you want that one, just ask.

Thanks to Nelson for the splendid design!

*edit – up to you to determine what kind of output capacitance you need. Don't rely on this schematic. Stick with Nelson's schematic when in doubt :)
 

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codyt

Member
Paid Member
2019-08-26 7:05 pm
IN, USA
B1-SRPP

As promised, here's my version of the standard SRPP design often seen with the 2SK79. Not sure who exactly to credit for the design, but rest assured it wasn't my creation :)

Same footprint as the B1 board above, and also uses an offboard PSU (I use the same Salas Ultrabib for mine.) With this board I also made provisions for a dual mono approach – if you use with a single supply you'll need to jump those positions referenced in the middle of the board.

*Before the jokes start, I'd just like to say that the two pair of 'fancy' caps on these boards represents my complete collection of 'fancy' caps. I had to make my own PCB just to fit them into a design! :D
 

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