One more JFET sensation from Russia

[DDD]

The article introduces one more attempt to emulate vacuum triode clipping with N-channel JFET.
The main idea is to use NJFET with low cutoff voltage as 0.2-0.6 Volts with low POSITIVE (not negative, but positive!) bias on its gate. Power supply is 9 Volts.
The article contains some practical preamp circuits and very impressive sound clips. It's worth to be written for sure.
Link:  http://gtlab.net/gtlab4/archives/415#more-415

[dschwartz]

IIRC, some peavey transtube amps have a similar approach, providing positive bias to some of the stages...although they later replaced fets for darlingtons for consistency.

[R.G.]

...although they later replaced fets for darlingtons for consistency.

Doh!!! CONSISTENCY!!!  OH!!!!

[DougH]

Another year, another 'new & improved' JFET circuit that emulates tubes...

[stm]

Thanks for posting the link Dmitriy.  The russian to english translation allows understanding some of the explanations behind.
I understand that from the 2nd JFET and beyond J201 is an adequate candidate.  What about the first FET (KP303B)?  I guess it has higher pinch-off voltage as it is located at the front-end.  Do you have a datasheet or some basic spec to find an adequate substitute? (like J202, 2N5457, MPF102, etc)

[DDD]

The stages with the positive gate bias utilize JFETs with the possible lowest cut-off voltage (0,2 to 0,6 Volts) and    I drain = 0,2 to 0,6 mA  at U gate-source = 0
Russian КПС104А matched NJFET pair is a very good candidate (pls see the article schematics). Author also recommends correspondingly selected J201, 2SK118R and 2N4338.
What about КП303В: it has higher cutoff voltage about 3 Volts and can handle guitar attack signal without full cutoff of the drain current.

[mac]

stm,
i use 2sk117 toshiba, avarage vgs=0.45v, more gain than j201, bf254a, mpf102.
also 2sk170/184 toshiba.

i tried viktor kempf diode trick between gate and source and i was pleased with the results.

mac

[stm]

Thanks for the info.  So, based Vgs(off) of 3V a 2N54598, 2N5459 or MPF102 would be adequate for the first transistor, where a J201, or even better, a 2SK117 would be adequate for the remaining stages.

Now the trick is where to buy 2SK117Y, as the Y suffix correspond to the ones with lower Vgs(off).

[DDD]

Pls don't forget of the FET drain current at Ug-s=0.
Also, the idea of the positive gate bias is based on the Victor Kempf's idea of shunting gate-source FET terminals with the Shottky or Ge diode (due to their low voltage drop). But here author utilizes positively (initially) biased gate-source junction instead of the discrete diode.

[mac]

Now the trick is where to buy 2SK117Y, as the Y suffix correspond to the ones with lower Vgs(off).

Just 8 blocks away from my apartment!

Do a search in Mendoza, (muchos mendocinos compran en Santiago y viceversa)
http://www.todoar.com.ar/r/Mendoza/Electronica/

mac

[stm]

Pls don't forget of the FET drain current at Ug-s=0.

Regarding the first JFET, you mentioned around 3V cutoff voltage but no Idss for Ug-s=0.

[DDD]

КП303В  NJFET
U cutoff = 1,5 to 4,0 Volts (most often 1,8 to 3 Volts)
Idss = 1,5 to 5,0 mA   for Ug-s=0

[caspercody]

Mouser shows NTE458 as a replacement for 2SK117Y??

[caspercody]

MAC, what diodes did you try between gate and source?

[mac]

Since Vcc/forward_voltage_drop has to be as higher as possible I used 1n5819. See my gallery.

mac

[stm]

Now the trick is where to buy 2SK117Y, as the Y suffix correspond to the ones with lower Vgs(off).

Just 8 blocks away from my apartment!

Do a search in Mendoza, (muchos mendocinos compran en Santiago y viceversa)
http://www.todoar.com.ar/r/Mendoza/Electronica/

mac

Thanks for the tip Marcelo!

[caspercody]

Does anyone think this method would be a big improvement in sound? I have made the JCM800, Dr Boogey, Diefet, and Plexizer and wondering if these ideas would change the sound any closer to a tube sound than those mentioned.

[mac]

IMHO you will get best results by going higher voltage than experimenting with exotic biasing techniques.

mac

[puzzle87]

Does anyone think this method would be a big improvement in sound? I have made the JCM800, Dr Boogey, Diefet, and Plexizer and wondering if these ideas would change the sound any closer to a tube sound than those mentioned.

You can find the answer to this question independently.  
Try, it is absolutely simple.

PS
The authors of article aspired to get a good sound from 9V power supplies.

[caspercody]

I have made up some pcb layouts for some of these in Eagle. I am not sure when I will get to them, but if anyone wants them pm me with email address.