Aleph J Schematic

[bobodioulasso]

Which voltage across Z1?

 

[schultzsch]

8.9V

 

[schultzsch]

I susspect the Jfets are fake

 

[vgeorge]

Did you measure the idss?

 

[Zen Mod]

gimme voltages across :

Z1,R8,R7,R18,R17,C-E of Q4

are you sure that your 2SJ109 are genuine ?

 

[Zen Mod]

while I was typing and preparing sch , several posts showed

definitely fake Jfets

try to find 2SJ74BL , or buy J271 , few dozens , and match them by your self

 

[schultzsch]

if I test theJfets with a fresh 9V baterry(idss measurement) the dmm reads 60ma and goes down slowly the fet is getting hot quickly, if the battery is used the dmm starts from 15ma and goes down to 8ma in 5 minutes and still goes down..

i will measure voltages in few minutes and be right back

 

[schultzsch]

z1 - 8.9v
r7 - 52mV
r8 - 8.4V
r17 - 0V
r18 - 0V
c-e q4 - 0.1V

 

[Zen Mod]

fake Jfets

leave entire shebang ( un-powered ) , until you find adequate ones

 

[AudioSan]

yes that is fake Jfets
as 98% of all j109 out there.

 

[schultzsch]

If I buy 2sj74BL do I need to thermal couple them?

 

[vgeorge]

I think Spencer has a new stash of 2sj74.
It' s no harm to tie them with a small zip tie.

 

[schultzsch]

I just sent him a message.

Thank you to all for your help

 

[schultzsch]

How can i tie them on Peter boards? laterally?

 

[Zen Mod]

I'll post a pic , as soon I find it

 

[6L6]

Once you remove the J109 you will see the diagram for how the two J74 mount.

 

[vgeorge]

Here you are. I used a small tie to keep them touching together on my build. You can use a small amount of thermal paste also.

 

[schultzsch]

Hello vgeorge
What kind of caps do you use for C1, C2 and C3 ?

 

[schultzsch]

I have found on ebay at alweit 2 pairs of J74BL at these matching specs : 2 very well matched pairs: I declare +/-2% to be on a safe side but really I select no more than +/- 1% (Vgs = 10V) - really, I allow maximum difference between pair is 0.03 mA(I know the matching between channels isn't necesary)
and spencer has this specs : As in the past, the match from part to part is within 0.2mA assurance but you will get normally within 0.1mA tolerance. The Idss accuracy is about +/-0.1mA due variation of equipment and temperature during test. Special requirements may or may not be fullfilled and it is a case by case discussion.
Guess the first ones are the best, right?

 

[Zen Mod]

.........

not so important

we are often obsessed without reason