2sk246 bias problem

[erez1012]

Hi
I use the pair 3sj103&2sk246 in the F5:
When I tray to bias , the 2sj103 bios normally, when I clock count the pot the current rise.
But when I try to bias the 2sk246 the max is 0.2A when the 2sj103 gives 1A.
I have many of this transistor and after replace the results is same.
thanks

 

[Soundhappy]

bios ?
do you want talk about bias ?

 

[kasey197]

It's best you approach these thing systematically.
Use the same dc supply voltage as shown in the f5 article.
Then measure important voltages eg Vgs etc and put them on this thread.

Others who take picture and use schematics with measured voltages written on the diagram tend to get quite a lot of help.

 

[Zen Mod]

it seems as matter of Jfet Idss

so , schematic please , so we know what to write

 

[erez1012]

hi
i remove the protaction transistor and r17,19,15.

 

[Zen Mod]

set both P1 and P2 to zero - confirm same measuring ZERO OHMS across R5 and R6

change R5 and R6 to 4K7

start biasing procedure - plenty of tutorials ( member 6L6 and others ) around

 

[erez1012]

thanks. i change the R5,R6 to 4K7.

 

[Zen Mod]

what ?

you will change ........ or you already change , prior to starting this thread ?

 

[erez1012]

no i will change and post the results.
sorry for my bed English

 

[KatieandDad]

You are completely changing the whole set up of the circuit.

 

[BobEllis]

EDIT: (Deleted potentially confusing part of my post)
EDIT: Re-reading your initial post, it sounds like you have a problem with the front end.

Before biasing the output stage, set the P1 and P2 to zero ohms as ZM suggests and P3 set to the midpoint. With no power applied, the resistance measured across R3 and R4 will be the same and slightly below 10R if the pot is centered. Then apply power and measure the voltages across R3 and R4. They should be the same, or at least very close. If not, figure out why. If they are close, then you can proceed to biasing the output stage.

 

[Zen Mod]

please - do not confuse him more

fact is that he can't open output mosfets with wimpy Idss , which his input Jfets certainly have

first thing first , later more complicated

 

[BobEllis]

Yes, I see that I misread the initial post. See my edit above.

 

[erez1012]

Hi
ZM is right.
I do a lot of time the biasing procedure, but now the 2sk246 do not change the current, just little.
When I switch to the original pair: 2sk170 the F5 work fine and the currant is equal.
After change the resistor I post the results.
Thanks a lot

 

[finwbu]

The pinout from the 2SK170 - SJ74 is different too the 2SK246 SJ103. Turn them 180deg 🙂

 

[Zen Mod]

smart to point that

I'm always so hysteric about pinouts , so I'm forgetting that other ppl maybe aren't

erez

just search for datasheets and you'll see

 

[flg]

170's are supposedly "short channel devices" which means you can swap the D-S, S-D...
Just sayin 😀

 

[BobEllis]

I tried that out of curiosity and got 50% greater Idss when reversed. The opposite of erez's problem.

 

[finwbu]

ZM you mean smarter than your Nelson?

 

[AndrewT]

I tried that out of curiosity and got 50% greater Idss when reversed. The opposite of erez's problem.

I too have measured a very few sk170 and compared Idss with DS swapped and conventional.

As far as I can tell the jFET does not appear to be damaged by swapping D&S for Idss checking. It reverts to standard Idss.

Usually the reversed Idss is a couple of percent lower. I have never found any that are more than 5% lower when reversed. Some measure the same in both directions.