Burning Amp BA-3

[Zen Mod]

chasing your tail

what's voltage reading across R3 and R4 , referring to enclosed schematic ?

P3 presumably in mid - position

 

[Hikari1]

I would suggest you find out the IDSS of your Jfets (which is what Zen Mod is trying to get you to do). Are they BL grade?

 

[stormfrontier]

Figured it out. To find my offset, I clipped a lead onto R13 where the two FETs feed and the other lead on ground. Working like a champ.

 

[stormfrontier]

Voltage across R3 and R4 is .035V, both sides.

 

[Zen Mod]

buy new fets

with Idss of ~ 3.5mA you're getting nowhere ........ not counting that they;re most probably fake ........ certainly fake if labeled as BL

 

[Zen Mod]

Figured it out. To find my offset, I clipped a lead onto R13 where the two FETs feed and the other lead on ground. Working like a champ.

unnecessary and missleading

leave everything as is

with proper parts , circuit is working as intended

 

[stormfrontier]

buy new fets

with Idss of ~ 3.5mA you're getting nowhere ........ not counting that they;re most probably fake ........ certainly fake if labeled as BL

Where can I get the 74 and 170 jfets?

Mine are the GR, not the BL. It says the GR grade is right around 3mA. Will they not work?

 

[biju55]

Matched JFET pairs (Grade B) - Parts

 

[Hikari1]

Matched JFET pairs (Grade B) - Parts

Are you existing Jfets labelled "BL" or "GR"?

 

[Zen Mod]

well , besides hanging with too young EE's ..... you didn't made necessary homework

 

[stormfrontier]

Matched JFET pairs (Grade B) - Parts

Are you existing Jfets labelled "BL" or "GR"?

The ones I have are GR. I found a post by 6L6 that says you can use them despite their lower Idss, but that you have to up the value of the pots (which I did).

So can I use these? And if so, what are the drawbacks?

The funny thing is, on the DIY store, the jfets with lower Idss values are "grade A" and sell for more money.

 

[Zen Mod]

BL grade is way to go

GR will work , with pots and fixeddrain resistors at least doubled in value , but that would be different (pre)amp , having different OLG to original ......... and everything what goes after that

 

[stormfrontier]

Damn. So I need to find BL jfets.

 

[Hikari1]

Another source is spencer at fetaudio.com

He has the jfets, toshiba parts at a little cheaper price than the linear systems parts. But you will pay more for shipping (he is in singapore) and wait longer.

If you got your jfets from Albeit in Isreal they are likely genuine (I've had good luck with him) but simply just the wrong grade.

 

[stormfrontier]

Yes, that's him. Wish I would have known earlier. Oh well. I bit the bullet and bought the grade B FETs from DIY. They're supposed to be between 6-10mA Idss.

Thanks for everyone's help. Will have to wait a couple days for the new jfets to show up.

 

[Russellc]

Yes, that's him. Wish I would have known earlier. Oh well. I bit the bullet and bought the grade B FETs from DIY. They're supposed to be between 6-10mA Idss.

Thanks for everyone's help. Will have to wait a couple days for the new jfets to show up.

You did fine. I dont know if Spencer has any matched sets, I havent looked in a while but last time I did he didnt have matched sets....I'll have to look again. What you got from DIY store is basically same range as BL. They have a different grading system than Toshiba used.
They should work perfectly.
Russellc

 

[AndrewT]

I would suggest you find out the IDSS of your Jfets (which is what Zen Mod is trying to get you to do). Are they BL grade?

I agree.
You need to identify the jFETs are what is psecified.
Measure the Idss and the Vpinchoff. and report back.

 

[stormfrontier]

I agree.
You need to identify the jFETs are what is psecified.
Measure the Idss and the Vpinchoff. and report back.

I already did, see above. I purchased my jfets from Alweit in Israel. He shipped genuine jfets, but they are GR grade, and the Idss is low at 3.5mA. That is why I had to bump the value of the trimmer pots.

It started biasing last night, but I decided to wind it all the way back, and I purchased the matched "BL" jfets from the DIY audio store. The grade B version.

 

[AndrewT]

What were the originals that have been installed when you had your problems?

 

[stormfrontier]

What were the originals that have been installed when you had your problems?

GR's. Don't know about Vpinchoff, but the source resisters were reading .035V