BF245 FET Matching

Hi all

I'm trying to find a suitable circuit to trace BF245 FETs for a G1176.

I'm not able to find a suitable pair just by replacing Q10 until it matchs Q1

Thanks in advance

http://www.gyraf.dk/gy_pd/1176/1176sch.gif

The Tracking and Zero pots should give plenty of adjustment for almost any two similar JFETs.

Using just the trims is perfect for mono unit. I needed very good stereo tracking and this does it. A bit laborious.

http://www.axtsystems.com/joomla31/index.php/projects/11-cat-1176ln-audio-compressor/4-art-1176-fet-matching

You still need the trims for initial calibration, and perhaps another one a decade or two later.

[edit]

axt article notes, "slope of the linear region of conductance is consistent across all devices". I just checked my own excel notes/graph of the batch I measured and indeed confirm the same. Why I didn't pay attention to this in 2009 I've no clue. 

To reiterate PRR, just follow the common 1176 calibration procedure and you will have close to perfect match with any similar FET pair. Probably doesn't need to even be from the same batch.

PRR said:

The Tracking and Zero pots should give plenty of adjustment for almost any two similar JFETs.

Click to expand...

Kingston said:

Using just the trims is perfect for mono unit. I needed very good stereo tracking and this does it. A bit laborious.

...

To reiterate PRR, just follow the common 1176 calibration procedure and you will have close to perfect match with any similar FET pair. Probably doesn't need to even be from the same batch.

Click to expand...

Well, the thing is I'm having a hard time to get it done, tried with different FETs with no luck.

When I make Q-Bias calibration I get almost no movement on the meter, no matter how I tweak the trimmers.

I tried changing Q10 until I got something similar in the meter, but not really the same.

There's a difference in Tracking Adjustment trimmer value in original schematic (100K) and Gyraf Version (500K, which I'm using) Maybe it's related to this?

fripholm's TTX OP-76 build guide has a method for matching fets in it that seemed pretty painless to me and very easy to set up. I'm not sure if it would be of any help here. ???

https://groupdiy.com/index.php?topic=68997.0

I've looked at Kingston's link and need to wrap my head around it a bit more but that seems very thorough???