BSIAB II Voltage Q's

[gotdabluz]

I just recently finished my first circuit build, the BSIABII.  I built it on stripboard using Torchy's layout as a guide (I had to add some space in a couple places to fit the components I had chosen) . . . and aside from some necessary trim-pot adjustment, it worked on the first try.

This thing really kicks!  Vocally, it is very similar to my BOSS BD-2 (which I like alot), though it has a bit fuller range, and way more output than the Blues Driver (perhaps more than I need)  . . . but,  I like it . . . yes, yes . . . I like it very much!!!

Anyhoo . . . to the point of my post . . .

Being the anally-retentive person that I am, I decided to test the voltages at the various tranny connections to make sure everything was working as it should.  I found that my readings varied quite a bit from the readings posted on the GGG site in some places . . . and well . . . this causes some concern for me (I want everything to be as close to perfect as possilbe) . . . like I said . . . anally retentive.

Here are my numbers:

Voltage into board: 9.01
Voltage out of diode: 8.33

Q1-
D: 5.11
S: 0.29
G: 0

Q2-
D: 8.33
S: 5.11
G: 3.97

Q3-
D: 4.51
S: 0.28
G: 0

Q4-
D: 8.33
S: 4.51
G: 3.96

Q5-
D: 6.48
S: 0.55
G: 0

They don't look quite right to me.

Are the GGG numbers based on readings from a circuit build, or are they mathematical ideals?

I used silver mica, poly and metal film caps for everything but the 100uf, metal film resistors, and the FETs are socketed.

I tested using a 9V regulated PS.

Am I just a victime of my own obsessiveness?

[petemoore]

The 'stacked' Jfets are Mu-Amps, the drain of bottom where it connects to source of top one should be around 1/2V of supply....looks like you have that going on. The top Jfet sets bias...
 For the last transistor...[   ]...hafta go look to see...but I'd guess it's pretty 'right' since it Kix..and you started with a "Verofied' layout !!1

[gotdabluz]

Thanks!

I tried swapping trannies around but it made no difference electrically.  I did however, discover that I like the sound better with 2n5457's across the board (probably helps compensate for the sharper high end of the silver-micas I used vs. ceramics) . . . it still has gobs of gain . . . just a little creamier to my ears

I guess I should just be happy that it works and it does sound really awesome.  I have a single channel Crate V15 tube amp, and this thing (BSIAB) is so tonally transparent it sounds just like my amp with the gain cranked way up . . . actually it sounds better!  I feel like I now have a 2 channel amp . . . Cool  


I guess my Boss BD-2 is going to be sitting on the shelf, now.

[petemoore]

Source is higher than the gate, and 6.48 on the drain is probably pretty good, is that a source follower? If it is, you're BSIAB is probably working just fine [enough output] if it isn't, it is the same story...BSIAB KIXX.
 Check at Geo or elsewhere for Jfet biasing info. 1/2v is 'usually' where these drains of Jfet amps like to sit, but maybe it's supposed to be higher bias Voltage on that Q.
  You could try different Jfets if socketted, and see if you don't get different voltage readings on the pins.

[petemoore]

Also you could measure R14 and R15 to ground, just to verify, and the schematic at GGG shows a trimpot on the drain...the transistor socket should provide good bias points if these resistors are working..

[gotdabluz]

Thanks again, Peter

Ok . . . here is what I am finding:

With Q5 socket EMPTY

Gate pin to Ground - 81.8k (R14 is supposed to be a 82k so this looks right)
Source pin to Ground = 5.57k (R15 is supposed to be a 5k6, so this looks fine, as well)

Now Q5 with populated by 2n5457

Gate pin to Ground - 81.8k (Again, no problems)
Source pin to Ground = 4.3k (WTF  :shock: )

Is this normal for the resistance at the source to drop like that?

[Ed G.]

The drain looks a little high on q5, but not excessively so. I'd try subbing different fets if you have a few extra. I tune it usually by ear, but it should get in the 4.5v range.

[petemoore]

Maybe thats the alternate current path I typed of...gets created when the Jfet is inserted.
  Ohh...it's HIgher value R...hmm you dont have the battery connected when testing R value by chance ?

[gotdabluz]

I usually test with a regulated power supply so that I can get a 'consistent' 9v voltage reference . . .

but no, these resistance tests were conducted with the power sources completely disconnected

I have a few 2n5457s left . . . so which ones should I sub to affect the change to Q5, or is it Q5 I want to sub out first?

BTW Ed, I am glad you checked in . . . just want to say that despite these minor issues that I am driven to understand (due to my anal nature), I think you have come up with a truly legendary pedal here.  It matches my rig perfectly.

[Ed G.]

Sub out Q5, see if different fets can get you closer to 4.5v on the drain of Q5. Fets are variable and sometimes you just get a bad one. J201 will work there too, it might be a bit more compressed and gainy.