Author Topic: Less gain out of a BSIAB II?  (Read 1036 times)

Bucksears

  • Transistor Tickler
  • **
  • Posts: 86
    • View Profile
Less gain out of a BSIAB II?
« on: April 27, 2022, 03:12:57 PM »
Is it possible to mod the BSIAB II for less gain?
I plugged in one I built years ago (went on the shelf due to noise issues, and I haven't gotten around to rebuilding), and it sounded really REALLY good, but has way too much distortion.
I THINK I have all 2N5457's in it now, but wanted to know if using MPF102s in the first Minibooster stage would bring the gain down enough, or if I should replace Q5 with something of lesser gain.

OR is there a way to bypass the 2nd Minibooster altogether?

Just spitballing here, as I love the tone, but not the gain (or the accompanying noise).
Thx!

zombie_rock123

  • Electron Doctor
  • *****
  • Posts: 511
    • View Profile
Re: Less gain out of a BSIAB II?
« Reply #1 on: April 27, 2022, 03:48:30 PM »
Just whilst you wait on the people who have a clue - is it the GGG board? Just for the designations. If it is, I'd be tempted to remove/socket the caps on the first stage (C3a+b) to see if lowering the capacitance helps, as well as playing with R3, maybe making that a trimmer to reign it in a bit.

I'm not sure if J201s would be drop in replacements, and full disclosure - I've no idea what I'm talking about, but the Vertex Steel String Clean Drive looks to have a similar base topology and I remember thinking it was quite low gain a while back so could be a good cheat sheet for toning it down.
I sometimes label builds rockwright
https://www.instagram.com/rockwrightfx/

JackSkellington

  • Solder Ninja
  • ****
  • Posts: 411
    • View Profile
Re: Less gain out of a BSIAB II?
« Reply #2 on: April 28, 2022, 02:23:37 AM »
If I remember well the BSAIB clean up very well with the guitar volume pot (but it even can reach a clean and sparkle sound at low gain setting with its Gain pot. The one was mine did it).

Maybe you can try to add a sort of Pre-Gain control, a simple pot in front of the circuit.

If you really want to decrease the overall gain (I don't know what kind board you built your circuit) you could try to add a voltage divider before the gain pot, a sort of fixed gain pot (at 50%), placed before the stock gain pot, I guess this will halve the amount of the gain.

Looking at the schematic of the BSIAB II and the Vertex Steel String Clean Drive, mentioned by zombie_rock123, I think a big difference does the caps and the resistor, on the transistor in the first stage, between the source and the ground. A bigger resistor and a smaller cap (it's enough one) will reduce the gain (and maybe the low end).
«Just because I cannot see it doesn't mean I can't believe it»

aion

  • Solder Ninja
  • ****
  • Posts: 432
    • View Profile
    • Aion FX
Re: Less gain out of a BSIAB II?
« Reply #3 on: April 28, 2022, 08:31:22 AM »
OR is there a way to bypass the 2nd Minibooster altogether?

This is where most of the drive comes from (the first stage boosts the signal which clips the 2nd stage) so if you bypassed the 2nd stage it probably would tame things a lot more than you want.

I would say biasing is key. Try raising the 2nd stage resistor to ground from 120R to 220R or even higher.

Also make sure they're genuine/known-good 5457s... the type used to develop the original would have a Vgs(off) in the 1.2 to 1.4 range, but if they're not Fairchild/ONSemi brand (or if they are fakes) then they could be wildly different.

Maybe you can try to add a sort of Pre-Gain control, a simple pot in front of the circuit.

The gain control is a volume control after the first stage (which shouldn't really distort on its own) so this probably wouldn't do much more than allow fine-tuning of the gain range - e.g. setting the pregain control to a place where the maximum gain setting is good. Better to try biasing it differently and only try this if tweaking the bias is a dead end.

Bucksears

  • Transistor Tickler
  • **
  • Posts: 86
    • View Profile
Re: Less gain out of a BSIAB II?
« Reply #4 on: April 28, 2022, 08:57:04 AM »
Thanks guys.
I've been using the General Guitar Gadgets PCB layout, which I have etched (they've gone through more than one layout over the years), but now building on using an etched version of the Perf & PCB Effects layout.

I'll give the biasing a shot before I box up this newest one.
Thx!

EDIT:  I'm using all Fairchild 2N5457s.
« Last Edit: April 28, 2022, 09:03:10 AM by Bucksears »