Caboose Overdrive (J201 BIAS ISSUES)

[ODA]

Caboose Overdrive - R16 Mod​

R16 is a 12K resistor on the PCB/Schematic
The mod is to change R16 to a 2K2 resistor

Mod Improves pedal output volume and improves bias adjustment/voltage range (1.5v - 9v) on Q2 J201 Drain

Had two issues with this circuit, changing R16 to 2K2 fixed both issues.

Issues (R16 with 12k resistor):
#1 low output volume
#2 could not achieve standard bias voltage (4.5v) on Q2 J201 Drain

Anyone else have issues here with this build?

 

[darwin999]

Just curious - hence 2 questions:
1. You're sure that your Bias trimpot is 50K, and not something smaller?
2. Is your J201 from a quality vendor (PPCB, Mouser, Digikey, Stompbox Parts, etc), or is it from something like Amazon/eBay? I.e., any chance it is something different (aka, a 'fake')?

 

[ODA]

Just curious - hence 2 questions:
1. You're sure that your Bias trimpot is 50K, and not something smaller?
2. Is your J201 from a quality vendor (PPCB, Mouser, Digikey, Stompbox Parts, etc), or is it from something like Amazon/eBay? I.e., any chance it is something different (aka, a 'fake')?

Good questions! Yes, I am puzzled too.

I first thought maybe I put a 5K trim pot in by mistake. Nope. It is marked (503) 50K, I even checked with ohm meter. It's ~15K when the J201 is biased to 4.5v. I double checked all the other resistor values, all correct. The J201 is a Fairchild purchased from Small Bear, I also swapped it with a SMT J201 from Stompbox, same results just a few milli volts difference on the bias. I even double checked the circuit on a simulator and my results should not be correct, but there it is. Something I cannot explain or understand.

What I have not tried yet is to build the J201 section of the circuit (output booster) isolated on a breadboard and test it.

 

[darwin999]

Wow, so J201 is definitely legit and trimpot is correct. Very strange then - I've got a Caboose PCB waiting on my workbench, I'll try to go to work on it in a few days and see what happens to mine.

 

[HamishR]

I have dozens of legit J201s and hate using them! It's a bit weird because every other JFET I have used has given me no trouble but J201s often do. I wish pedal designers would never use them. It's a pain because some J201 based designs sound amazing.

 

[lrgaraujo]

I can't simulate the circuit right now, but 12k is a little high compared to what we typically see used with j201s. Maybe the original used (borderline) out-of-spec jfets

 

[temol]

Caboose Overdrive - R16 Mod​

R16 is a 12K resistor on the PCB/Schematic
The mod is to change R16 to a 2K2 resistor

Mod Improves pedal output volume and improves bias adjustment/voltage range (1.5v - 9v) on Q2 J201 Drain

Had two issues with this circuit, changing R16 to 2K2 fixed both issues.

Issues (R16 with 12k resistor):
#1 low output volume
#2 could not achieve standard bias voltage (4.5v) on Q2 J201 Drain

Anyone else have issues here with this build?

In the case of J201 and a 12k source resistor, a 50k trimmer may be too low. You could try 100k. The higher the RS value, the larger the RD value needed to get the same drain voltage. Maybe the original has a larger trimmer? Or maybe a different JFET? Or 1.2k instead of 12k?

 

[temol]

I just uploaded the schematic to ltspice. RS = 12k and J201 with Vp -0.7V requires RD of 105k to get 4.5V on drain. With RS 1.2k RD is 20k (for the same transistor).

 

[ODA]

Thanks temol

Appreciate the confirmation. Obviously, the sim I was using was not as good or I was using it wrong.