Cobalt / BD-2

[Erik S]

Build Rating

5.00 star(s)

Blues Driver!

I grabbed this out of the pile yesterday thinking it would be a nice quick one to put together. Took a little longer than I thought since I hadn't accounted for "balancing" the JFETs. Not sure how critical that is myself, but I read through Chuck's notes here: Chuck Bones take on BD2, and he said we want within +-10mv of vref on the collectors of the two 2n3906s so that's what I went for. Coming up with resistor values to hit that was pretty finicky. I ended up combining some values to get 7.1k in R6 and 6.3k in R7. That got me to -7.4 mV difference on Q5, and -.03mV on Q6.

Since this board doesn't have solder pads for SMD jfets, I decided to finally use up some of these through hole-j201s I've been sitting on. I had gone through and measured them at some point, so (also per Chuck's advice) I grabbed the closest matched set I had and put them in.

I've never played a Boss one, and I can't say how much of a difference the matching and balancing made, but it sounds great, so I'm happy. I was a little suprised about the tone control - not a huge range there. Sounds good, just doesn't get very extreme at either end.

 

[Giorfida]

Nice build Just did one myself a couple weeks back, also thinking it was an easy day…wasn’t. But for me, the big time sink was trying to use 1/4w resistors
Honestly don’t know why it’s taken me this many years to really try one!

 

[Erik S]

@Giorfida - I hear you on the resistors. Pretty sure I missed that this was an 1/8w layout when I ordered parts. Got lucky and had almost all the values in 1/8 though.

Just hit your profile to check out what you did with yours, why no build reports!?

 

[jcpst]

It looks great, inside and out. The offboard wiring is hot.

I thought this was on the trickier side ppcb boards. It’s tight, and doesn’t have smd pads for the j201s.

I like it either low gain single coil, or higher gain humbucker.

 

[Erik S]

Thanks @jcpst!

I guess it is kinda tight, and smd pads would definitely be cool. I agree the layout isn’t quite as slick as some of Roberts other stuff. Some little things stuck out to me like the grounds not being labelled on the silk screen.

I just looked it up - Cobalt is PCB #78. The stuff in the coming soon section looks like it’s getting close to 700! That’s a lot of PCBs. Seems like they keep getting nicer!

 

[Giorfida]

@Giorfida - I hear you on the resistors. Pretty sure I missed that this was an 1/8w layout when I ordered parts. Got lucky and had almost all the values in 1/8 though.

Just hit your profile to check out what you did with yours, why no build reports!?

I’m lazy and need to get on them! I’m notorious for never fully completing anything. I build it, test it, box it up and throw knobs on it. By the time it comes to labels and build reports, those seem to never happen. If you add up all that I have here now, I’m have around 50 completed builds here, with over 30 of those being PPCB boards built within the last year and a half (and that doesn’t count the ones I built years ago and may have sold etc)…so yea, I have some homework to do!

 

[swelchy]

Ive have this pcb for a while and need to dive in…. Im just now getting experienced enough to try and match jfets by taking measurements with the dca75… I’m a bit Intimidated with jfets readings and being able to figure out what resistors to make everything right..

 

[Erik S]

I’m lazy and need to get on them!

I’m waiting…

Jk jk

If my count is right, I’m short about 20 build reports myself, and I’m certainly not gonna go back and catch up. It does feel nice to post them though. The archive has gotten me out of a few jams. Hopefully documenting my struggles and screw ups helps somebody else out.

 

[Erik S]

Ive have this pcb for a while and need to dive in…. Im just now getting experienced enough to try and match jfets by taking measurements with the dca75… I’m a bit Intimidated with jfets readings and being able to figure out what resistors to make everything right..

Definitely give that Boneyard post a read through! As far as the balancing part, Chuck’s write up covers exactly what to measure, and then finding the right resistors was just trial and error.

I’m jealous of that dca75! I’ve looked those up a couple times, but haven’t justified buying one yet. I set up a breadboard at some point and measured and labelled a bunch of parts, but one of those testers would be way more convenient.