Sabbath Distortion

[BuddytheReow]

Hey guys,

Just finished the circuit on a sabbath distortion. I alligator clipped it to test and I get virtually no signal coming through. The signal that does come through is incredibly faint and sounds not distorted. The pots appear to sound correct (volume, gain, etc). I did test the pots before soldering them in. Here are the voltages for the trannies, pins reading left to right:

q1 0 2. 8.7
Q2 0. 1.6 3.7
Q3 2.9 4.8 8.8
Q4 0 0.6 4.2
Q5 2.9 5 8.8
Q6 2 5 8.8

Any assistance would be greatly appreciated. I know I can count on you guys to help with this.

 

[Nostradoomus]

What kind of JFETs did you use and where did you get them?

 

[BuddytheReow]

Pn4393 from NTE Parts Direct as a substitute for MPF4393. The BC184 is from Tayda.

Edit I learned real quick to steer clear of eBay for most parts

 

[Untro]

I have a sabbath build that also doesnt work, but because of the transistors that I havent ordered! The bc184lc from Tayda however is a reversed pinout I believe, so you have to mind that? I also understand mouser makes reliable pn4393s, thats what im waiting to order.

 

[moonlightpedalbuilds]

Need to check the bc184 pinouts. I believe the ones from tayda is bc184L which has a different pinout.

For 4393s check aionfx. They’re selling them in sets of 5

 

[BuddytheReow]

I have a bc184lc from tayda. I’ll do some research in the pinout and see if that works. How can I tell what the pinout is supposed to be on the board itself?

 

[Mcknib]

I use the shop pcb image where you can see some traces and compare to the schematic e.g. you can see the BC184 has a 4K7 on the emitter in the schematic and image 10K collector so EBC left to right

With the fets looks like GSD left to right from the fet with the 470K, 820R and 1K coming off it 2nd from the right (Q4 in the schem I think)

Just for info the shop images can be handy if the schematics not been posted yet doesn't show every trace but if you look at the 22u ground pad you can see it's got faint lines surrounding it so you can suss ground pads too

 

[BuddytheReow]

Definitely need to print those out as well.

I tried flipping the bc184 around and no signal at all

Audio probe is good on pin 2 of the bc184 which I think is the collector if not mistaken.

q2 and q3 I get nothing from any pins.

q4 I get signal on pin 3.

q5 signal on pins 1 and 2

q6 signal on pins 1 and 2.

does this make sense? I’m hoping I didn’t buy fake 4393s....

 

[Mcknib]

Might just be me but I'm finding your transistor voltages hard to read

Are they from left to right so Q1 would be

E 0v B 2v C 8.7v

I've checked several 4393s from different manufacturers and they all have this pinout which is the same as the MPF4393 with the BC184 being E B C

 

[BuddytheReow]

Yes. Everything is read left to right. Apologies for how I typed it. It was on my phone and did a few autocorrects.

 

[Mcknib]

Yeah I'm on my phone at work right now, obviously I use the term 'work' very loosely

I'm thinking any images I post will be massive when I have a look on my PC anything posted by phone is usually humongous

So your 1 2 3 is left to right too gotcha

 

[Mcknib]

I don't think Q1 emitter should be 0v you may have a short around there and I suspect the collector voltage is a bit high with the 10K coming off VCC I'd check around your transistor socket C4, R4 and 5

Unfortunately I'm not proficient enough to work out what the emitter voltage should be but I'd think a volt or 2ish

Q3 ,5 and 6 gate voltages should be nearer half VCC (8.8V) they've all got equal resistance voltage dividers from VCC to their gate pins

Audio probe is good on pin 2 of the bc184 which I think is the collector if not mistaken.

When you say Q1 pin 2 do you mean the middle pin (base) or the one to the right (collector) as you look at the pcb image I ask because you don't mention if you get audio on the other pins which would tell us if it's going in or out, if it's going in but not out that'd confirm it's a problem

 

[Chuck D. Bones]

Hey guys,

Just finished the circuit on a sabbath distortion. I alligator clipped it to test and I get virtually no signal coming through. The signal that does come through is incredibly faint and sounds not distorted. The pots appear to sound correct (volume, gain, etc). I did test the pots before soldering them in. Here are the voltages for the trannies, pins reading left to right:

q1 0 2. 8.7
Q2 0. 1.6 3.7
Q3 2.9 4.8 8.8
Q4 0 0.6 4.2
Q5 2.9 5 8.8
Q6 2 5 8.8

Any assistance would be greatly appreciated. I know I can count on you guys to help with this.

Q1 is cutoff. Make sure R1 & R2 are the right values, check solder joints, the usual. And of course make sure Q1 is facing the right way. Transistors will work when E & C Are swapped, their hFE is just a LOT lower. Pull Q1 out of the socket and measure the voltage on the middle pin of the socket. Also make sure the transistor leads are snug in the sockets.

You have a cheap DMM, it's loading down the circuit when you measure Q3-G, Q5-G & Q6-G. Ignoring the bogus readings, it looks like the JFETs are all working correctly.

 

[Mcknib]

I notice tayda have the BC184 and 184L listed together and they have very different pinouts and I've just noticed you say you've got a BC184LC

The BC184L / LC has the base as an outer pin so it wouldn't make a difference just turning it around 180° you'd need to turn it around and bend the B and C pins, B to the middle C to the right, from the image below if you put it in per the silkscreen it'd be Base, Collector, Emitter and you want Emitter, Base, Collector

Meant to say I'd checked all your resistors, caps etc all looked correct but as @Chuck D. Bones says check them again...and again....then ask your neighbour to check them!

 

[Chuck D. Bones]

That would explain the problem, good catch! Turn the transistor approx 90 deg, form the leads so collector & base are swapped, plug it back in and you should be good to go.
Alternatively, use a different transistor for Q1, nothing special about BC184. Any NPN Si tranny will work. I recommend a low noise tranny like 2N5088, 2N5089, BC549, MPSA18, etc. Mind the pinout of course.

 

[BuddytheReow]

Would I need to wrap any of the pins in electrical tape? Going to try this afternoon

Can you tell I’m learning ?

 

[Mcknib]

Just use insulation you've stripped from wires, transistor trousers as they call it!

Who isn't....learning I mean!

Did somebody say....Chuck!

Well ok then

But who else.....isn't learning

 

[Chuck D. Bones]

Wire insulation is ok, but air is a fine insulator. Think in 3 dimensions, you can easily form the leads so they go into the right holes and don't touch. Rotate the transistor 45 deg and you'll see what I mean.

 

[BuddytheReow]

Success!! Sounds much better now. I didn’t realize the pinout was different for a bc183lc vs bc184. I move the pins and put some electrical tape in between.

When I crank the gain to max I get a loud warble for lack of a better word when I’m not playing. When I play it goes away. Anyone else get this? The work around is I don’t max out the gain? When fully CCW it’s still pretty gainy

 

[Chuck D. Bones]

Grounding and lead routing are critical with high-gain pedals. The wires between the board and stomp switch need to be short and the input kept away from the output. The wires to the input and output jacks should be routed away from the board, along the sides of the box. If your box is painted inside, then you need to sand or scrape the paint around the pots, switch and jacks to ensure they all make metal-to-metal contact. Input and Output jacks should both have ground wires to the board.