Need Help with Tube Screamer Build

[CYOUL8R]

I built a Tube Screamer on a PCB from GGG using their schematic. I've also audio probed the circuit and checked the solder joints with a magnifier. After installing everything I can't get it to work. It works fine when bypassed but I get nothing when the effect is turned on. I made four part substitutions from the original schematic.

• 47 pF silver mica cap in place of 51pF ceramic cap
• 0.022uF cap in place of 0.02 cap
• 20KG linear pot in place of 25K linear pot (tone)
• 2N3904 transistors in place of 2SC1815

I have a strange situation with the opamp. I'm using an IC socket. When I remove the opamp from the socket the voltages at the socket seem right but when I install the opamp the voltages increase. The Vr and transistor voltages also increase. These differences occur using different opamps, JRC4558D or NTE778A (a cross to the RC4558)

Opamp Removed      Opamp Installed

V+ = 9.07v               V+ = 9.07v
Vr  = 4.56v               Vr  = 6.26v

IC                             IC

P1 = 4.51v               P1 = 8.43v
P2 = 4.22v               P2 = 7.99v
P3 = 4.51v               P3 = 7.49v
P4 = 0                      P4 = 0
P5 = 4.51v               P5 = 8.24v
P6 = 4.52v               P6 = 8.54v
P7 = 4.42v               P7 = 8.52v
P8 = 9.07v               P8 = 9.07v

Q1 & Q2                  Q1 & Q2

C = 9.07v               C = 9.07v
B = 2.68v               B = 3.60v
E = 3.09v               E = 4.50v

Where do I go from here?

[woulfer]

I just spent a couple of days debugging a screamer. My voltages before I found the problem were similar to yours.

See this thread. http://www.elixant.com/~stompbox/smfforum/index.php?topic=42934.msg310738#msg310738

What I ended up finding was a small solder bridge to ground.

It is a frustrating process, and being a noob myself, I can't help much but to say stay calm, and keep looking and checking things out. Using an audio probe helped alot, too. It is very rewarding when you do finally get it working.

[woulfer]

Another thing I notice, if I'm not mistaken, is that your voltages are a little off for your transistors. Your emitter should be a little lower than your base. I would definitely recommend using an audio probe to trace the signal from the input, and see where it stops.

[CYOUL8R]

Thanks Woulfer,

If my problem was a solder bridge why would the voltages differ with the opamp removed as opposed to installed? I'm just not understanding this but I'll keep checking and probing.

[woulfer]

I'm not implying it is a solder bridge necessarily, but it is really amazing how easy it is to miss, even if you've systematically checked it over many times. My very uneducated guess is that your problem could be before the first transistor because of your voltages. But I wouldn't bet the bank on it.

Hopefully someone who knows what they are doing will show up and offer you some real advice.

[gitarrmongot]

I also built a GGG TS-808 clone.You should use MPSA18 transistors.Check the pinout on the PCB(its for MPSA18).
My pedal sounds great with JRC4558 IC.Good Luck...

[johngreene]

It's kind of confusing because the layout calls for 2SC1815 but it's not laid out for them. 2SC1815s are ECB.
http://www.semicon.toshiba.co.jp/td/en/Transistors/Bipolar_SmallSignal_Transistors/en_20030328_2SC1815_datasheet.pdf

But since you used 2n3904s, the layout should be ok. However, your voltages are not ok. The emitter should not be higher than the base. Check the orientation of your transistors and the connections, especially to the base and emitters. Since they are both the same, I'd suspect they may be in backwards.

--john

[twabelljr]

     Here is a very interesting thread about tubescreamer transistors and voltages. Don't know if it will help, but it's a good read.

http://www.elixant.com/~stompbox/smfforum/index.php?topic=37858.0