Help request - Vulcan build problem

[jpritts]

Any help would be appreciated.  My problem:

So I threw Joe Davisson's Vulcan together on a breadboard, and it worked immediately.  After quickly satisfying myself that the Vulcan was my new favorite overdrive pedal, somewhere in the transfer of all the components to perfboard, I screwed up.  When I hook up the finished circuit, the tone is great - just as smooth as the prototype - but it sounds as if the gain is biased REALLY, REALLY high - i.e. if I open up the gain pot the tiniest bit, it goes from silence immediately to heaping tons of gain with an amp-killingly high output volume.

I built the circuit verbatim from Joe's latest schematic (http://www.diystompboxes.com/analogalchemy/pedals/vulcan.gif).  I've triple-checked the layout and component values, though I could of course still be missing something.  The diodes pass the DMM test.  I've tried two sets of 2N5089's.  My guess is that I probably overheated a cap near Q2....

Can anyone recommend what parts I should start swapping out or any other troubleshooting tasks?  Please let me know if voltages at any other nodes would be helpful.  Thanks very much.


Vbatt = 9.2
Q1 C = 3.78
Q1 B = 1.125
Q1 E = 0.54
Q2 C = 3.96
Q2 B = 1.08
Q2 E = 0.52
Q3 C = 4.08
Q3 B = 1.06
Q3 E = 0.51

[jpritts]

bump.

[Joe Davisson]

The voltages look close enough, it's probably a wiring problem. Make sure the input cap goes to the diode and not the transistor. This goes for the other transistors as well, the .0022/470k after the gain control, and .1/470k before the last stage go to the diode. Accidentally going past the diode will increase the gain tremendously, making me think that's the problem. Let me know how it goes...

As far as the gain control problem, make sure there are .1 caps before and after the pot.