Quiet LaVache

[warriorpoet]

Hey y'all.

This is my first Madbean pedal, but I've built up about 6-8 GuitarPCB boards, so be gentle with me

I just troubleshot an EXTREMELY quiet signal, only to discover I had first a bad transistor, then an improperly oriented one (thank God for sockets...).  Anyway, now that I have it sounding like an angry Tweed, I'd need to troubleshoot another issue.

When I run the volume 100% and the gain 100%, I'm roughly at unity.  With the boost engaged I'm just over unity.  I have checked, re-checked and re-rechecked the wiring, and it all looks good.  I'm wondering if there might be something in the signal path that's a common boo-boo here.  Component values have also been re-rechecked.

I noticed while troubleshooting the transistor that these double-sided boards make solder joints harder to evaluate; is there a trick to it?

Thanks!

[mgwhit]

I breadboarded this circuit a couple weeks back, and I remember that the Gain knob only had a very small useful range at the end of the dial.  It's certainly not a standard overdrive Gain control, and I'm thinking about trying smaller values or a reverse log taper on that pot.

Yours does seem like it might be too quiet, though.  Check that your R4 is 330R and not 330K (or some other value).  A higher value there would certainly kill your volume, although I would expect that to disappear in Boost mode.  Also, make sure you used a B100K Volume pot and not an A100K like you would expect.  Oh, and which transistor are you using?

I know you've checked the component values, but extra eyes never hurt.  Post some good pics and several of us will go over them.  Good luck!

[warriorpoet]

Yep, used an audio pot.  I'll remedy that this afternoon, though I don't think that could be the entire issue; could it?

To make a log pot from linear, you solder a resistor ~20% of the original pot's value between 1 and 2, so to make it reverse log you do the same between lugs 2 and 3?  I'll give this a shot on the gain pot, if that's the case...

Will post pics later; thanks for the help!

edit: Oh, and I'm using the suggested transistor, 2N3565

edit2: only other modifications are BAT41 in z1 and z2.  Using 47uf 35v audio electrolytics in c2 and c4, and they're HUGE.  I don't think this is part of the problem, but there it is

[mgwhit]

I was going to try a reverse log, or C taper, pot for Gain.  I think a standard audio taper here will just make it worse.

[warriorpoet]

I was going to try a reverse log, or C taper, pot for Gain.  I think a standard audio taper here will just make it worse.

Right.  I'm trying to figure out the conversion from linear to reverse log.  I know how to do a log, just thinking out loud reversing it might, er, make a reverse log pot

[midwayfair]

I was going to try a reverse log, or C taper, pot for Gain.  I think a standard audio taper here will just make it worse.

Right.  I'm trying to figure out the conversion from linear to reverse log.  I know how to do a log, just thinking out loud reversing it might, er, make a reverse log pot

Sorry, man, you can't fake a reverse audio pot. The only way to get one is to buy it.

The boost should be blowing up your amp. It's a BIG boost. (I left it off mine because it was just too much.)

[warriorpoet]

I was going to try a reverse log, or C taper, pot for Gain.  I think a standard audio taper here will just make it worse.

Right.  I'm trying to figure out the conversion from linear to reverse log.  I know how to do a log, just thinking out loud reversing it might, er, make a reverse log pot

Sorry, man, you can't fake a reverse audio pot. The only way to get one is to buy it.

The boost should be blowing up your amp. It's a BIG boost. (I left it off mine because it was just too much.)

Ah nuts.  I'll go through it again tonight, then.  Thanks!

[mgwhit]

The audio-taper Volume pot explains why you have to dime it, but it doesn't explain why it's so weak when dimed.  When you have a chance, please post your voltages on each pin of Q1.

[warriorpoet]

The audio-taper Volume pot explains why you have to dime it, but it doesn't explain why it's so weak when dimed.  When you have a chance, please post your voltages on each pin of Q1.

You bet.  I'll do it tonight.  Thanks.

In the meantime, are suggested ranges for each available?

[warriorpoet]

As requested, pics and readings:




Transistor (top to bottom):

.614
.622
1.331

[mgwhit]

Thanks for posting these.  I don't see any bad component values, but I think your Q1 voltages are off.  I'll go recheck mine, but if I remember correctly, the collector (bottom pin) should be quite a bit higher and the emitter (top) needs to be a smidge lower.  Can you measure the voltage at the positive pin of C4?

Edit: Here's what I've got on Q1 -- E:0.16, B:0.80, C:7.26.

Edit 2: You mentioned this in your first post, but I forgot about it.  Several of your solder joints look grainy and are probably cold. I'd reflow any that don't look smooth.  You won't always get the perfect little Hershey's Kiss joints on through-plated pads, but the solder should at least look smooth and not grainy or pitted.  You can also disconnect power and do continuity tests between component leads that should be directly connected on the board.  Good luck!

[warriorpoet]

Thanks for posting these.  I don't see any bad component values, but I think your Q1 voltages are off.  I'll go recheck mine, but if I remember correctly, the collector (bottom pin) should be quite a bit higher and the emitter (top) needs to be a smidge lower.  Can you measure the voltage at the positive pin of C4?

Edit: Here's what I've got on Q1 -- E:0.16, B:0.80, C:7.26.

Edit 2: You mentioned this in your first post, but I forgot about it.  Several of your solder joints look grainy and are probably cold. I'd reflow any that don't look smooth.  You won't always get the perfect little Hershey's Kiss joints on through-plated pads, but the solder should at least look smooth and not grainy or pitted.  You can also disconnect power and do continuity tests between component leads that should be directly connected on the board.  Good luck!

Thanks man.

I will give 'er another go later.  Your readings are especially helpful.

[warriorpoet]

9.29 at power
9.29 at c4
9.29 at z1
1.33 at r2 and r3 (!)

I redid the solder joints using a scadload (technical term, that) more solder than I use on a one-sided board.  All the joints are solid, but I'm missing something...

The negatives aren't labelled on those electrolytic caps; wonder if I reversed 'em.  Any way to check without desoldering?

[warriorpoet]

Got it.

fiddled with the transistor again.  Had a a tranny from another project laying about and it worked with it, so I redid the recommended transistor (again), and for whatever reason, it works this time.

Nice sounding circuit

[mgwhit]

Cool!  Glad you got it going.