Simple mistakes 101 : Rat and wah examples

[brett]

Hi
Please feel free to laugh out loud.  Especially noobs who think that us old farts always get it right.

My Rat was going ok for a year, but I decided to swap out the TL072 op-amp for the original components: an LM308 and a frequency compensation cap (33pF).  I expected it to sound better, but it was far worse.  The treble was really weak, especially at high gain.  A year after tossing it aside I checked the guts the other day and found that the RFI bypass cap is 1000pF instead of 100pF. (102 instead of 101)   Funny how it didn't affect the TL072 as much.  Anyway, for those interested in calming a Rat, you might consider increasing that bypass cap and rolling off some treble

I put an output buffer in my wah about a year ago, so it would work ahead of my faceface.  While I was in there I also paralleled the 33k resistor that sets the narrowness of the filter, figuring that a less "quacky" wah would be good (I used a 47k resistor).  Instead, it quacked like hell.  Because I have a memory like a seive, I forgot the resistance mod and spent a year wondering how the hell the buffer was making the wah so quacky.     The other day I open it up, saw the resistance mod and instantly realised that LESS resistance makes MORE quack. Of course, I had pulled out the buffer before I saw the resistor!

Doh! x 2

[george]

what a drag it is gettin' old ....

[Renegadrian]

I'll use this ol' topic to keep the forum clean, but yeah it's the same subject after all...

Just wanted to share with you a couple of mistakes and things you learn (and it seems you never stop to learn...)

OCD - I built some of them for friends, it seems it's very popular and it's quite a good pedal. So it wasn't the first I built. This last one presented a high hiss, an earpiercing whistle...Checked EVERYTHING, but no the hiss remained...Then I looked back at the schem HERE and see that I made a small mod to it...Instead of the 2 10µ caps before and after that 10k res. in the power supply, I thought "well, it's just two caps to ground (filter?!)" so I went with only one 22µ before the res. - switched back to the original, put the correct caps and I have it good...
So strange, after all those two caps should only filter the PS...why only one cap makes it noisy...

ORANGE SQUEEZER - this one too is not my first. I use one myself with bass and am pretty satisfied.
But you know stupid errors are everywhere...So I just inverted the way the IC is in its socket. So positive pin went to ground and viceversa...I think I fried it...Gotta get some new one!!!

n00b!!!

[jasperoosthoek]

Last week I found out that the inverters in all my Millenium C bypass circuits are actually buffers (4009 instead of 4049). I probably accidentally miswired the first one which worked. Then the following 10 units I built the same way worked too. That was actually a positive mistake.

Some years ago I built a class D power amp to go with my battery powered boom box. The amp worked out fine and it used much less current than the old TDA class AB amp. So more battery life! The night before I wanted to use it for 5 days (drinking beer on some island) I demonstrated it to a friend. I always use black and red wires for power wires but my DMM has them too. I used the 10 amp function of the DMM where it has a 0.1 ohms resistor inside. I connected the red wire of my DMM to the + of the 12 volts battery. There was a black wire left on the DMM which I connected to the black wire of my new amp. (See it coming already?) So there was a red wire left on my amp which I connected to the other side of the battery. Which was the,  ,  - side of the battery.

The chip on my amp exploded with a bright flash. My friend remarked with an evil smile that this was probably not what I wanted to show him.

I just had enough time left that night to reconnect the old class AB amp. The battery did ran dry that year... 

Ever since I've always used a power diode in series with that amp, right now it does have a class D amp running at 12-0.7 volts.

[PRR]

> the inverters in all my Millenium C bypass circuits are actually buffers (4009 instead of 4049)

Both parts are "hex inverting buffers".

The 4009 takes two power supplies to convert digital from one voltage to another.

The 4049 just has the one supply voltage.

Oddly, it sure appears that if you fail to connect 4009 pin 16, a 4009 will work about the same as a 4049. There may be some difference in maximum drive but that's not important in your work. There may be some difference in distortion, but not major, and maybe just as melodious.

[jasperoosthoek]

Hmm, you are right! Didn't see that one coming.

[Cardboard Tube Samurai]

I don't know how many times I've completed a pedal, it doesn't work, I start debugging... scratching my head... getting frustrated... walk away and leave it for a day... keep getting frustrated... THIS THING SHOULD WORK DAMMIT!!!... walk away... oh look! I forgot to connect the input to the board   

[Top Top]

"Why won't it work?"

Oh, no ICs in the sockets 

[jasperoosthoek]

Happened 2 weeks ago. Why is that Inverter still on the table???

[Ronsonic]

This one from a few years ago. How come my tele, the one in the picture is cutting out sometimes. Deoxit on the switch didn't help, open it up and there's a wire lead twisted onto a volume pot terminal without benefit of solder. It had been like that for years.

[jasperoosthoek]

I built a tube amp for my dad some years ago. The local electronics shop sold me a jack socket looking like this:

But it was the very very cheap variety held together by spot welds on the farthest ring. One day I accidentally pulled the guitar cord sideways. The tiny little spot welds in the socket broke and the socket fell apart inside the amp! All the little rings were scattered inside the chassis including two metal rings. 260 VDC inside, glad nothing happened.

[valdiorn]

My Rat was going ok for a year, but I decided to swap out the TL072 op-amp for the original components: an LM308 and a frequency compensation cap (33pF).  I expected it to sound better, but it was far worse.  The treble was really weak, especially at high gain.  A year after tossing it aside I checked the guts the other day and found that the RFI bypass cap is 1000pF instead of 100pF. (102 instead of 101)   Funny how it didn't affect the TL072 as much.  Anyway, for those interested in calming a Rat, you might consider increasing that bypass cap and rolling off some treble

I'm trying to understand the effect of that 100p capacitor on the LM308... Can you explain to me what it does, what it control and how it affects frequency response?
Also, you say "Funny how it didn't affect the TL072 as much". I wasn't aware that the TL072 has pins for a compensation capacitor; in fact, I know for a fact it doesn't... What do you mean by that statement? (The TL072 is a pretty standard component that can usually be thought of as "being ideal")

I'm actually working on a digital model of the Rat, right now I'm modelling the op-amp as an infinite-gain saturating amplifier, but I'm trying to understand this slew rate stuff and the effect it has on the sound. It isn't even mentioned in the datasheet and I can't find any info on it, and googling LM308 turns up nothing but threads on the Rat , but with no substantial electronic info

[ParlorCitySound]

"Why won't it work?"

Oh, no ICs in the sockets 

along the same lines... "no sound off...no sound on?!" make sure you plug the guitar into the pedal.

[Scruffie]

One little thing regarding the original post but... isn't the LM308 a Single Op-Amp and a TL072 a Dual Op-Amp? I'm sure it was just a typing error and you meant TL071 but... I suppose it might have worked with a Dual Op-Amp and explain the Drastic tone change 

[burdt]

The worst gaffe I ever had was with my build of the Guv'nor. I spent 3 hours trying to get rid of a buzz, I think I even desoldered all my pots and attempted all types of esoteric shielding and grounding. Then I finally tried turning down the gain and treble and moving around, away from my fluorescent light source. And what do you know, the magical buzz of death disappeared.

It wasn't the last time either.

So remember, TURN DOWN THE GAIN AND TURN OFF FLUORESCENT LIGHTS when you test high gain pedals. Otherwise you'll spend 3 hours trying to ground something that's already grounded.

[Renegadrian]

 

[Ronsonic]

The worst gaffe I ever had was with my build of the Guv'nor. I spent 3 hours trying to get rid of a buzz, I think I even desoldered all my pots and attempted all types of esoteric shielding and grounding. Then I finally tried turning down the gain and treble and moving around, away from my fluorescent light source. And what do you know, the magical buzz of death disappeared.

It wasn't the last time either.

So remember, TURN DOWN THE GAIN AND TURN OFF FLUORESCENT LIGHTS when you test high gain pedals. Otherwise you'll spend 3 hours trying to ground something that's already grounded.

Perspective is a bear. Start chasing a noise and it is so easy to have the thing cured, but be listening so deep into the noise floor that nothing is going to sound right any more.

[petemoore]

  It's like plumbing with invisible water.
  Where's the leak ? Can't see any.
  'It' has gotta be going somewhere.
  Maybe we can find it by measuring the pressure and flow...before and after..
  Wait !...there's no pressure...that must have been the problem all along, and it turns out there was no leak in the first place !