How exactly does an overdrive pedal work?

chris77

I recently returned home from a much needed holiday and was greeted at the door by four cats and a new Class 5 amp. (Thanks Matt at Musicmadness!) But 5w is still way too loud to crank up and naturally overdrive the amp. So out came my old sd-1 pedal, followed by a lot of grinning soon after. So how does it work? Does it amplify to guitar signal, or does it do something with the amp? Ive heard that it drives the tubes to breakup earlier, but how? It works just as well on my s.state amps, so are tubes really part of the equation?:-S

[deleted]

chris77 wrote: I was greeted at the door by four cats and a new Class 5 amp.

I read through this sentence and couldn't decide exactly how to decipher it. There are a few (admittedly unlikely) possibilities:

1) MusicMadness is now selling cats. (I don't approve, if they're using courier companies.)
2) The amplifier also greeted you at the door. (Manufacturers are obviously achieving the darndest things these days.)
3) The cats actually did greet you at the door. (This is surprising enough in itself, since my experience with cats is just the cold shoulder, if I've been misbehaving (going away).)

While I'm not the best person to be imparting wisdom about overdrive pedals, I'll make the suggestion that you contact Mr Parfett about a TubeTamer, which could allow you to reach those levels of natural overdrive without permanently damaging your ears/neighbours/cats?

Squonk

@ Strat

My cats greet me at the door. ? But that's just because they want food, after that it's the cold shoulder again

That tubetamer does look good. www.tubetamer.co.za

[deleted]

Yes, mine rolls around in the parking lot when I pull up, all smiles...but if I've been away for TOO long (according to whatever calculation), I get an icy reception. It must be a bell-curve phenomenon...

ezietsman

Overdrive pedals aren't too sinister. They're meant to amplify your guitar's signal. In the bad old days, when people realised distortion is cool, it was a pain in the, eh, ear to make an amp overdrive. They had to crank the amps until the power stage valves couldn't deliver the increased power any more and started to give non-linear amplification (. We hear this as overdrive. The increase in harmonics sounds cool. ? An overdrive pedal boosts your guitar's very weak signal so that the signal that is amplified by the valves are also stronger and therefore they started doing their thing at lower volumes. Modern overdrive pedals actually simulate (or try to) the effect a valve will have on the input signal.

See this article and the image. I believe valves tend to give soft clipping which we hear as that nice warm overdrive sound. Please correct me if I'm wrong. The end-result is that your sinusoidal wave (from your guitar, which is in fact not really sinusoidal) gets 'modified' by the effects and amplifier to something non-sinusoidal, which if done in certain ways sounds good.

http://en.wikipedia.org/wiki/File:Clipping_compared_to_limiting.svg

Gearhead

chris77 wrote: 5w is still way too loud to crank up and naturally overdrive the amp

Depends on the room. It is actually too little to keep up with a drummer while clean, without mic'ing up. With 15W tube you will be able to do that, but 15W overdriven at full tilt will be too loud for most non-guitarist fellow band members. Other guitarists do understand ?

chris77 wrote: how does it work? Does it amplify to guitar signal, or does it do something with the amp? Ive heard that it drives the tubes to breakup earlier, but how?

The link from Wikipedia tells most of the story. To clarify: any amp (there's one in your overdrive pedal as well) multiplies the incoming signal with its amplification factor. Now if the incoming signal when you play loud as you can is swinging between -1V and +1V and the amplification factor is 5, it should put out a signal swinging from -5V and +5V. If you power this amp with a nice 9V battery, the best it could ever do is swing between -4.5V and +4.5V, which now is 10% short and clipping will occur. Should you play a little less loud, it would however not be short. I am ignoring a lot of side effects to keep the story sorta understandable.

The difference they try to explain between soft and hard clipping is not entirely right. Solid state amplifiers are sort of linear up to the power supply voltage causing hard clipping or fuzz. Tubes are more nonlinear the further they are away from the 'middle', the voltage they put out when the input is at 0V. If you keep a tube on non-clipping voltages (actually with tubes it's current), there can still be up to 5-10% distortion, what they call harmonic distortion: exactly like the overtones of your guitar strings themselves. Because these amp overtones are applied to the entire guitar sound, it just fattens it up; this is why clean tube tone sounds better to most people than clean solid state tone. The tube amplifying nonlinearly also makes it work as a very subtle compressor.
Since there is compression in the tube amp and since there is only soft clipping, it is much harder to tell when a tube amp starts being overdriven and when it is still clean. This makes it easier to 'play' the overdrive; you play lower notes or you play louder, and you get more distortion.

The one thing you amp will be able to give you (well, not a 5W single ended amp but push-pull amps do) is crossover distortion (not in the graph). This is not at the +peak and -peak voltages, but smack in the middle. A push-pull amp has a phase splitter and pair(s) of output tubes. Every output tube in this amp does only the positive voltages or the negative ones, so at 0V they must 'take over'. This does not happen so lekker, which sounds similar to soft clipping but still different.

One more thing is completely overlooked in the Wikipedia graph: solid state designers need a lot of negative feedback to make transistors kinda linear; this has its sonic downsides called TIM. Basically when the signal changes quickly (as in music instead of sine waves) the feedback messes it up. Attack becomes attack or something :-\

chris77 wrote: It works just as well on my s.state amps, so are tubes really part of the equation?:-S

Imho it does not, but it does take time to start hearing the differences. This is a dubious point to get to, since you will be easily able to distinguish between sand and bottle, but the downside is that you will now have more expensive taste.
I think the above will sorta explain why tube amps treat overdriven pedal tone differently: they are not really clean.

AlanRatcliffe

To add a little simple stuff to what has already been said... There are two ways to use an overdrive/distortion pedal: either adding it's own distortion to the mix or as a volume boost to drive the amp harder.

To use the pedals distortion, you crank the gain/drive on the pedal and turn the level/output down so the level is more-or-less the same as when the pedal is bypassed. This is the best way to get drive at low volumes.

Turning down the pedal's drive and boosting the output level so that the guitar is louder when the pedal is on "kicks" the amp harder and makes it drive more. But that's more for getting a volume/drive boost for solos when playing and isn't good for keeping the volume down.

chris77

Thanks for the great answers Gearhead, ezietsman and Alan.? I think I understand it a bit better now. @ Gearhead, what is a push/pull amp?? @Stratisfear - hehehe. Read it again myself and it doesn't make sense does it :$ After a nasty incident involving the neighbour's boerboel and the missus's favourite, they aren't allowed outside when we aren't home. So the overly friendly reception at the door probably had more to do with going outside than seeing us! Still a nice sight though.

TomCat

Regarding OD pedals and tube amplifiers...one thing that I enjoy on my amp is that I use a Rat into the OD channel and drive an overdriven signal into the high gain of the OD channel of a tube amp for wicked sustain and distortion........I have a level control after the gain stage to keep levels sane......(At least for me that is..... ?)

What I'm saying here is that you can boost your signal with an OD and then push that through more tube gain (Preamp Gain that is) for a really rich tone.

Gearhead

Reading back I realise I never answered your question :-[
So the way that they emulate soft clipping in solid state is by taking advantage of the fact that diodes in forward voltage conduct badly up to about 0.7V (actually it differs per material). So they amplify and buffer the input signal, feed it through a pair of diodes (one for the positive swings and one for the negative swings) and then amplify the voltage over the diode. Since the voltage will not become much more than 0.7V because the diode will start conducting, you now have 'soft clipping' at that voltage. By playing around with the pre-diode gain you get more or less distortion and by playing around with the gain after the diode you get more or less output volume.
In an experimental mood I once (>20 years ago :-[) built a distortion pedal with two pairs of diodes, slightly offset. The second pair goes into the transition of gradually conducting more at the point where the first one really starts conducting already: more soft clip. Sounds alright but I stopped playing through it when I got better amps 8)