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Pedal Friendly Amps – Myth or Magic?

Amp_SR_15_head_front_3_lrg.jpgThe term Pedal Friendly is very trendy at the moment.  What does it mean?  Are certain amps more palatable with pedals?  Stay tuned and we’ll find out!

Before we dive into this month’s topic, here’s a disclaimer: I certainly am not a pedal expert.  The following discussion is based solely on my knowledge of tube amp design and the pedals I personally have shoved in front of them.  My plan is to analyze the technical aspects of the ‘pedal friendly’ – or PF – phenomenon and see what we can learn.  Here goes!

It seems to me that PF is increasingly becoming a key decision making criteria regarding tube amps.  Many dealers that I talk to, and customers that seek advice directly, ask about how an amp reacts to having pedals put in front of it.  Often, there is not a lot of additional information provided regarding exactly what the questioner is looking for.  However, a little probing typically uncovers that what is meant by PF is the amp’s ability to sound good when some sort of boost, overdrive or distortion pedal is plugged into its input.

So, the first boundary that we will put on our investigation is to delve into what happens when this sort of pedal is placed in the signal chain before the input of a tube amp.

This makes sense (to me!) as other types of effects – time based such as delays, choruses, flangers, phasers and the like – simply modify the sound coming from the guitar and do not typically alter the gain of the fundamental tone.  In my experience any tube amp of reasonable quality handles these kinds of effects well.  Further, if the amp in question has an effects loop, time based effects almost always sound better in the loop rather than being plugged into the amp’s input.

So what could make an amp friendly or unfriendly toward a ‘BOD’ pedal (boost, overdrive, distortion – yeah, I like coming up with stupid acronyms!)?  First, let’s examine what that type of pedal does. 

A BOD pedal does at least one thing and maybe two.  All types increase the gain or the level of the signal reaching the amp.  That means it takes the signal from the guitar and makes it bigger.  Overdrive and distortion pedals then add harmonic content by distorting the signal – making it more complex and usually adding more gain in the process.

So, what’s hitting the amp’s input is a larger than normal signal and one that may contain additional harmonic content due to the signal being distorted by the pedal.

What happens at the amp’s input?

All amps have input circuitry that passes the signal at the input to the first preamp stage.  Usually, that means feeding the input signal to the grid or input of ½ of a 12AX7 tube.  In between the amp’s input and the AX7’s grid is a resistor or two and maybe even a capacitor that block radio frequency interference, reference the input signal to ground and, depending on the input circuitry design, attenuates – reduces – the input signal.

Many amps have more than one input jack often labelled “high” and “low” or something similar.  One input passes the input signal to the first tube stage without attenuation; the other input attenuates the signal and sometimes filters out frequencies – as in the case of a “bright” input that reduces the amount of the incoming bass frequencies thereby making the sound brighter (essentially a fixed bass roll off control).

By itself the input circuitry isn’t affected by the presence of a pedal – it simply passes the signal on either as it sees it or at a reduced level.  It doesn’t care how big the signal is.

It gets interesting at the first preamp stage (well, at least it does for tech freaks like me).  

A preamp gain stage serves two purposes.  First, and most obvious, it amplifies the relatively tiny signal coming from the guitar.  Second, the designer has the option of altering the harmonic content of the signal by causing the preamp stage to overdrive or distort the signal.

By selecting the value of various components associated with the tube the amount of gain and the stage’s frequency response can be determined.  The more gain, the more the signal will be amplified and, if the designer so chooses, the more the signal will be distorted.

This is where the meeting of a BOD pedal and an amp can get troubled.  In my experience, the more gain that is designed into the first preamp stage – or multiple pre stages if they don’t have a gain control to tame subsequent stages – the more difficult it will be to find a BOD pedal that will do its thing AND play nicely with the amp.

Think of it as running two BOD pedals into one another.  We’ve all done that and found that sometimes it sounds good and sometimes it doesn’t.  Again in my experience, and I bet you’ve found the same thing, some tweaking is required to make two BOD pedals in sequence sound good together.  Usually, this requires tweaking the gain of one or both pedals to get a good sound. 

Simply put, if there is too much gain in the signal chain the sound can get out of control in a hurry – the specific tonal carnage will depend on the pedals and their individual gain characteristics and frequency responses.  However, we’ve all winced in pain when we chain a couple of overdrive/distortion pedals together and the result is NOT pleasing (not to say that this kind of tonal mayhem isn’t exactly what some players are looking for, but for the purpose of this article I’m approaching the topic from the perspective of players looking for rich, thick, harmonically complex overdriven and distorted tones – and, yes, that’s what I like and since I’m writing this article I get to steer the car in the direction I want to go!).

So, we’ve seen that a BOD pedal and the first preamp stage of an amp can be considered the same as putting two BODs in sequence.  This means that the matching the gain of the two stages is critical to getting a pleasing tone.

If you buy that, then it is logical to conclude that the design of the amp’s first preamp stage will determine how good it and any given BOD pedal will sound.

High gain amps that are designed to create preamp distortion typically strive to get as much gain as possible from each preamp stage.  Therefore, designers tweak component values to maximize the distortion that each stage creates.  These amps are very sensitive to the input signal level.  Since the first and subsequent preamp stages are designed for maximum gain, a small change in the input signal will cause a very large signal at the output of each preamp amp stage.  Depending on the gain controls provided with the amp it may be impossible to compensate in the amp for the added gain produced by the BOD pedal – after all the pedal will likely sound best when its gain is in the ‘killing range’.  Result: too much of a good thing!

The other possibility of BOD pedal and amp disharmony is related to frequency response.  If your pedal emphasizes frequencies that your amp either further accentuates or attenuates the result may be less than you hoped for.

For example, if the pedal produces lots of bass and the amp is voiced to accentuate bass frequencies you may once again get too much of a good thing.  The reverse can also be true: running a treble booster into an amp that is naturally bright may appeal to local dogs and cats, but probably not the humans in earshot. Conversely, if you are running a treble booster into an amp that tends to be dark sounding you may need more treble than you can boost to get the tone you’re after.

Of course, frequency response can be altered throughout the signal chain using tone controls.  However, some pedals have limited or even no tone controls.  Without the ability to alter the frequency response of the pedal it may be difficult to alter the result after the signal passes through the first preamp stage.

So, what is the profile of a successful BOD pedal/amp match? 

Here’s the recipe I believe gives you the best chance of success when matchmaking BOD pedals and amps:

1.    An amp NOT designed to create gobs of preamp distortion.  For example, most Fenders are designed with medium gain preamp stages that amplify the input signal sufficiently to drive the power amp while not overly overdriving the signal within the preamp.  The tube component selection ensures a nice warm tone through each preamp stage – the key to classic Fender tone.  MANY amps are designed with a similar goal in mind.  In fact, virtually every tube amp that strives for classic tube amp tone – rock and blues tones – feature preamp stages that don’t create a ton of distortion on their own.

The other benefit of this type of design is that in many cases a well designed preamp will be sensitive - characteristics of a dynamically sensitive preamp are that it responds quickly to picking technique and changes in picking attack and that the amp will clean up nicely when the guitar's volume is rolled down.  Amps with well designed and sensitive preamps tend to behave extremely well with all manner of BOD pedals.

2.    BOD pedals with both gain AND tone controls.  This ensures that you can tweak the incoming tone to match the amp’s voicing and avoid the ‘too much of a good thing’ problem described above.

That’s it.  In my mind there are only two ‘simple’ requirements to reaching BOD pedal nirvana (aside from the quality of the pedal itself).  Only one of them has anything to do with the amp.

I believe ANY tube amp that isn’t designed to create massive amounts of preamp distortion can be ‘pedal friendly’.  If you can get a nice clean sound out of the amp at reasonable volume levels chances are you’re in good shape.

Not much to it in my mind.  Conclusion: the ‘pedal friendly’ label is really a bunch of hype – another guitar gear myth!

As always, I’m curious to hear what YOU think – especially since this may be a controversial topic!

Don Mackrill

www.MackAmps.com