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Topic: What actually is "overdrive"? |
Paul Seager
From:
Augsburg, Germany
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Posted 23 Jul 2014 8:08 am |
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I feel I should know but I must confess I don't.
I come from the bass fraternity and have always used a transistor amp and a clean tone. Now that I am more of a steel player (non pedal), I'd like to try a tube amp, nothing too wild - probably a Fender Deluxe Reverb or maybe a Bassman - doesn't matter, just a classic tube amp which stays clean most of the time.
But then I think a mild overdrive tone is not unpleasant for a steel. Then it occurred to me, what actually is overdrive? I'm not talking about the effect pedals that simulate it, just what is actually happening on the amp and why do tube amps do this and trannies not.
Will high output pickups, (I have Ricky horseshoes on mine) overdrive an amp or does it come from within the amp itself?
\paul
_________________
\paul
Bayern Hawaiians: https://www.youtube.com/@diebayernhawaiians3062
Other stuff: https://www.youtube.com/@paulseager3796/videos |
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Lane Gray
From:
Topeka, KS
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Posted 23 Jul 2014 8:37 am
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Both tubes and transistors have limits to the max voltage (the height of the peaks of the sine wave).
As long as you stay within those limits, a sine wave stays a sine wave. If you exceed the limits either by turning up the gain or by boosting the incoming signal, they'll TRY to make the wave as high as you ask, but the limits "clip" the tops off, so it looks like a sine wave with flat tops and bottoms.
Not all fuzz boxes are overdrives: the Boss Tone does something with diodes.
(It's been years since I studied this stuff, so I MAY have some stuff "differently correct.")
EDIT: Solid state amps DO achieve overdrive, but it's not as pleasant. If you have a Peavey, dime the pre gain and turn the master down to 1 or 2. Or of you have some monster boost like an old Izzy Plus, dime that (24dB gain) and use the volume pedal loop.
_________________
2 pedal steels, a lapStrat, and an 8-string Dobro (and 3 ukes)
More amps than guitars, and not many effects |
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Kevin Raymer
From:
Chalybeate, Kentucky, USA
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Donny Hinson
From:
Glen Burnie, Md. U.S.A.
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Posted 23 Jul 2014 2:32 pm
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In a nutshell:
Overdrive is simply the distortion/compression caused by overloading (overdriving) an amplifier stage. All overdriving will cause some sort of distortion, but you can also have distortion without any overdriving taking place, since distortion can be classified as any change in the electrical waveform, other than amplitude. So, in the purest sense, any tonal change is also a form of distortion.
The reversed diodes used in a Bosstone (and in many other devices and amps) serve only to "clip", or flat-top an amplified waveform, thus increasing (but not necessarily enhancing) enhancing the distortion. (The overtone or harmonic content is what determines whether or not the distortion has a pleasing sound.) |
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Michael Hummel
From:
Ottawa, Ontario, Canada
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Posted 23 Jul 2014 6:10 pm
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Hoo boy, this is a subject where the experts (and the less than experts) like to weigh in, and the info presented can get all over the map.
There are so many things that come into play to explain the differences between tube and transistor -- but considering only the amplifying devices themselves, and the circuit topologies generally used:
Generally speaking: transistors tend to be "hard clipping", which means reaching the hard stop all of a sudden. Tube transfer curves are a little more gentle, so they start to round off somewhat before they hard clip. This generates more pleasant harmonic distortion.
In my experience, the positive peaks and negative peaks of waveforms saturate or clip unevenly in both types, but more so in tube amplifiers. This means the resulting amplified waveform is more "asymmetric" in tube amplifiers. The resulting harmonics seem more pleasing, at least to my ears.
This hardly touches the surface of the subject but explains at least some of the first-order effects.
Oh how I miss my Traynor YBA-1 that I used in my teenage years in Toronto!
Mike
_________________
MSA Classic 5+4
Too many 6-strings and amps to list |
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Steven Husting
From:
Germany
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Posted 24 Jul 2014 3:36 am
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Here is a somwhat but not too technical discussion of tube triode gain stages (like a 12ax7) if you can get through it. He discusses overdrive and distortion about halfway through. Well worth it if you want to better understand what is going on in a tube amp. Math isn't really important for what you want to know and you can skip it.
What Mike said about uneven clipping is true; in a tube triode, the top half will clip due to cut-off (the tube can't amplify anymore) and the bottom half will clip due to grid current.
Steven
http://www.valvewizard.co.uk/Common_Gain_Stage.pdf |
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Paul Seager
From:
Augsburg, Germany
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Posted 24 Jul 2014 4:45 am
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Thank you all so much - it certainly isn't TMI as this becomes a reference for all in the future.
I get now how the Gain and Volume controls offer so much more control over this than a single Volume control. It also means that my potential choice of a Deluxe Reverb is probably better as I think I only really want that overdrive to come in on peaks.
Once the piggy bank is filled I'll let you know!
\ paul
_________________
\paul
Bayern Hawaiians: https://www.youtube.com/@diebayernhawaiians3062
Other stuff: https://www.youtube.com/@paulseager3796/videos |
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