G.A.S.

Amp Projects

2x12 Tweed Cab

Amps

Tech21 Trademark 60
Fender 75

Effects Pedals & Mods

COMPRESSORS
BBE OptoStomp
MXR Dynacomp
Boss CS-2
Boss CS-3

DISTORTION (etc)
Big Muff
Boss BD-2
Boss DS-1
Boss SD-1
Digitech Bad Monkey
Fuzz Face
Ibanez TS-808
MI Audio Blues Pro
MI Audio Crunch Box
Proco Rat B
Tube Driver
Tube Works Blue Tube

MODULATION
Boss CE-2
Boss CE-3
Elec Mistress Deluxe
MXR Phase 90
Univibe Stereo Chorus

OTHER
Boss GE-7
Morley Wah

Effects Projects

Valve Preamp Imitation
Line Buffer
Buffered Switch Box

Effects Settings

Guitar Projects

Red Strat/EMG DG20
Blue Strat/Custom 54s
P90 Strat
Sunburst Strat/L280s
Rickenbacker 4001
SX Precision Bass
Legend Fretless/EMG P

Guitar Shielding

Rants

Vintage Stagger
Pole Spacing

valve preamp imitation

 

What follows is an investigation into the reproduction of the overload characteristics of triode preamps, using common source JFET amplifiers and single-sided clippers. It was originally published as part of my engineering thesis in September 1997, but has been abbreviated here to skip the boring bits.

I remember there being some heated debate back then as to whether the key to the "valve sound" was preamp distortion or power amp distortion. There were commercially available valve preamp stompboxes out there, as well as hybrid guitar amps with 12AX7s located in the preamp somewhere. And there were scientific papers floating around arguing that the even harmonics produced by overloaded valves (and not transistors) made valve distortion more pleasing to the ear. Since most of these even harmonics were generated in the Class A preamp, and not the Class AB power amps of the Fenders I liked, I decided to concentrate on preamp distortion. Not only was it measurable, there were also good psychoacoustic arguments for it. Which was good for an engineering thesis.

Of course today I think most people agree that there's no one secret behind the distorted "valve sound". There's

• preamp tone - specifically the EQ settings and any clipping that may occur
• a clipping phase inverter (sometimes)
• a clipping power amp
• the playing dynamics from rolling back a guitar's volume while plugged into an overdriven output stage
• interaction with the output transformer and speaker
• clipping in the output transformer and speaker
• the low-pass filter introduced by the cabinet
• the "sag" from a valve rectifier
• the treble roll-off caused by the capacitance of a guitar cable
• and so on

And despite the psychoacoustic arguments, I've heard plenty of good distortion out there generated by Class AB power amps overloaded by clean preamps.

So this project was intended as only one part of an overall solid-state approach to replicating valve amp characteristics. In the end I came up with a circuit that reproduced the triode preamp overload characteristics pretty well. (And asymmetrical clipping is currently the preferred option in boutique distortion pedal mods!)

Click here for the rest of the document (a crude html conversion from word)..

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