THE SOUND OF VALVES

Many factors went into the creation of the Clapton Marshall sound.  The most significant and ignored is the technique of the man himself combined with the humbucking sound of the pickups.  The SG provided greater microphonic feedback then the Les Paul and the Firebird a much thinner sound compared to both of them.  The ES335 provided its own unique tone closer to the Les Paul.


GEC KT66

The Valves

Valves have a different distortion spectra (emphasis on even order which is less harsh on the ears) compared to transistors and they also have three other characteristics that creates their sound.

1.  Linearity - Distortion in valves largely increases in a linear fashion ie as the power output increases over the optimum output level (typically say .3% at half power, 1% at rated), distortion steadily increases until the valve cannot put out any more power and begins to "clip".  The clip point is when the valve cannot reproduce the wave form accurately. 

Transistors have low distortion and show only modest increases as maximum power is reached (ie .08% to .12%).  At maximum rated output, distortion grows exponentially and very quickly begins clipping.

2.  Clipping - Valves clip (run out of power) in a much less harsh way then transistors.  They reduce the wave form by rounding it down where transistors cut of the top of the wave.  Transistor clipping is very harsh on the ears - the diagrams below literally represent the effect.

3.  Microphonic Effect - because valves are not solid state but made up of metal components sealed in a glass vacuum tube, they are physically effected by vibrations.  The high volume environment of rock music most definitely would have produced significant microphonic effects.  Also different valves had different characteristics with the KT66 having a great propensity then the EL34.

4.  Transformers - Valves are high impedance output devices and require a transformer between them and the low impedance speaker.  Transistors virtually direct connect with only a resistor/capacitor network between them and the speaker.

 
EL34

Transformers

Transformers are long lengths of insulated copper wire wound around a metal block, with current being transferred between coils by induction.  Transformers are complex to build and always represent design compromises.  High powered guitar amplifiers require large transformers and that means large lumps of metal in the cores and heavy wire to handle the power.  That size effects their frequency responses with most amps having frequency roll-offs starting at 50 and 15,000 Hz (hi-fi amps are flat 20-20,000+).

Transformers have two further effects - they add their own distortion and saturate at high powers.  Saturation is when the electric current flowing through the copper wire creates a powerful magnetic field that saturates the metal core ie it becomes magnetised.  When this occurs the frequency response is altered (loss of bottom and top end) and distortion increases further.

 
1959 Chassis and transformer layouts - left is a very early aluminium chassis, far right is the 1968 config. 
Note the changes in power transformer (bottom position) and the change of orientation of the output transformer
(middle of chassis)  on 2nd from right.

Pickups and the amps

Clapton always used the flat input on the Marshall Amps unlike Hendrix and Townshend (and most guitarists at the time) who used the treble input which boosted higher frequencies.  I believe he did this cause he didn't like a prominent high frequency and because it gave him greater control from his guitar.

The use of a splitter would have effected the frequency response as the pickup would have been seeing a halved impedance (what the actual effect was I am unsure but I believe a high end roll-off).  The higher capacitance of the very long leads he used would have also impacted on high end response (additional roll-off).  The effect on sound can be heard on the early bootlegs where the guitar/amp sound is hard with a prominent treble and that of late '67 which is much sweeter and rounded.

Speakers

Speakers can also be overloaded which can result in physical failure (a common occurrence in Cream's time).  However if it is done carefully they become overdriven ie cannot physically move any further and begin to compress.  I believe this was also a component of the woman tone.  Unfortunately, as they evolved to the higher rated CG1230M speakers, that effect was diminished.

How did Clapton Achieve his sound.

Eric Clapton took advantage of these effects in combination by:

Turning all amp controls to ten which gave maximum gain and had all the valves at maximum load or near overload.  He then controlled the sound from his guitar where small changes in control settings, picking pressure or fret work would have a significant sonic effect. 

Ran the amp at a sweet overload level (KT66 valves) where the distortion enhanced rather then impeded the sound.  The distortion became part of the sound.

Selectively overloaded the output stages and saturated the output transformers.  This can be heard on the live woman tone with the guitar's treble rolled off and bass up, producing a heavy thick singing sustain.  The distortion is massive but sweet on the ears.

Clapton's sound was a product of these factors in combination plus the sonic interaction with the guitar.  The pickup - amp interaction even further complicates the picture. The 1964 SG humbuckings must have had a nice sound - Clapton selected his guitars on sound and feel.  

The sound evolved based on Clapton's technique and use of the available technology.  It began to change as the technology rapidly developed and Clapton wanted to try a different sound. 

Can the Clapton Cream sound be reproduced?.  

No, for the following reasons:

Clapton's technique

Today's amps are much more refined and subject to tight quality control

Today's valves are much more refined and subject to tight quality control

Today's guitar pickups are much more refined and subject to tight quality control

Today's speakers are much more refined and subject to tight quality control

Even the "accurate reproductions" use much higher tolerance components  than those of the sixties

Yes, by experimenting with effects rigs and using dual stacks on full!

 

Loud & Louder

  Graeme Pattingale, 2004