Kolme klassikkoartikkelia Björn Juhlilta

Björn Juhl saapuu marraskuussa Suomeen pitämään mestarikurssia. Juhlistaaksemme tätä julkaisemme kolme hänen vanhaa kolumniaan, joita ei ole ollut aikaisemmin luettavissa.


Fender Amps and the magical six

It was some time in 1979 when one of the "big guys" showed me how to dial in that Fender or Music Man amp that always seemed to be on stage where ever you played: "Easy" he said. "It's the magic six". Volume to 6, Treble to 6, Middle to 3 and Bass to 2 (6, 6, and 3x2=6). Bright should be on, reverb set for two and master volume (if one) set so that you can be heard above the drums without drenching the vocals. "The rest of the knobs you can just ignore" because no one used tremolo in late 70's and the footswitch was always disappeared…

The effect was almost like having the key to Holy Grail - I have to admit that he was right and I joined the Magical Six Choir.

It wasn't until years later, when I was absorbing schematics that I understood why there is magic in the six. In an amplifier with the topology input stage, tone control, volume control, voltage amplifier (tremolo- and reverb amplifier if fitted), phase splitter and output stage it is the first four controls that will set what is going to distort and how much.

The first stage runs essentially clean if nothing else but a guitar is connected.
The voltage amplifier will hit the ceiling as volume is turned pass 4.
The tone controls are placed before the volume control and will control what is being fed into it.

The purpose of the setting Treble=6, Middle=3 and Bass=2 is to:

1. Cut a bit of bass to get balance between high E-string and low E-string distortion. The bass-frequencies will have overtones far up in the midrange and may therefore build into disturbing high overtones that can make certain chords (D7) sound out of tune on the top three strings. Some amps will even oscillate when exposed to the combination of high amplification and lots of bass.

2. To control "The Mud Frequency 249,5Hz". Too much of this frequency and the tone will be muddy and too little and it will sound hollow and thin.

3. Cut some midrange (approx. 340Hz) to gain output power while keeping inter-modulation distortion at a minimum. Amplification factor will also be highest in the treble (above 2 kHz) and that is compensating the treble-loss of passive pickups. Open E-strings (high and low) played at the same time should now have approximately the same volume level with little more distortion on the top-string. Both strings should be clearly recognizable.

Inter-modulation distortion is a sort of mathematical distortion in that the sum and difference of two incoming frequencies are obtained as harmonics and will therefore build up so called "ghost notes" (these are notes that are not played and that are not in musical relation to the notes played). This can make certain chords sound dissonant as opposed to what we know as harmonic distortion where overtones follow the pattern of 2x, 3x, 4x.. in upper frequency, something that makes the probability of so called ghost notes being produced a lot lower.

The risk of inter-modulation distortion is quite high on Fender amps, mostly because of the working-points of the different stages. Ironically enough some of the modifications that were performed at the factory to reduce the total amount of distortion will, while reducing total distortion, rise the amount of 7th overtone, which is said to be the least pleasing of all.

4. Obtaining the correct treble response on most amplifiers made for guitar the treble-control does not work the same way as on your Hi-Fi, regulating the amount of treble. On guitar-amps the position of Treble-knob will very much affect to bass and midrange and it works more like a balance between bass and treble. In fact the tone stack system as used in most amplifiers is derived from a so called "Baxendall type" Hi-Fi circuit. Treble has been set to full while an isolation resistor often placed between the treble and bass controls is used to control treble (or actually tone balance between treble and bass).
Balance is obtained when the treble pot is in it electrical center - that is between 5 and 7 depending on the taper on the pot. Final regulation of treble is best to be done with the bright-switch.
The bright-switch seems to work best if the volume is set to between 5 and 7.

5. If the volume-control is set to 6, stage #2 will be fed from highest possible impedance and there will be certain current limitation. Stage #2 will also be fed with as much as it can take without getting some rather odd distortion as feedback (Durchgriff) through B+ tree that is appropriated as the true distortion generator is the stage #3 (reverb-mix amp)
Where some peculiarities take place such as rectifier action between grid and cathode and glazing-spikes because of cathode decoupling etc…

6. And so 6, 6, 3, 2 (2x3=6) The Magical number 6.

Treble boosters, why boost treble?

The human ear is very sensitive to changes in treble. The amount of treble to an instrument will determine the position of it in a mix. Echo reflections are distinguished in distance as their treble content for instance. Any change in treble response will have a great impact of final sound. Up to fatal levels of volume an increase in output level will also appear to be an increase in brightness because of the way we hear things.

Consider having an amplifier with bright switch (capacitor connected to volume control). This amp would sound best when volume is set to 6. With Little Red Trebler (LRT) you can have adjustable treble boost at any levels, so you will get the correct treble at all playing volumes.

If you connect a distortion pedal to an amp set for a bright, singing clean tone you very often end up with very bright shrill tone. There are not that many pedals with kind of construction and tone controls that can handle an amp with bright switch on.

If you place the LRT in front of your distortion you will give more sparkle to your distorted tone without affecting the overall treble-balance and at same time give correct treble for your clean sound.

You might have an amp that has a soft treble tone and any attempt to increase treble results in a loss of bass and/or honky higher midrange instead of brilliant high-end.

This amp has her own beautiful voice and you might be happy until the second guitar player in your band comes with his new amp with treble of silver and your amp sounds bit dull next to it.

The LRT will let you dial in brilliance without changing the magical positions of your amp that makes it sing like angels (but not heard). You can have the shimmer when ever you need it by footswitch and now it's easier to cut through in a mix and still have the right balance of bass and middle that are so important to your tone.

On the other gig you have set your amp for light breakup with your single coil pickup guitar.
For some songs you need to plug your old trusty "Paul" with humbuckers

After a little fiddling with the controls sound doesn't seem to be right. Lowering the volume helps correcting the break up, but treble response seem to suffer- focus is shifted either way you turn the tone controls as midrange and bass are also affected. The LRT might make things a little easier in this situation. String balance may be aided with the LRT - especially so with a jazzy neck pick up.

The LRT may also give a new life to the tone controls on a guitar - most tone controls on guitars are of the lowpass kind where a capacitor reacts with choke/s in the pick up bleeding highs to ground. The combination of this choke based low-pass filter and treble boost give rise to a resonant effect, somewhat like a wha set in between. (Some tweaking to the tone-control cap may be needed to get your exact in between sound)

This effect is all the more evident with distorted sounds - some combinations may result in your ordinary treble roll off being converted into a resonating, sweep able mid-control. Note that LRT does not give this effect but enhances it, making it more evident. Some guitarists have used this resonant effect (tone control, treble booster, distortion) with an almost infinite sustain, with a flute character as a result.

Some old fuzz pedals may benefit from having a bit of treble boost before them - making them more contemporary (while still organic) and all settings more useful. Your rehearsal amp is a -70's transistor radio - a pack of slightly worn batteries to get the radio distorting and a treble booster may be all you need. The LRT will suit fine in this application.

Consider having a non master-volume, tube amplifier with bright and normal inputs. This amps break up into nice distortion when volume is advanced to 6. Workable range is volume 5 to 8 - above that sound gets fuzzy, and below sound is clean. Distortion devices on this amp work excellent up to volume 6 where distortion gets distorted. With the LRT on the normal channel (which has slightly more gain) volume 7 makes the amp trip to 10 without getting to fuzzy.

The treble booster trick to this kind of amp is the embryo of later master volume models from more than one maker of such amps, and has been used by many a notable player.

18 volt power on pedals

" If your fuzz unit isn't raspy enough, you can increase both its volume and distortion capabilities with the addition of an extra battery. However, you are risking possible damage to the transistors, so experiment with caution. (Tom Wheeler, The Guitar Book)"

The Fuzz- , Distortion-, Overdrive effects are often set around a specific voltage range.

A classic way of obtaining these effects is to take a well biased circuit and lower its supply voltage until you get the desired effect. An example of such a circuit is the OP-amp.

Many popular effects rely on the blend of distortion products of an OP-amp in conjunction with limiting in some form. A great many OP's are designed to work at a supply voltage of no less than 9 Volts or performance is compromised - meaning high levels of distortion can be expected from the OP as internal bias supplies may be offset. The op-amps were not originally designed to be used on overdrive or other pedals and the optimum voltage for them is often higher than 9 Volts.

In an OP-amp based circuit with clipping diodes, higher voltage means less influence of the OP, it works more linear, up to a point when something fuses - often a filtercap. Maximum distortion-less gain in a circuit powered by 18Volts, driven by an ordinary humbucker is however 35 dB as opposed to 18 dB in the same system powered by 9 Volts.

In a discrete design ( only transistors or tubes ) the working point of internal stages is set within a limited span. Raising or lowering supply voltage beyond the limits of the span directly affects linearity of the circuit, and the exact overtones produced. Discrete designs are usually less tolerant to supply voltages outside the design limits - very unpleasant sounds may be the result when using OP-amp based pedals with non-standard voltage.

Most pedals designed for 9 Volt supplies have components rated safely above 9Volt, but rarely safely above 18Volts. Electrolytic capacitors used in powersupply filtering, are made with specific voltage ratings e.g. 10V, 16V, 25V, 35V , 50V. In 9Volt powered effects 16V rated components are often used because they are physically smaller than say 25V types. This may be of significance if space is limited.

Applying higher voltage than rated to such a capacitor will seriously shorten its life and may result in an explosion. One of the design criterions of BJF- pedals where that it should be feasible to power them from a 9 Volt battery at good economy. This was achieved by low current draw or ability to work from low voltage.

Extensive tests were made to set acceptable voltage range to each pedal. Performance has been optimized for a supply ranging from 7,5V to 12V, taking into account that a fresh battery may leave 10,5 Volts, but would be considered empty at 7,5Volts.

It would be likely that a pedal sometimes would be powered from a 12V supply, since many
pedalboard supplies can deliver that (like Voodoo Lab Pedalpower-2). You would get more output dynamics and overally cleaner sound. This however does not work if supply is increased further as bias systems may be overridden or even blown.

Had the standard been 18 Volts instead of 9 Volts the pedals would have been made differently because a designer would try to make the best out of the voltage range given and it is actually much harder to make things work at 9 V than at 18V. I remember having a hard time with pedals requiring 18V because batteries were (are) expensive. There were solutions like electronically fabricate a negative 5V supply from a 9Volt battery giving a total of 14 V but that would typically be 1V low from lowest supply acceptable with these 18V effects, that were very often regulated internally for 12V to keep bias steady.

3.10.2016 Björn Juhl
Writes is knows as "Mad Professor" and has designed effetcs for Mad Professor, BJFE and One Control



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