The Cyclops is a light to medium gain based on the correct biasing of JFET's, purposed to simulate and obtain the qualities and characteristics of vacuum tube break up tonalities. A sound full of harmonics and character, this pedal will add a full sounding weight and drive to the sonic representation of your instrument. The controls on the surface of the device have been utilised to sound useable and proficient at the either-side of their parameters, making useable sonic qualities through ought.
JFET'S AND TUBE SIMILARITIES
As notated in the pedal overview, there are numerous Jfets being utilised to replicate tube performance as there are several characteristics and traits that are exhibited and shared between the Tube and the Jfet. This leads to the JFET being a suitable replacement within this circuit.
One of the first characteristics shared between the two devices is there mode of operation. Both of these devices operate as transconductance devices. Transconductance is the ratio of change in Drain/Plate current in relation to the change in the Gate/Grid voltage. This factor alludes to how the outputted current is governed by the amount of incoming voltage. There will be transconductance rating (Given in gm) in the data sheets of each device, giving the engineer the relevant information on this matter.
Another similarity is the value of input impedance displayed by both devices. A Jfet device can display input impedance values of hundred of millions of ohms, when circuit analysing a Jfet circuit the input impedance of the Jfet is often notated as near 'infinite.' Similarly a vacuum tube is in possession of near these values. Despite some of these discussed similarities there is one noticeable difference between the devices, this factor is the law that governs the performance of devices and the subsequent results as a factor of this.
JFET'S AND TUBE DIFFERENCES
Not including some of the more obvious difference between the devices (size, power supply requirements etc) the main and noticeably relevant difference between the two devices is the law that governs their performance. The JFET’s performance is dictated by a square law determining behaviours impacting the output current, whereas the vacuum tube is governed by the three halves power law.
The JFET is a square-law device due to the Transconductance curve being parabolic. Square law alludes to one of the values varying at a square value of another value. This can be seen to determine the behaviour of the JFET when examining the gate to source voltage in impacting the output current. The output Drain current follows a square law and will increases up to a certain point called Pinch-Off point.
This is a key characteristic of the JFET and this performance trait impacts the harmonic content of the transistor. Due to the transistor being a square law device this means that theoretically only 2nd harmonic distortion is produced.
Alternatively the Vacuum tubes performance characteristics are determined by a 3/2 power law which associates their Plate current with the incoming grid voltage. The plate curves follow quite closely a 3/2 power law, in which the current increases as a function of the 3/2 power of either the grid or the plate voltage.
Due to the vacuum tube performing in this way, their harmonic detail and performance differs from the Jfet. The harmonic content of an overdriven tube devices will display a plethora of 2nd order, 3rd order harmonics and some 4th order harmonics. This result is a key factor in the tube obtaining some of the infamous buzzwords associated with this device e.g (full, warm, thick etc.)
These difference can be mitigated through a biasing scheme utilised within a JFET circuit altering the Law governing its performance and obtaining results closer to that of a vacuum tube. This will result in a more desirable resemblance in the harmonic content.
The Gain stages and properties of the Cyclops circuit is rooted in the usage of Jfets being utilised in numerous stages, gradually adding amplitude to the signal similar to that of a vacuum tube amplifier. All of the Jfet's gain values have set gain properties excluding the first Jfet. This Jfet consists of a potentiometer located at the Source terminal. This potentiometer is the gain 'knob' that can be altered external on the surface of the device. Varying the value of resistance at this terminal subsequently affects the amount of current flow through the Jfet (providing the depletion region permits this factor.)
As each Jfet stage amplifies the signal there will also be an amount of of 'clipping' added to the signal, this occurs due to the utilised JFETS low pinch of voltage adding early saturation to the signal. In some applications this isn't a desirable effect of the utilized JFET, however in the application of an 'overdrive' pedal this is an ideal byproduct. This amount of gain added at each JFET works in conjunction with configured diode selection (see next section) to slowly add clipping to the signal with each amplification stage.
Another feature of the circuit that would impact the sonic perception and interpretation of the pedals performance is the clipping diodes. The diodes are configured ‘shunt to ground’ (had clipping diodes.) There are several stages of the diodes through ought the circuit, gradually clipping the signal in conduction with the Jfets. The first stages are configured in symmetrical clipping. Symmetrical clipping refers to the impact the clipping will have on the incoming signal, with both the negative and positive portion of the incoming AC signal receiving the same amount of clipping. Symmetrical clipping can be known for having a smoother sound. However, the final stage of clipping diodes is configured as asymmetrical, asymmetrical clipping can offer a more textured alternative sound. This is utilised to replicate the tubes asymmetrical clipping features as the device is being driven, this alludes back to the buzz words associated with the vacuum tube ‘textured’ ‘dimensional’ etc. Only one stage of asymmetrical clipping is utilised as the waveform would appear too “lopsided’ by the output if implemented at each stage.
Another important implication of the diodes is the material they are constructed of, as silicone, germanium, Schottky and Led diodes all have different forwards voltages and other characteristics that will impact the clipping characteristics on the signal. As this pedal is opting for tonalities associated with vacuum tube (full, warm etc) the choice of utilising germanium diodes seemed the obviously option. Germanium diodes are often associated with a vintage and warm tones. When their forward bias voltage is achieved or exceeded germanium diodes ‘turn on’ with a knee slope that is smoother and softer than their other diode counterparts.
The tone control in the cyclops consists of a passive low pass filter control, utilising a capacitor value fixed to ground in conjunction with a potentiometer altering how much high end frequency content is attenuated. The Cyclops isn’t a noticeable high end present device however offering this tone controls allows some modification in frequency content. As the value potentiometer value is increased with the tone ’knob’ externally being rotated counter clockwise, frequency from the midrange onwards begin to become attenuated with the filter point moving closer to the midrange as the ’knob’ continues further to be rotated.
One of the principle visions when conceiving this pedal idea was for the pedal to sound more linear and ‘full’ in comparison to other drive pedals. This circuit is intended to be rich in low range and midrange content and to simulate the frequency characteristics of the ‘warm’ ‘full’ and frequency rich vacuum tubes. The capacitor values through this circuit allow for a multitude of low midrange and low end frequencies to pass through the circuit being amplified at each stage.
Another implication of vacuum tubes specifically triode tubes, is the interelectrode capacitance. This capacitance occurs due to the factor of there being two or more conductive elements within a tube separated by an insulator. This factor was utilised by engineers.
Vacuum Tube performance can be problematic at high frequencies which in turn can cause feedback and oscillations negative impacting the performance of the tube. To combat this, engineers designing vacuum tube products utilising a 'grid stopper' resistor. This resistor limits current in series with the grid but mainly forms a low pass filter with the capacitance of the tube to filter out frequencies above, near and around 20khz, that would potentially cause issues with the tube. This causes a vacuum tubes circuit designs frequency response not to be linear in this region.
To recreate this pattern within a JFET circuit design the capacitance has to be recreated through the addition of a capacitor as the JFETs capacitance is not as high of that as a tube.
This capacitor value is then introduced via a capacitor placed going to ground to simulate the grounded tubes capacitance the incoming signal would perceive.
This figure of capacitance was then included in the RC filter equation to calculate the resistor that would need to be utilised in series with the gate terminal of the Jfet.