Specifications

The coupling capacitor removes the high voltage DC oset from the output signal of the tube stage. This
high-pass frequency is usually between 5-10 Hz, but if it is set higher, a user can change the tonal and dynamic
properties of the stage.
The grid resistor value is an all-in-one resistor used to calculate the eects of the stage’s band pass limitation.
Inside each tube there is an internal capacitance called “Miller capacitance.” The greater the resistor, the lower
the bandwidth. The bandwidth can be calculated at 1/(6.28*RC) where R is the “grid resistor” and C is the Miller
capacitance (usually around 150pF). In a normal triode stage with a 12AX7 tube, the bandwidth is around 15
kHz if the grid resistor is 68 kOhm.
The slider for Class A/B is used for pentodes in push-pull conguration only. (The Bias voltage slider can be
used as an alternative.) This sets the bias of the tube stage such that it corresponds more to a Class A or B
stage.
Some of the non-physical tweaks:
Normally, a tube’s output swing is determined by the VPP, along with the various loads in the cir cuit. Setting
Clip Limit to something higher than 1.0 gives the circuit a greater dynamic range. The output gets higher and
the distortion on one side decreases. Clip softness is used to smooth out the sharp dynamic edge and gives
the distortion a smooth sound.
The default value of the Dynamic Bias oset amount is 1.0, and scaling this value changes the dynamics of
the signal. The signal is usually not entirely free to move through the tube, and strong transients and high
volumes aect the bias of the tube, moving up and down, as a function of volume.
Cathode biased treble boost (where applicable) changes the “lter eect” that the cathode resistor & capaci-
tor have on the frequency response.
The Bias adjust slider simply adds or subtracts a certain voltage to or from the tube’s bias. Otherwise, this bias
is dened by the tube and its surrounding components.
Extra input headroom gives the stage more dynamic headroom, making the tube distort later if given a
greater headroom. For a triode this means the signal is “attenuated” before the input and “amplied” after the
output, thus restoring the signal strength to the following tube.
The tube Tweak GUI features the following graphical displays: A “bode plot” (frequency& phase response) an
oscilloscope (with input and output signal display overlaid), and a “transfer function” (a snapshot of the
static input-to-output relationship).
The whole tube component can be disabled via the Disable circuit checkbox.
46