The bright channel of the Vox AC30 Top Boost preamp begins with a traditional 12AX7 preamp connected to a volume control with a bright bypass capacitor. This is followed by two more stages to prepare the signal for the tone stack. The normal channel starts with the same preamp but without a bypass capacitor and its output is connected directly to the phase inverter. The low-gain input jacks for both channels are connected to an inverted-L network that acts as a voltage divider to attenuate the input signal by half, or 3dB.
The high-gain input jacks connect the guitar circuit to a gamma network formed by the grid resistor RG and the grid-stopper resistor RGS.
The resistance Ro is the output impedance of the guitar and represents the ability of the guitar circuit to drive the amplifier input. An output impedance of zero ohms signifies that the guitar is a pure voltage source that can supply any amount of current to the next stage. A non-zero output impedance represents the decrease in voltage magnitude that occurs depending on how much current is required. The other parts values are
RGS = 34k
RG = 1M
(RGS represents the effective value of two 68k resistors in parallel.) The gamma network exploits the capacitance between the tube's electrodes to attenuate radio frequencies, as demonstrated by our Grid Stopper Resistor Calculator (when we use in advance a voltage gain of 74 that we will determine shortly). The calculator shows 11dB attenuation at 128kHz but less than than 1dB cut at 16kHz. For good treble response this is quite sufficient for a guitar amp.
Here is the first preamp circuit for the normal channel:
The parts values are
RL = 220k
RV = 1M
CG = 0.047uF
RK = 1.5k
CK = 25uF
The bright channel adds a bypass capacitor to the volume control, which we will discuss in a moment. At DC all the capacitors are open circuits, so the bright and normal channels have identical DC operating points. Under DC conditions the cathode resistor RK carries the current of two triodes, so its effective resistance per triode is 3k, double its nominal value of 1.5k. For a plate supply voltage of VPP = 275 and a plate load resistor of RL = 220k the load line (red) and grid lines (blue) for RK = 3k are plotted here:
The lines intersect at a DC grid bias voltage of minus 1.8 volts. The DC operating point for the bright and normal triodes is thus defined by a quiescent grid voltage, plate voltage, and plate current of
VGQ = -1.8 volts
VPQ = 190 volts
IPQ = 0.6 milliamps
When two triode amplifiers share the same cathode resistor we want to ensure that RK is fully bypassed by CK. Otherwise the two triodes become a common-cathode differential amplifier instead of independent amplifiers. The 25uF capacitor used by Vox effectively shorts all audio frequencies to ground, thus completely bypassing the resistor and providing maximum gain. This is demonstrated by our Cathode Bypass Capacitor Calculator. For RG we use 1M, because it refers to the grid resistor of the following stage, representing the load that the preamp is required to drive. In the case of the AC30 preamp this is the volume control RV. The calculator shows that the voltage gain is 74 for for all audio frequencies.
The output impedance of the first stage, which represents its ability to drive a load, depends on the plate load resistor value RL and whether the cathode resistor RK is fully bypassed. Our Preamp Output Impedance Calculator shows that the first stage has an output impedance of 49k. This compares to only 38k for a preamp with a 100k plate resistor.
By using a higher resistor value than in a typical Fender or Marshal, the AC30 has greater voltage gain and a higher output impedance. The calculator shows an unloaded gain of 78, representing the voltage gain that is achieved if the preamp is disconnected from the volume control. It also shows an output impedance of 49k, which causes the voltage gain to sag to 74 when connected to the next stage. This is not much attenuation, because the 1M volume control doesn't demand much current. If the load were an electron-hungry tonestack then the situation would be quite different.
The coupling capacitor CG blocks the high DC voltage at the plate but is designed to be large enough to pass all audio frequencies on to the next stage. If the value is too small then the stage suffers bass attenuation. If it is too large then the amp could succumb to unwanted effects like blocking and motorboating. Our Coupling Capacitor Calculator shows less than 1dB attenuation at 10 Hertz, well below the lowest note on a guitar with standard tuning, which is 82 Hertz. This demonstrates that the capacitor value is substantially more than what is necessary for good bass response.
When the volume control is at maximum the 100pF bypass capacitor CBP is shorted, so the bright channel has the same response as the normal channel.
At lower volume settings the control becomes a voltage divider that attenuates its input. The bypass capacitor shorts the top of the control at high frequencies and thus provides treble boost (or perhaps more accurately stated, less attenuation for treble frequencies). Here is a screen shot of our Bright Boost Capacitor Calculator when the volume control is set at 50 percent.
At low frequencies the signal is attenuated by 6.4dB. (The extra 0.4dB attenuation is caused by the output impedance of the driving stage.) Treble boost begins to get significant above 1kHz. Many amplifiers use 220pF or 500pF in this position, which allows more high frequencies to bleed through. The Marshall Model 1987 uses 0.005uF.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
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