The Top Boost Preamp

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 R_G and the grid-stopper resistor R_GS.

The resistance R_o 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

R_GS = 34k

R_G = 1M

(R_GS 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 this screen shot from our Grid Stopper Resistor Calculator, where we've used in advance the voltage gain of 74 that we will determine shortly.

The attenuation is substantial above 100kHz but the gain is down by less than 1dB 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

R_L = 220k

R_V = 1M

C_G = 0.047uF

R_K = 1.5k

C_K = 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 R_K 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 V_PP = 275 and a plate load resistor of R_L = 220k the load line (red) and grid lines (blue) for R_K = 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

V_GQ = -1.8 volts

V_PQ = 190 volts

I_PQ = 0.6 milliamps

When two triode amplifiers share the same cathode resistor we want to ensure that R_K is fully bypassed by C_K. 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 a screen shot from our Cathode Bypass Capacitor Calculator.

The calculator value R_G refers to the grid resistor of the following stage, because it represents the load that the preamp is required to drive. In the case of the AC30 preamp this is the volume control R_V. 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 R_L and whether the cathode resistor R_K is fully bypassed. Here is a screen shot of our Preamp Output Impedance Calculator which 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. Notice that the unloaded gain is 78, representing the voltage gain that is achieved if the preamp is disconnected from the volume control. The preamp's 49k output impedance 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 C_G 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. Here is a screen shot of our Coupling Capacitor Calculator showing less than 1dB attenuation at 10 Hertz, well below the lowest note on a guitar with standard tuning, which is 82 Hertz. This shows 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 C_BP 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).