This puts a total of 100kΩ + 94kΩ = 194kΩ plus the bias resistor in series with the tube. The equivalent circuit for one triode doubles the resistor values. The resistors connecting the cathodes to ground carry the current of two triodes, soįor the purposes of determining the DC operating point, In fact, there is no negative feedback in the Vox AC30!.The tail resistor is not split to provide an insertion point for negative feedback.The non-inverting input is connected to AC ground.The phase inverter is a long tailed pair in its purest form: The crossover point for the other three traces is at 780Hz. Maximum scoop for the orange trace is at 480Hz. When the normal channel volume control is set to maximum. Load is 544kΩ, which is approximately the case This plot of gain in dB versus frequency in hertzĪssumes the source impedance is 614Ω and the output The tone stack has an architecture that is similar to the Fender Bassman, know your design works by measuring performance at every point in the amplifier. Guitar Amplifier Electronics: Circuit Simulation This is very low, making it ideal for driving a tone stack that demands a lot of current. In an environment of resistances measured in kiloohms, Indicates that the stage has a gain of 30dB and an output impedance of 68kΩ.Īccording to the Cathode Follower calculator, The Preamp Gain and Output Impedance calculator The latter is not an issue, because the voltage amplifierĭrives a light load that does not demand a lot of current. The context of system input sensitivity and the stage's position in The former is a design choice to create the amount of gain desired within This reduces gain and increases output impedance. The cathode resistor for the second stage voltage amplifier is not bypassed by a capacitor,Ĭreating negative feedback from cathode degeneration. design your amp using a structured, professional methodology. Gain is down by 2dB at midrange and almost 18dB at 82Hz.įundamentals of Guitar Amplifier System Design The bright channel also uses a much smaller coupling capacitor for a huge amount of bass cut. The Bright Boost Capacitor Calculator shows the effect of the 100pF capacitor when the volume control is atĥ0-percent rotation (10-percent resistance). The bright channel uses a much smaller coupling capacitorĪnd adds a bright bypass capacitor across the top of the volume control. master the basics of preamp, power amp, and power supply design. Guitar Amplifier Electronics: Basic Theory Gain at 10Hz, well below audio, is down by more than decibel, which contributes to inter-stage decoupling. attenuation at 82Hz is less than a tenth of a decibel. The large 0.047μF coupling capacitor creates a flat response The load of the 500kΩ volume control attenuates the output by 0.8dB, The unloaded gain and output impedance are 37.8dB and 49kΩ, respectively.Īccording to the Coupling Capacitor calculator, New! Guitar Amplifier Electronics: Fender Deluxe - from TV front to narrow panel to brownface to blackface ReverbĪccording to the Preamp Gain and Output Impedance calculator, This is well beyond the maximum output signal level from a typical pickup. This makes the input headroom 1.4V peak ( -0.1dBV). The DC grid bias is slightly cooler than neutral: 1.4V from cutoff according to the AC load line. The calculator indicates that the DC current load on the power supply is a modest 0.5mA. The AC load for each channel is a 500kΩ volume control. The 12AX7 calculator shows that the DC grid bias is -1.6V. So its effective resistance per triode is 3kΩ, double its actual value.įor a 275V plate supply and a 220kΩ plate load resistor, Under DC conditions the cathode resistor carries the current of two triodes, That share a 1.5kΩ cathode resistor and 25μF bypass capacitor. The first stage preamp contains two voltage amplifiers, one for the bright inputs and one for the normal inputs, There are vintage tone monsters and then there's the tone goddess - the AC30." "Few vintage tube amps have attained the gilded reverence heaped upon the Vox AC30.Ĭertainly, the late 1950s Fender tweeds and Marshall's JTM45 and Plexi amps are equally lust-worthy in the eyes and ears of many players and collectors, but JMI's creation is often discussed with an added catch of the breath, an excited charge in the air.
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