Guitar Amplifier Electronics: Basic Theory

Guitar Amplifier Electronics: Basic Theory

Richard Kuehnel

Our new book uses these calculators to design preamp stages, cathode followers, tone stacks, power amps, phase inverters, negative feedback, and power supplies.

calculatorLaunch me!

guitar amp cathode bypass capacitor circuit

What does this calculator do?

The cathode resistor in a typical triode preamp is bypassed with a large capacitor to eliminate a form of negative feedback known as cathode degeneration. This substantially increases gain.

When the capacitor is large enough, it acts as a short circuit for audio frequencies, eliminating the negative feedback, but as an open circuit for DC, thereby maintaining DC grid bias. Treble boost can be introduced by using a lower capacitor value, one that acts as a short circuit for high frequencies but allows negative feedback to attenuate bass. This technique is often used for the preamp's bright channel.

The calculator plots gain versus frequency based on the characteristics of the tube, resistor values, and the capacitor value. It does not account for coupling capacitor bass attenuation.

How Does It Work?

The calculator implements a formula provided by F. Langford-Smith, editor, Radiotron Designer's Handbook, 4th edition, (Harrison: RCA, 1953), page 484. The formula calculates the frequency response based on the triode's amplification factor and plate resistance. The following tube parameters are assumed:

  μ rp
12AT7 60 10.9kΩ
12AU7 17 7.7kΩ
12AX7 100 62.5kΩ
12AY7 44 25kΩ
5751 70 58kΩ
6SL7 70 44kΩ
6SN7 20 7.7kΩ