Jim Marshall crafted guitar amplifier masterpieces without the assistance of computers, software or the Internet... In 1963 "graphical methods" meant drawing Bode plots with a pencil and ruler... Times have changed. ● Master the basics ● Design your amp ● Know it works Amp Design for the 21st Century

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What does this calculator do?

guitar amp cathode bypass capacitor circuit

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
6386 17 4.25kΩ
6922 33 2.64kΩ
6SN7 20 7.7kΩ