The key to understanding this circuit lies in the fact that the tip of each input jack is making electrical contact with the arrow-shaped terminal when no plug is inserted. Fig.3 shows the typical input circuit of a Fender tube amp. High and Low gain inputs are often noted on the faceplate as 1 and 2, respectively. OK, now let's go: three, two, one, lift-off!
The high voltage supply voltage is replaced by a low voltage, usually in the 9 to 18V range as used in effect circuits.This is supported by the fact that both the vacuum triode and the JFET are voltage controlled devices which change their transconductance according to a (usually) negative control voltage. The triode is replaced by a JFET with the following correspondence of terminals: plate to drain, grid to gate, and cathode to source.The steps involved can be summarized as follows: This circuit was developed by applying the well known Triode-to-JFET translation to the first stage found in a typical Fender tube amp, as illustrated in Fig.2. The original Fetzer Valve circuit is presented in Fig.1.
While the circuit has its merits, we decided to further explore it to determine if it was a truly faithful solid-state version of the familiar input stage used in many tube amps. In the two years since the Fetzer Valve was first published in the midst of our adaptation of tube amps for use as distortion pedals, the circuit has enjoyed popularity as a standalone booster and as a building block in larger circuits. This project is rated 1 out of 3 for the level of complexity. FET booster based on a vintage Fender 12AX7 input stage
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