The Fetzer Valve

Posted on Feb 7, 2014

Tthe circuit has enjoyed popularity as a standalone booster and as a building block in larger circuits. 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. This circuit was developed by applying the well known Triode-to-JFET translation to the first stage found in a typical Fender

The Fetzer Valve
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tube amp, as illustrated in Fig. 2. The triode is replaced by a JFET with the following correspondence of terminals: plate to drain, grid to gate, and cathode to source. 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 plate resistor is changed for a variable resistor (usually a 10k to 100k trimpot) which is used to adjust the JFET`s biasing to a value close to half the supply voltage. High and Low gain inputs are often noted on the faceplate as 1 and 2, respectively. Fig. 3 shows the typical input circuit of a Fender tube amp. 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. When a plug is inserted, the contact with the arrow terminal is opened, thus interrupting this previous connection. As shown, the Low Gain input is in fact a resistive divider that introduces 6 dB attenuation and presents a relatively low impedance to the guitar, which is 68k + 68k = 134k, hence its "Low Gain" designation. When a low impedance source is connected to the amplifier, the grid of the valve "sees" an equivalent resistance of 68k | 68k = 34k. The High Gain input introduces no attenuation and presents a 1 Mohm impedance to the instrument. Again, a 34k equivalent resistance...

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