The Fender Champ 5E1 output transformer secondary supplies feedback to the second-stage preamp via a feedback resistor RF.
We determined in our study of the 5E1 preamp that second stage voltage gain, including the load of the power amp's 220k grid resistor RG, is 24. For the Champ's power amp, a 19 volt rise from idle at the 6V6 grid causes a 280 volt drop in plate voltage, for an effective voltage gain of 15.
Total voltage gain from the grid of the second preamp to the output transformer primary is thus (24)(15) = 360.
The voltage gain across the 7k transformer equals the ratio of the number of windings in the secondary to the number of windings in the primary. This is equal to the square root of the impedance ratio. For the Champ's 4-ohm output it is
G4 = sqrt(4 / 7000) = 0.024
The voltage divider formed by the 22k feedback resistor RF and the 1.5k cathode resistor RK in parallel with the 12AX7's cathode impedance attenuates the feedback signal by an appreciable amount. The cathode impedance is
(RL + rp) / (mu + 1) = 1.6k
Putting this in parallel with the 1.5k cathode resistor gives us an effective resistance of 770 ohms. The gain from the output transformer secondary to the cathode of the preamp tube is thus 770 / (22k + 770) = 0.034. The portion of the output transformer primary voltage fed back to the second stage preamp input is thus (0.024)(0.034) = 0.0008. Increasing the 12AX7's cathode voltage by 1 volt increases its plate voltage by a factor of 52, so the loop gain is (52)(15)(0.024)(0.034) = 0.64. This means that the voltage gain for the two stages decreases from 360 without feedback to a value of
360 / [1 + 0.64] = 220
with feedback. This represents only 4dB of attenuation. Unlike their high-fidelity cousins, guitar amplifiers generally use only modest amounts of negative feedback.
The Champ's input sensitivity, which is the signal amplitude at the amplifier's input jack needed to drive the power amp to full power, rises from 13 millivolts to 21 millivolts. Even by modern standards this represents respectable gain and the amp can still be easily overdriven with an off-the-shelf Stratocaster. The feedback reduces the power amp's nonlinear distortion somewhat and flattens the frequency response. It also lowers the power amp's output impedance, making it easier to handle the frequency dependent nature of loudspeaker impedance.
Overdriving the power amp causes output clipping and clamping, which represent a lack of output response to a changing input signal voltage. The output provides the source voltage for negative feedback, so these effects reduce it. This creates more closed-loop gain, which drives the amp further into an overdriven state, producing even more clipping. The result is that as the amplifier is overdriven, negative feedback from the output transformer accelerates the transition to a more distorted state.
1Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).
2Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Power Amps, (Seattle: Pentode Press, 2008).
3F. Langford-Smith, Radiotron Designer's Handbook, 4th Ed., (Harrison: RCA, 1953).
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