Negative Feedback

The Fender Champ 5E1 output transformer secondary supplies feedback to the second-stage preamp via a feedback resistor R_F.

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 R_G, 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 R_F and the 1.5k cathode resistor R_K in parallel with the 12AX7's cathode impedance attenuates the feedback signal by an appreciable amount. The cathode impedance is

(R_L + r_p) / (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. This reduces the voltage gain for the two stages from 360 without feedback to a value of

360 / [1 + (0.0008)(360)] = 280

with feedback. This represents only 2dB 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 17 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.

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).