The Preamp

The Champ 5E1 preamp uses a 12AX7 dual triode, the first with a fully bypassed cathode and the second without a cathode bypass capacitor.

The parts values are

C_G = 0.02uF

R_G = 1M

R_GS = 34k (two 68k resistors in parallel)

R_V = 1M

R_K = 1.5k

C_K = 25uF

Since the power amp cathode resistor is fully bypassed, the power amp needs only a 19-volt signal amplitude to be driven to full power, minus the effects of feedback from the output transformer secondary. From our Cathode Bypass Capacitor Calculator the first and second stage voltage gains are 59 and 24, respectively. (To create the conditions for the second stage unbypassed cathode resistor we simply enter an extremely low value for the bypass capacitor, specifically 100pF.) The total preamp gain is

G = (59)(24) = 1416

So the input sensitivity, which is the signal amplitude at the input jack needed to drive the power amp to full power, is

V = 19 / 1416 = 13 millivolts

This is quite respectable by modern guitar amp standards, because standard pickups can easily produce 13 millivolts and overdrive the power amp without external boost. (This is why modern-day Champ clones usually omit the negative feedback used in the original 5E1).

The two stages have identical plate supply voltages, plate load resistors, and cathode resistors. So the DC operating points are the same. The load line (red) and grid lines (blue) are plotted here:

The lines intersect at a DC grid bias voltage of minus 1.4 volts, which is close to Fender's measurement of minus 1.5 volts. The DC operating point is thus defined by a quiescent grid voltage, plate voltage, and plate current of

V_GQ = -1.4 volts

V_PQ = 170 volts

I_PQ = 0.9 milliamps

For the high gain input the grid stopper resistor consists of two 68k resistors in parallel, for an effect resistance of 34k. From our Grid Stopper Resistor Calculator we see that for a low-impedance guitar pickup there is negligible attenuation at 16kHz but over 9dB attenuation at 128kHz. This is exactly what the gridstopper is designed to do: filter radio while leaving audio frequencies intact.

¹Richard Kuehnel, Vacuum-Tube Circuit Design: Guitar Amplifier Preamps, 2nd Ed., (Seattle: Pentode Press, 2009).