Fender Champ - comparing the 5F1 and 5E1 circuits

In modding their Champion 600's, some people are trying to get closer to earlier versions of the Fender Champ circuit, the 5E1 and 5F1. This post is to clarify the differences between those two early circuits. There are two significant ones.

The first is the missing cathode bypass capacitor in the 5F1. Eliminating this cap reduces the first stage gain. Reducing the value instead of eliminating it completely will give a bit of a treble boost (see my earlier post "Champion 600 cathode bypass mod" for a chart showing how different capacitor values will effect the response).


Secondly, in the 5F1 champ the choke has been eliminated and a screen supply node consisting of a capacitor and resistor has been added. This saved Fender some cash, as a choke is a good bit more expensive than a cap and a resistor. In a Class A amp a choke doesn't contribute to sag they way it does in a Class AB amp, but the change still has an effect. The screen supply resistor in the 5F1 drops the voltage for the screen below that of the plate. Since the screen voltage actually has more effect on plate current than plate voltage, this changes the character of the output section. If you really want to hear if the choke makes a difference between the two circuits you need to put the choke in series with the screen supply resistor, not eliminate the resistor completely.