Special High/Low Inputs

This post is about the high low input setup in my Nervosa amp.

I was building a tweed bassman type amp but wanted to have an option for adding an extra triode infront of the normal preamp. However, I wanted to do it with an extra input jack, not a switch. I looked at the setup in a JCM800 which basically does that but I didn’t like how the low input went right to a volume pot and then to the cold biased 10k stage. I wanted a way to alter the circuit based on something being plugged in (or not plugged in) to the input jacks.

After looking around a bit the switchcraft 14B seemed to fit the bill. The 14B an extra switch on it that I could use to connect or disconnect something from ground. If nothing is plugged into it this switch is closed to ground. So, using this jack in a JCM800 type amp, it’s possible to change the cathode resistor on one of the triodes. In this case, the cold clipper.

This is the schematic (click for bigger):

The way it works is if you plug into the normal input and nothing is plugged into the gain input, which uses the switchcraft 14B jack, the 1.8k/22uF is connected to ground, parallel with the 10k cathode resistor, on the “normal” input triode. So the normal input is now 1.5k/22uF, basically like a JTM45. You could instead put a .68uF and a 3.6k resistor in parallel with the 10k to get 2.7k/.68uF.

If the guitar is plugged into the gain input the 1.8k/22uF is disconnected from ground and the normal input triode is now biased with just the 10k cathode resistor. The gain triode then cascades into the normal triode through a switched jack on the normal input. This produces something more like the JCM800 topology.