Dummy load box for guitar amp


2008-07-18 1:09 pm

We would like to build a dummy load box for our guitar amps. We have a 25W, 35W, and a 85W guitar amp.

The dummy load would have a switch to select 4R, 8R and 16R.

It will also have speaker output and volume control to play through the speaker cabs.

The purpose of this load box is to allow us to (over)drive the output stages of the guitar amps, and still listen at comfortable volumes. We have run some tests and decided that 5W is the absolute maximum for playing back and/or mic-ing in a bedroom or living room.

Our dummy load will not be fancy, just a simple load resistor, so the output stages and transformer of the guitar amp will not be driven realistically. But we are happy with this choice because we will be able to hear some of the sound shaping of the output valves / transformer at various levels of (over)drive.

As I sat down to design the circuit I have hit upon a number of issues. I cannot think of a way to make it totally passive, in other words to be able to divert up to 5W max to the speakers and with a volume control.

I decided I would need to have a small, 5-10W audio amp inside the load box, setup to play up to 5W under all circumstances, never to be overdriven or clip. This amp would be driven by a small transformer tapping the dummy load resistor.

As there are so many possible configurations (selected dummy load and amp power), I would need different windings ratios so that I do not accidentally overdrive the active 5W amp.

This is where I am stuck at the moment. Am I on the right track ?