While restoring a Fender twinreverb I needed something to use instead of speakers. The amplifier is capable of delivering 135W into 4 Ohm. But it is no problem to use a load with more resistance . I was in doubt between making a dummy-speaker or a resistive one so I measured the impedance of the speakers and to my suprise that had a much greater real part as I would have expected. The major part is R with a rather small +jX part. Impedance was around 7 Ohm at 1 kHz and over 20 Ohm at 20 kHz.
So a pure resitive one will be great for aligning and testing at 1 kHz but will load the amplifier much more if frequency increases. The amplifier normally sees a 20 Ohm load at 20 kHz but now a lot less. That was easy to see when I used a square wave. The left part, the leading edge, of the square went linear up, just like the speaker impedance. So you would think a dummy speaker would be better. The problem is, you must in that case know the impedance of the speaker.
Soldering the 10W 120 Ohm resistors
Because speakers have an inductive part too I used wirewound resistors. I had 14 NOS 10W 120 Ohm resistors. I mounted them on two pieces pcb and because the resistors had a metal ring I could use that too, to join them together .
Ready for use
While testing it was dissipating 105W and I needed no forced air cooling. I made a pair of connections for the input and a second pair to connect a TRMS voltmeter. The bnc is a 100x attenuated 50 Ohm output to connect to a spectrum analyser (via an external attenuator and dc blocker for safety. This for checking harmonics while balansing.
The dummy is 8.721 Ohm at 1 kHz and 8.59 Ohm for DC.
The patient, a 1979 Fender Twinreverb.
Twin reverb