Wattage Measurements

[Leftydave]

I’ve used a scope and a dummy load on many occasions to determine if an amp is working properly and is putting out its rated power. I’ve made a spreadsheet that converts the AC voltage at the load box to watts, based on whether the box is set for 4 or 8 ohms.

I’m wondering if I’m doing this correctly. Should my calculations be based on the dummy load’s resistance or the resistance rating of the output transformer? Using a blackface Bandmaster with a 4 ohm output transformer as an example, and connected to an 8 ohm dummy load, should I base my calculations on 4 ohms or 8? And does the mismatch effect the amps ability to put out full power?

[loudthud]

Use the value of the load that is connected. There are some speaker cabinets that are rated at 6 ohms (they're actually 5 2/3 ohm). If you want to know how your amp will perform, connect a 6 ohm load, measure the voltage and use the E squared over R formula to know how much power your amp will produce with that load.

Tube amps will typically deliver more voltage into higher impedance loads. Solid state amps, not so much.