With the big KT88 amplifier nearing one year old, it occurred to me that I had never performed a frequency response measurement on the whole amplifier.... I'd done plenty of them when I was prototyping the preamp and driver stages, but I hadn't done anything other than cursory checks with the oscilloscope over the entire amplifier.
Today I set about rectifying that omission. My method was simple. Move the amplifier to my (newly tidied) workbench, attach the function generator to the input, dummy load to the output, set up a sine sweep from 50Hz to 50hKz at 200mV RMS, open the taps until the amp is producing about 5v RMS on the output, then start the FFT process.
This resulted in the following:
So the yellow trace reveals that the input signal is not exactly flat... about 1dB down by 30kHz. This is why I showed the input signal, for a reference.
The blue plot is the speaker terminals. the load is an 8 Ohm carbon resistor on a heatsink.
So what does this tell us? I read this as the amplifier being reasonably flat (within 1dB) to 40kHz whereafter it drops rather quickly.
My 'scope does have a "Math" facility where I can supposedly reference one signal against the other. This functionality seems buggy however, which is why I avoided it.
I am happy with this frequency response.