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I dug out my SG-8. I had modified it somewhat. I remembered that I had changed the audio connectors to RCAs and changed the RF output to a BNC.
I also remembered tweaking the audio oscillator to 400 Hz, which is a standard test frequency, and adding a tube to improve the modulation. What I didn't remember was how I changed the modulation switch hookup. I think you need to have the modulation switch set to "INT." for the internal audio oscillator to work.
At any rate, you should be able to get up to 20Vpp ( that means 20V peak-to-peak) out of the audio output.
Now a little education, in case you don't understand the subject, 20V peak-to-peak is what you would see with a oscilloscope, 20V between the positive peak to the negative peak. Since a sinewave with no DC on it is symmetrical around zero volts, the peak voltage (Vp) is half that or 10V. What most meters would display is RMS (Root-Mean-Squared) AC volts, the equivalent DC value. That means it produces the same power (and current) in a resistor as that DC value. For a sinewave only, the RMS voltage value is equal to 70.7% of the peak voltage. So for a sinewave with a peak of 10V the RMS value is about 7V.
That is a big signal, way bigger than what the battery powered amplifier you tried is intended to deal with. What you probably are hearing is hum due to a bad connection, maybe the ground. The audio oscillator (6C4) may not be working at all.
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