Try to check, if in "low distortion" amp the 300B filament "+" and "-" in the same pin as in "high distortion" one.
Try to "increase" carrier level in measuring set to -10dB.
If then the 100Hz component at -70db (-60dB relative to carrier) it's enough good.
Try to "increase" carrier level in measuring set to -10dB.
If then the 100Hz component at -70db (-60dB relative to carrier) it's enough good.
The 50 Hz thing could be nothing more than a ground loop formed between your test generator & the amp.
Crank the test gen amplitude down, if 50 Hz thing is still there start looking for a way to break the loop.
People long enough in the biz or hobby often have an audio transformer that can be wired in to provide a test signal path.
And the ground loop is broken,
😀
Crank the test gen amplitude down, if 50 Hz thing is still there start looking for a way to break the loop.
People long enough in the biz or hobby often have an audio transformer that can be wired in to provide a test signal path.
And the ground loop is broken,
😀