Any Theories on 6U8A Hum?

Discussion in 'Tube Audio' started by OBMG74, Feb 18, 2018.

  1. 6DZ7

    6DZ7 Super Member

    It must be amazing and impossible then to have a @100% quiet (common inobtrusive PS ripple/ear to the speaker hum excluded) amp with AC heaters lifted by only 7v of DC from output tube cathodes with minimal bypassing.
    Last edited: Apr 16, 2018


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  2. Pio1980

    Pio1980 AK Member Subscriber

    Eastern Bamastan
    silvertone-sears 528.69160.jpg
  3. Retrovert

    Retrovert AK Subscriber Subscriber

    New York City, NY
    Instruments will reveal the modulation.

    Besides, the issue is not merely hum, but degradation of the insulation leading to failure.
  4. 6DZ7

    6DZ7 Super Member

    It would take a significant amount of signal not bypassed. The cathode of a driver or PI would never see a tiny residual signal on its heater. The "foreign signal," if any in this context, doesn't create a problem.

    1. As signal, it would join the signal already there. It's not "noise." If that signal got to the cathode it would be the same as another source of feedback, in phase and raise the gain, or opposite and reduce the gain. It wouldn't present as noise.

    2. If it was significant, coming from a common cathode point, it would also appear on the cathodes of the alternate channel tubes and become crosstalk on the cathode of the other output tube cathodes "modulating" them, also. This is why bypassing the common cathode system is necessary. Check for crosstalk by inputing two frequencies, one into each channel, then look for them as a complex frequency on the output. You won't find any worth mentioning in amps working well.
  5. gadget73

    gadget73 junk junkie Subscriber

    Southern NJ
    I suppose there has to be something nice to say about it :)

    I really need to do this on my amp with the LTP inverter. It runs some 90 volts on the cathode, and the heater string runs right to ground. 12AX7's, maybe not as hum-prone but at the same time why abuse them? No room for a filament trafo so I'll probably just elevate it 30-40 volts and call it better than it was.

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