I am trying to troubleshoot my 12b4 Test bed and measurement system. I am finding a hum/noise problem that I can't track down. The test bed is currently a 12b4 with DC filament at 6.3V. Ripple is <1mV. I am running 300V from my Kepco 400B bench supply into a Simplistic HV shunt reg dropping to 280V B+. The shunt is passing 62mA. The Gain stage is a Waveborn Gyrator with the anode voltage set to 140V. Bias Current is 18.1mA. With no signal applied I get the FFT seen in the first attachment. Switching to scope mode the harmonic is clearly seen. The vertical scale is 200uV/div and the timescale is 10mS/div. I couldn't see this before with my Tek2430A scope as it is below the resolution of it.
What do you think would cause this kind of waveform?
FYI The soundcard is a EMU 1616m and the software is audio tester. The baseline curve with the input cable disconnected from the testbed is seen in the third attachment.
I am thinking that it is an artifact from the incoming power I have here. On a scope my AC line looks like a square wave.
What do you think would cause this kind of waveform?
FYI The soundcard is a EMU 1616m and the software is audio tester. The baseline curve with the input cable disconnected from the testbed is seen in the third attachment.
I am thinking that it is an artifact from the incoming power I have here. On a scope my AC line looks like a square wave.