Audio Noise Tube Tester

I am interested in testing some of my audio tubes, and Ive put this circuit together in order to test for hum, white noise, and microphonics and connect it to a simple amplifier, I have a few features incorporated such as a HV switch so that I can change tubes out with out having to power the down totally, and a switch for the heaters to switch from parallel and series connections. Ill be using about 220VDC for Testing,as well as Direct Current on the Heaters. One of the mains questions I have would be, do I need a simple opamp to buffer the output? Have a pic below Thanks.
 

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First -- there was an Audio Engineering Society article by one of the forum members who discussed this topic -- not at the tip of my fingertips at the moment.

Second -- for tubes, the instrument you are using to measure noise must have high impedance

Third -- you've got to pick an operating point which is consistent among tube types.

Fourth -- to specify noise you have to bandpass filter and convert back to nano-Volts per root Hertz.
 
First -- there was an Audio Engineering Society article by one of the forum members who discussed this topic -- not at the tip of my fingertips at the moment.

Second -- for tubes, the instrument you are using to measure noise must have high impedance

Third -- you've got to pick an operating point which is consistent among tube types.

Fourth -- to specify noise you have to bandpass filter and convert back to nano-Volts per root Hertz.

I'm just going to be using sound tests through an audio amplifier, I saw a YouTube video of a gentleman using just a simple box connected to an amplifier the only input was a HV and LV for heaters.
 
Perhaps it would be usefull to be able to measure the cathode heater insulation.

The cathode heater insulation resistance (not capacitance) has been measured using the test circuit in link.
http://dalmura.com.au/projects/Heater-cathode%20conduction%20plots.pdf

Measuring that resistance when the tube is also connected in to a common-cathode amplifier circuit may be achievable, but the measurements may best be done separately.

Although I haven't done it, powering the heater from an AC source would also cause capacitive current to flow, from which the heater-cathode capacitance could be determined and assessed across samples for variation.

Note that the total bulk heater to cathode resistance and capacitance do not cause hum current to flow, but rather that hum current flows in the outer arms of the heater relative to the heater mid-point (which is at humdinger mid-point), and driven by the voltage differential as the heater AC waveform completes each cycle.
 
Perhaps i'm thinking too simple but, if you would wire a neon bulb from the high voltage to the cathode. This neon would light up if the insulation resistance is insuficcient.

Here I tossed together a circuit i thought might give you some ideas.

Adjustable CCS as a anode load. Adjustable cathode resistor. And a switch that cuts out B+ and connects a neon through the cathode.

whilst you're at it you might even include some cheap chinese digital panel meters to measure the static anode voltage and cathode current.
 

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