'The Music Room'

The ShermanAudio - Educational Series


ShermanAudio created these slides for a person who asked some questions about  how to go about measuring vacuum tubes and interpreting the results.

As I haven't come across a textbook that presents, in clear graphical form these methods, these slides I made in PPT will show how a particular Vacuum Tube is behaving and how the voltages are settling.

It is always best to have a Tube Manual handy to relate the measurements to published specifications. By taking some static AC and DC Voltage measurements on gains an insight into how the tube is operating.

Of course current measurements are possible but they almost always require a disruptive circuit intrusion, de-soldering, etc. and not recommended for the average person. One can choose to install permanent "Tip-Jacks" with an SPST switch soldered across to allow for simple current measurements.

But due to the intrusive nature of current measurement, I would stay away from drilling perfectly good production amplifier. Unless you have your own DIY home made amp where modifications are typically seen as enhancements instead of a permanent value affecting damage to the equipment.


Note the common mistake made when measuring Plate Voltage. Plate Voltage is the "potential difference" measured between

the Plate and Cathode connections, not between Plate and Ground, of course unless the tube uses Fixed Bias and the Cathode

is connected directly to DC Ground. I make this clear in my explanation above.


The measurement above applies to Cathode Bias and Cathode Current sensing resistors.

The Cathode current is calculated from the DC Voltage measurement and the value of the cathode resistor.

It's always a good idea to remove the power tube and measure the resistance of the Cathode resistor to

verify that it has an acceptable value according to the tolerance of the resistor.


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