Principles
A tube tester is a measuring instrument designed to test vacuum electronic tubes. These instruments can be divided into two groups:
Basic models that allow classifying a tube as good/bad with a color scale from 0 to 100%.
An apparatus of this type is presented on the page « Eurelec tube tester ».
Advanced models that allow testing the characteristics set by the tube manufacturer statically and dynamically under specified conditions.
Basic models
- Testing filament continuity.
- Detection of short circuits between filaments, cathode, grids, and/or anode.
- Measurement of cathode emission capability. Here, pumped tubes are identified. Often the tube is tested as a diode, and it is the average rectified current that allows determining its emission capability. The tube tester manufacturer provides result tables that they have established experimentally.
Advanced models
On these more advanced models, the operator sets the static conditions specified by the tube manufacturer.
They set the nominal voltage values for grid G1, grid G2 (and more), anode, then verify that the anode current is within 20% of the announced value.
fig.1 triode
The dynamic characteristics of a tube are evaluated using 3 parameters:
transconductance or slope (gm), amplification µ, and plate resistance Rp.
The transconductance or slope gives the plate current variation ΔIp caused by a grid voltage variation ΔVG1 for a constant plate voltage. The plate resistance Rp is a dynamic internal resistance given by the relation below. The amplification µ is evaluated at constant plate current. On variable-mu tubes (certain pentodes for example), this amplification coefficient varies greatly with the bias voltage VG1. This property is happily used in automatic gain control of vintage radios by making the sound volume independent of the HF carrier intensity.
transconductance or slope (gm), amplification µ, and plate resistance Rp.
The transconductance or slope gives the plate current variation ΔIp caused by a grid voltage variation ΔVG1 for a constant plate voltage. The plate resistance Rp is a dynamic internal resistance given by the relation below. The amplification µ is evaluated at constant plate current. On variable-mu tubes (certain pentodes for example), this amplification coefficient varies greatly with the bias voltage VG1. This property is happily used in automatic gain control of vintage radios by making the sound volume independent of the HF carrier intensity.
- slope or transconductance is expressed in mA/V or micromhos or µSiemens
- the amplification factor µ = is dimensionless
