Triode Mode Cards on L1 and L3 tester

Testing in Triode mode

Matching in Triode Mode, or in Pentode mode, will give somewhat different results. So a perfect pair in pentode mode, may give the same in triode mode, or just only a "good" pair. Reason for this is, in pentode mode the influence of the screen grid on the parameters another. Meaning, a two tubes which gives identical Gm in pentode mode, may (or may not) test quite different in triode mode.

Triode mode vs pentode mode. Some background.

Some pentodes give exceptionally nice triodes. . Good examples aree given by the best pentodes ever, the C3m and C3g. These have triode curves, which are lot better than almost any other small signal tube. Another interesting example is the Russian GM50 pentode, which is (now in 2019) very low cost, but in triode actually tests like a 300B. Only GM50 is 50 Watt peak, and 40 Watt continuous. Or, also the RS291, a magnificent looking German WWII transmitter pentode, can make a nice triode, with resemblance to 300B also.

In Triode mode, the screen grid is connected to the anode. The suppressor grid (G3) is connected to the cathode, just as with a pentode as well. This eliminates of course the "screening" function of the screen grid. It's name already tells what it is intended for. It keeps the anode away from the grid field. Both in triodes and pentodes, the cathode-grid field is controlling the plate current of course. However, since grid and anode have 180 degrees phase shift electrically, the anode field works against the grid field. This is not just "working against it", but in fact it is added in negative polarity. So effectively, this is the same as feedback, and this is the reason why triodes have: Lower gain, lower output resistance, less input capacitance, and less distortion. This follows the classical laws of feedback theory.

In a pentode, the screen grid reduces the influence of the electrical field of the anode, so it can no penetrate into the grid1-cathode cavity. Since the screen grid is nearer to the grid1-cathode , the screen grid wins, so to say from the anode AC signal. The feedback effect as present in a triode is now gone, and we get the same as with every amplifying element of which we remove the feedback: Gain goes up, output impedance goes up, and distortion and input capacitance go up.