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While restoring a Sobell 515P I re-valved the set but the sound output was very distorted at anything above half the rotation of the volume control. Everything looked right, bias and other voltages all spot on so could it be the EL84? I had done a precautionary check of all the valves as received on the Sussex and all looked fine. So I tried the 50 odd year old blackened EL84 in the set and presto! Magic sound. So what was wrong? Why did the Sussex tell me all was right with the new valve when clearly it was not.
Now it was always regarded that a valve tester cannot reproduce the same conditions as the actual circuit the valve sits in, but it was annoying to think that the Sussex had given this valve an erroneous Green light.
One thing you can do with the Sussex is record the Ia at different settings of the Grid Voltage control, then the results can be put into a spreadsheet like Excel, plotted and compared with the manufacturers factory curves.
I have done this countless of times when matching up valves so I decided to try this with the suspect EL84. As you can see by the attachment, the valve sits at zero Anode current for quite some time (Red graph) before eventually getting started which is contrary to the makers curve (shown in Green). The valve then runs out of steam as more current is demanded, as if the cathode itself has insufficient coating to provide the electrons required. In contrast, the other attachment shows the chart from the blackened and worn out Mullard. It still just passes the AVO spec figures being the equivalent of 'just in the Green' but it does start conducting at the low end compared to the new valve. The Red line strays away from the Green due to the poor thing trying to give emission that it clearly cannot do, but it does a good job of keeping with the curve shape.
So after all of this, I am now inclined to do a much fuller check on the valves I test, happy in the knowledge that if I used a 'quick test' instrument I would still be wondering what is wrong. The Sussex has given me a greater insight into how some of my valves actually perform and is a tip I am happy to share with other owners.