Few areas of hi-fi are as apt for tweaking as valve amplifiers. At the very least, changing valves is easy since they almost invariably plug in and out. Alternative and/or upgrade valve types are widely available for the commonest varieties. Valves wear out, too, making replacement a maintenance job (though for most signal valves a life of several years should certainly be expected).
But how can one check the characteristics of a valve? Not only is it good to be able to check basic performance, but in many circuits it is highly desirable for both sections of a double triode (and double triodes are by far the commonest varieties in audio use) to be well matched. Enter the Moth Tube Imp.
It brings the power of modern integrated circuits to the business of valve testing and with remarkably little fuss allows you to test all the main parameters of normal, double triodes – gain, transconductance and current at various settings.
Ideally, you still need to know something about the valve type you’re testing and the circuit it goes in, but the supplied instructions make it easy enough to do a basic test. And there is no denying the benefits of putting a really good valve in any decent circuit – benefits that can readily be both heard and measured.
We tried a couple of phono amps, one line preamp and a power amp, all of which used variations of the common ECC81/2/3 types in different configurations and, in each case, were easily able to correlate overall performance with conformance of the valve to the original data sheet.
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