For many owners of valve gear there's always an element of DIY when valves need changing. It's inevitable. But how do you know they need changing?

Power valves sometimes make it obvious by glowing bright and then melting, but small-signal types just fade gently away and only provoke suspicion in the owner, impossible to confirm or deny without either specialist test gear or tedious hours of valve swapping. Neither of which is much fun.

Tube Imp to the rescue! This simple little device - a modern-day successor in some ways to the venerable AVO valve tester of old - is capable of testing the vast majority of commonly used small-signal valves, giving direct numerical readouts on the display of gain, transconductance and cathode current. Unless you're a practised designer of tube gear those specs won't actually mean much to you, of course, but it doesn't really matter.

Thanks to the well-written instructions, and with a little help from the valve data (available from various websites), it's possible to check whether a particular valve is up to scratch or not. It's also trivial to switch from one section of the valve to the other (since the Imp tests exclusively double triodes) and check for side-to-side balance.

This may all seem a little nerdy and there's still much to be said for having valve kit professionally serviced every few years, but if you have ever had the slightest urge to tweak a valve amp you'll find this quite invaluable. Richard Black