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Essential Test Equipment, The Valve Tester

Old valves still showing red ink

Essential test equipment, the valve tester is a must have bit of kit when you service / repair as many valve amps as I do! Not everything is what it seems and a really good valve tester ensures that the valves in my customers’ amps are good and healthy. A valve tester quickly highlights any issues and does so with the valves removed from the amp.

A regular customer recently dropped off a Mesa Boogie Mark 2 C Plus Combo. There wasn't anything specifically wrong with the amp but with seven years since its last service, Keith just wanted me to give it the once over.

Mesa Boogie Mark 2 C Plus Combo
Keith's gorgeous Mesa Boogie Mark 2 C Plus combo in for a regular service.

While we chatted, I explained that the legend on JJ Tesla valves should be a bright red, thus indicating that the devices are healthy. When the writing turns white, then you really need to change the valves. This characteristic is not common to all manufacturers' valves. The legend on the valves in this amp were indeed a nice shade of red.

Well, regular customers will know that I don’t take anything for granted and I tested the valves anyway.

Different valves have different specifications (of course) and the 6L6s inside this amp, should have a plate current somewhere between 12 mA and 40 mA. One of the valves is slightly out. In fact, after about half-an-hour, the reading fell to 10.1 mA.

Essential test equipment, the valve tester shows that the plate current on one of the 6L6s is less than the recommended 12 mA
The valve tester clearly indicated that one of the valves had a plate current slightly under spec'.

Another important valve parameter is transconductance and for 6L6s this should be than 2.0 mMhos. As you can see, my digital valve tester is showing that the suspect valve falls short. Just like the plate current reading above, after about half-an-hour, the transconductance fell to 1.7 mMhos.

The transconductance for 6L6s should be greater than 2.0mMHos
Indeed, the transconductance for the suspect valve was out of range, too.

Essential test equipment, the valve tester in this example isn't cheap. My Maxi Matcher 2 cost about 900 USD several years ago and took a couple of months to be delivered. On the other hand, customers like Keith; discerning musicians who look after their gear and get it serviced regularly, make it all worthwhile. Remember, there wasn't anything particularly wrong with Keith's amp. He just wanted a service. 🙂

On this occasion, Keith's lovely little Mesa Boogie combo (which weighs a ton, incidentally), gets a late Christmas present; a brand new matched quad of burnt-in, premium Harma 6L6s. 🙂

Matched quad Harma 6L6s in Mesa Boogie Mk2 C Plus Combo
A happy amp and don't those valves look lovely!