How can I test whether my preamp tubes have gone dead?

My wife has an Ampeg SVT4Pro bass head that has (3) 12AX7 tubes. From time to time we have replaced them after consulting the manufacturer about a loss of output volume. But as they all apparently do different functions in the preamp ckt I was wondering if there was a way I could tell if not all had worn out.

Dr.Bill8 years ago
I find that if the silver stuff at the top of the tube is black its lost its vacuum and also a continuity test of the filament is a good thing to do too.
appolo8 years ago
The 12AX7 tube is also known in the UK as an ECC83 double triode. The simplest answer is to keep a known good replacement and substitute each one in turn. These tubes or valves as we call them in the UK are generally very reliable and rarely fail. I just wonder if you may have other circuit problems! you may have an intermittent tube base holder or poor solder connection, often happens to mobile equipment. If you have this problem again gently tap around the chassis and tube bases. REMEMBER YOU ARE EXPOSED TO HAZARDOUS VOLTAGES IF YOU PROBE INSIDE!! There's no other way to check the tubes out unless you are knowledgeable in electronics and able to construct a tester.