[TA Forbes]

There is a lot of discussion about the real value of the "dynamic conductance" testers like the Eico 666 and 667, both pro and con.

I like 'em. They are easy to work on and calibrate, and give in my opinion a valid and reliable result. They are simple to operate and the "shorts" test is always done before the tube test is done. After reading the manual I was convinced that the combination of plate emission and mutual conductance (Dynamic Conductance) was a valid way to test tubes. HOWEVER, Alan Douglas in his book did some very interesting tests comparing some Dynamic Conductance to other testers and his results indicated otherwise. (read -unreliable)

The only bad I have to say is that they: 1. Have no gas, noise, or "life" test; 2. Tend to "struggle" a bit when testing high power tubes; 3. The roll charts are notorious for having errors.