And what does it sound like when it does? One of the fundamental laws of tube operation is that the signal exists within the boundary or limit set by the bias voltage. If the signal exceeds the bias, the grid goes positive, this is class AB2, A2, B2. There is a technical reason that limits Fender type tube amps to operate in class AB1, A1, B1. The goal here is to be able to gauge the correct signal level at the power tubes for any amp, based on the amp’s bias voltage. The second goal is to find out if Fenders go AB2, A2 or not. I did quite a bit of signal testing two years ago using a guitar to make the signal and quite a bit again last year using a signal generator. The tests with the guitars, mostly consisted of measuring the signal at different volumes with tone controls set in the middle. The results were that the signal rose linearly with the volume control setting, and that the max signal consistently ands up being a little bit more than half the bias. The most interesting results with the signal generator is that you can accurately measure both the input and the output anywhere and easily and accurately figure the gain. Another interesting result is that blackface tone controls affect the gain a lot and a simple tone control does not affect the gain at all. The other interesting result was that signal level anywhere in an amp strictly follows these two basic rules. If the path is not a path to ground, then there is no signal loss. If the path is a path to ground, then there is signal loss. Now it’s time to put the two tests together and see if a Fender amp will go class A2 or AB2. I think the answer is yes. Somewhere in the testing I have done, I think it happened and it wasn’t my focus at the time, so I disregarded it. But I think I can easily reproduce it and see if it is true. This is the test. One blackface champ. Measure the signal on the plate at extreme playing conditions. The bias is around 24 volts. I think the signal is going to go higher. Maybe 30. 30 but not a steady 30.