Tube stuff

Here's a nice diagram and cool explanation of signal flow through a common tube circuit.

http://www.el34world.com/charts/currentflow.htm

Video on biasing a Sound City L-B 50. By connecting one end of your multimeter to the centre tap of the output transformer and the other to pin 3 of the output tube. Read the current here. Should be around 42mA.

http://www.youtube.com/watch?v=PYuFsZhFIe8

Great site and forum for vintage gear

http://vintageamps.com/

Simple circuit to switch between tube rectifier and solid state rectifier in a tube amp.

Tubes 101

A tube amp works by letting 2 different types of voltage pass in 2 different directions, while not letting the 2 pass the same way together. The guitar signal which is a low AC voltage goes from the input jack toward the speaker output (getting bigger along the way) while the high DC voltage that powers the tubes runs back in the reverse direction often along the same connection points that serve as stage inputs for the AC signal.

Signal runs straight to pin 2 of the input tube (12AX7).
Pin 2 is known as  the Grid.
Pin 1 is the output which is also known as the Plate.
The flow of electrons from grid to plate increases gain.
Pin 3 is the cathode. This determines the operating level.