How to Connect a Tube to an Amplifier
Using a tube is an excellent way to learn about how electricity works. You can also build a simple amplifier for yourself given the right parts and basic knowledge. But if you’re new to electrics, it’s a good idea to take a little time to study the amp schematic and understand how things work first.
The first thing you’ll need to know is that vacuum tubes have three components – the anode, cathode and plate. Anodes attract electrons to them and cathodes emit electrons. The plate and grid are between the anode and the cathode and come together to form the vacuum tube circuitry.
The anode of a tube is the plate with a positive charge. It surrounds the hot cathode in a tube and collects electrons that are emitted by the cathode as they pass by. The plate also picks up your guitar signal when it’s passed by the cathode and passes it on to the next component in the circuit – the control grid.
The cathode of a tube is the centre part of the vacuum tube. It’s a conductive metal that absorbs electrons and emits them back out to the plate and control grid when they hit it. The cathode is where the electrical signal from your guitar enters the vacuum tube, and it’s also where you get your tone (the timbre of the sound).
The plate inside the tube has a low electrical charge. This causes electrons to be drawn to the plate by the power supply, and as they are ‘drawn’ they stack up on one side of the load resistor – this is what creates the voltage drop across the resistor. The plate’s current (the amount of electrons that flow) is then amplified by the tube’s control grid, which’releases pressure’ or ‘lowers’ the voltage on the plate.
The control grid inside a tube has an electrical charge of either negative or positive depending on whether the incoming audio signal is positive or negative. This electrical charge acts as a barrier between the hot cathode and the plate, preventing electrons from escaping the grid to the plate, thereby amplifying your guitar’s signal.
The load resistor of a tube is the piece of metal between the power supply and the tube’s control grid. Its purpose is to transform the circuit from a current amplifier to a voltage amplifier. When the voltage on the control grid increases, this increases the load resistor’s current.
When the voltage on the control grid decreases, this reduces the load resistor’s current. When the voltage on the control grid is equal to zero, this reduces the load resistor’s resistance.
A load resistor is necessary in all tube circuits, but it’s most important in the amplifier section of the circuit. The load resistor is the piece of metal between the power supply’s 6.3 volt AC supply and the tube’s control grid.
It’s called a “grid leak resistor” because it “leaks” off unwanted DC voltage created when free electrons from the cathode hit the grid. This prevents a grid voltage buildup that would affect the bias voltage between the cathode and the grid.