On a peavey 5150 original there is knob inside the amp to left of tubes What does it do?

The knob inside the Peavey 5150 original to the left of the tubes is the bias adjustment control. This control allows you to adjust the amount of bias voltage applied to the power tubes, which affects the way they operate. Specifically, it adjusts the negative voltage on the control grids of the power tubes, which in turn affects the amount of current that flows through them.

Here's a simplified explanation of how the bias adjustment control works:

* When you turn the bias adjustment control clockwise, you increase the negative voltage on the control grids of the power tubes. This causes less current to flow through the tubes, resulting in a cleaner and more defined sound.

* When you turn the bias adjustment control counterclockwise, you decrease the negative voltage on the control grids of the power tubes. This causes more current to flow through the tubes, resulting in a warmer and more distorted sound.

It's important to note that the bias adjustment control is a sensitive control, and even small adjustments can have a significant impact on the sound of the amplifier. Therefore, it's important to experiment with the bias adjustment control to find the setting that best suits your playing style and musical preferences. If you're not comfortable adjusting the bias yourself, you can take your amplifier to a qualified technician to have it done.