14N7/14AF7 Loktal Preamplifier
Here’s a long time workhorse in my setup. Four line-level inputs are on the selector switch, and active/passive mode is selected by the toggle switch in the center. Can easily be rewired to accept 7N7 or 7AF7, so that more types become usable. Collecting tubes is more fun when you can use some!
The Schematic. This preamp has its two 14N7 or 14AF7 tubes connected as CCS (Constant Current Source) loaded single gain stages. The idea here is to be able to use one stage of gain without feedback. Using the CCS load makes the stage linear enough that it does not require feedback. This does nothing for hum & noise of course, so the power supply is critical and thus somewhat complex. To read more about CCS loading, check out section 11-7 of Vacuum Tube Amplifiers by Valley & Wallman:
Evil Science Audio 