Replacing KT88 with 12E1 Electron Tube in a valve Hi Fi Circuit
1. PIN 3 to be connected to top cap of valve 12E1. Keep lead as short as practicable. Use good quality, well insulated flex with a good high temperature stability. Use an insulated top cap as voltages in excess of 450V DC are common on this application. It is suggested that a small hole 5mm is drilled adjacent to PIN 3 and an appropriate grommet inserted to cover bare metal edges. Wire from PIN 3 to top cap passes through this hole and is soldered to PIN 3.
2. Due to higher gain of 12E1 it will be necessary to increase bias to valve. To achieve similar standing current, bias will need to be about -40V. An alternative solution is to reduce anode voltage by about 50V. This is achieved by adding a decoupling R/C network in the HT feed to the output transformer.
3. When set up correctly the 12E1 gives good base drive and excellent transient response. As the valve has lower output impedance than a KT88, it’s dynamic performance will be a noticeable improvement in the adjusted circuit.