Here is an article to show how I designed my 6SN7 preamp with CCS based on DN2540N5 or IXCP10M45S D-MOS transistors. The article is an addition to my design which showed a 6SN7 tube in parallel connection. Here I will continue to develop the schematic. I will introduce a popular solution for a reliable constant current source.
The constant current source
Initially I settled on a current source based on this topology. This solution is relatedly easy to implement, however it requires more components. In comparison a current source based on DN2540N5 or IXCP10M45S is much easy on the parts count. It requires only one external resistor and that’s it. A good info on these devices is available in W. Jung’s Sources 101 Part 2. The current source looks like this:
Connecting the 6SN7 to DN2540N5 or IXCP10M45S
As I showed in my previous article this 6SN7 preamp is based on parallel connection. Each 6SN7 triode will run at approximately 6.5mA which means a total of 13mA for the constant current source. I’ve got a 10M45S on hand and that is what I’m using for this schematic. Since there is no clear formula to allow me to calculate the exact value of the current set resistor I will use the following graph:
In addition a P-spice model for IXYS IXCP10M90S is available online. I actually used this model and came at a resistor value on 220R for a plate current of 13mA for both 6SN7 triode sections.
The complete 6SN7 preamp schematic
So we arrive on the schematic for the actual preamp itself. For this project I decided to explore the design environment of KiCad so this one is created on this platform.
A PDF version with all the details is also available for download.
The 6SN7+DN2540N5, IXCP10M45S preamp PCB
I know that tube purist might find it inappropriate but I use a PCB for my project. The goal is to enclose everything in a slim elegant enclosure. Therefore I chose a PCB. The main design feature is to have a PCB that sits vertically in a case in order to keep a low-profile. The PCB measures 38 by 135 mm and fits perfectly in a very slim enclosure.
Gerber and drill files are available for download. Use JLCPCB or any other of your favorite PCB manufacturers to get the boards made for you. The 3D model of the octal socket is available for download at my GrabCAD page.