Here is my design of a 6SN7 (6N8S) based preamp with parallel sections of the tubes. I plan to use just two tubes, one for each channel. As we all know 6SN7 is a double triode and for my design I will tie the two sections in parallel. The main design parameters fall into three points – low-distortion, low-gain and low output impedance without using a output buffer/cathode follower. In order to achieve some of these goals I will be using an active load – constant current source(CCS).
Setting the bias point
First of all lets begin with the obvious – setting the tubes into the desired operation point. Lets consider a fairly typical power supply voltage of around 300V and a 30kohm plate resistor which results in a plate current of 10mA. From these three values it’s easy to draw the load line, so here’s what it looks like:
This leads me the next point in the process – choosing the bias point of my 6SN7 preamp. As you see from the picture above I settled with an anode current of around 6.5mA. That gives me an anode voltage of around 110V and a grid voltage of -2V. From these numbers it’s easy to calculate the cathode resistor R=2V/0.0065A=307Ohm. The preamp will operate in a relatively linear region of the anode characteristic of 6SN7.
I usually prefer to simulate my circuits whenever possible. This is of course in no way indicative of the sound quality. It’s merely to check my math and ensure the thing is electrically working. So here’s the quick simulation. My calculations are correct.
Getting both sections of the tube in parallel connection has several advantages line reduced noise, lower distortion and of course lower output impedance. There are some drawbacks of course like increased input capacitance as well as increased power dissipation in the anode resistor. Both sections of the tube need to be reletevly similar in order to ensure proper biasing. For this reason I choose to use separate cathode resistors and only adjust down the value of the anode resistor to 15kOhm. The schematic now looks like this:
The active load – constant current source (CCS)
For this project I will be using a transistor based constant current source. Now there are plenty of ways to implement this type of source. I settle for transistors because it’s simple to implement and requires less power than a tube based CCS. 6SN7 is a rather power hungry device if you put into account the amount of current needed for heating. I just don’t need two more power sucking devices that’s all.
Choosing the right topology for the CSS that fits my 6SN7 preamp was not an easy task. Fortunately Mr. W. Jung has some great articles in Audioxpress that helped me settle on a schematic with good results and optimal complexity:
In my case I use a slightly modified version to fit my 6SN7 preamp. The higher working voltage requires some different types of transistors. For my case I use MPSA42 and TIP48G devices. The voltage reference is TL431.
Using a CCS as an active load has one major advantage. It allows for a much lower supply voltage while keeping the bias of the 6SN7 unchanged. I must note that there is still a large voltage drop across the CCS. For this project I will settle with a supply voltage of 200V which gives a voltage drop across the CCS of around 90V. In those conditions the power dissipation in the power transistor is still significant. It this case it is P=Vdrop*I=110V*0.013A=1.43W. The transistor requires a heatsink!
So this leads me to the final topology of this 6SN7 preamp which is this:
Coming up next – detailed schematic and complete project files. PCB design and manufacturing files.