This is my DIY 6N16B line stage board. I seldom build or use a preamp or line amp in my system , not even in my own DIY R2R DAC. Recently, I built one for my local friend who have problem driving his class A solid state amplifier with passive preamp.
I choose 6N16B because its convenient to solder directly onto PCB. I love point to point hand soldering method , and PCB comes second choice for my vacuum tube projects but since this preamp need to be build into a smaller aluminium casing , so I used PCB instead. The whole PCB is very small ,about a palm size. I ordered a small aluminium casing for this little pre-amp and still wait for it.
6N16B is a Russian military subminiature tube, this small double triode are very durable, low microphonic, with µ of about 25 and a transconductance of about 5mA/V. From the schematic below, the 6N16B act as a gain stage and driving the second stage of cathode follower stage to provide low output impedance. I exclude the input coupling capacitor C001 and C1001 because I always want to solder it directly on RCA jack, so that I can swap it easily to experiment different types of capacitors. The coupling capacitor can be omitted by connect the R103/203 to the ground directly, instead of to the 4.7K resistor, but need to re-adjust the working point of this stage to suit though, because the two stages are actually directly coupled.
I include a delay relay to mute the output for around 30 second when switch on, with simple RC delay circuit. The delay time can be adjust by changing the value of the capacitor C601. The power supply of this delay circuit are actually tapped from 6V DC filament power supply with simple CRC filter. I lift off the filament potential to several 10’s of volts. Some tubes will sound better by doing so, I’m not sure for 6N16B and I need to experiment further to confirm with this.
power supply for B+ was smoothed out by using Mosfet capacitor multiplier, and since current are very low, a small heat sink seems more than enough for the purpose.
For test, I add an ALPS 100K volume pot at its input , hooked up a class A amplifier and power it up. I experiment with different bias point and only small changes can be heard. This small line stage sounds very good and very warm , very quiet background. I also try to mod it by remove the R106 /206 and connect a 12K from cathode pin 2 to pin 6, tweak a little bit bias, and thats formed a feed back circuit , sound quality became more ” Hi-Fi”ish , the warm of tubes reduced a little but very clean and dynamic. I switched back to original design in the end though, I love the warmer sound out from it.