I have been cautiously powering up the preamp over the last two days, and the circuit is starting to take shape.
While contemplating the power supply design and the practical aspects of the parallelled 6N1P gain stage after the volume control, it occured to me that the design on the schematic above relies a lot on the voltage regulation in the power supply to give a noise free output. Having two triodes available for a single output stage, could be used more efficiently and have a better power supply rejection. The favourite with me at the moment is the so called CCDA that is blogged about on the Tube Cad Journal.
The other advantage is the possibility to affect the gain of the preamp by using 12AU7 is stead of 6N1P for a lower gain output. We might need this in matching the pre amplifier with the power amplifier/speakers/room. This would be a plug-in-and-play option. Ideal for the non technical user.
Here is a photo (a bit blurry) of the filament wiring in place:
Next I soldered the earth rail to the turn table earth point. This is also the star earth of the whole pre amplifier.
Here you can see the two line inputs and the output RCA's
Here is a close up view of the CCDA and the two 100ohm resistors that creates the virtual centre tap for the filament reference. To clarify: the heaters are powered with 6.3V AC, but the filament circuit has to be referenced to earth somehow. If you do not do that, the sound suffers.
Here is another close up of the filament junction. I use a polypropylene capacitor to AC earth the virtual centre tap. This stops a lot of nasty noise getting into the circuit. I recommend the capacitor on all pre amp circuits with AC heaters. The blurry cap in the fore ground is soldered onto the 100ohm junction and the earth rail.
Usually I reference the filaments 10 or 20V above ground for regular circuit, but in this case the cathode follower of the CCDA is 100V above ground. A reference closer to 50V would put the filaments reference half way between the voltages reflected on the two cathodes..
I used a 220k and a 47k resistor for the job:
A last detail. I put a common mode choke between the voltage doubler and the next filter capacitor. This is to filter the high frequency noise from the diodes and mains power.