I have just converted my 12B4 tube pre-amp to this Audio Note M77 Clone. The 12B4 / 12B4A circuit was my reference bettering all other tube pre-amps I have owned in the past but this just betters it in so many ways. The circuit was linked from another overseas forum over at DiyAudio. Someone had opened up a real Kondo M77, took photos and traced the circuit plus measuring all voltages sketching this onto lined paper. I have re-drawn the original circuit in X-Circuit and the supply I used. The M77 is built to such as high standard using quite special parts, with an equal special price that most of us could never afford.
Tubes (Single or Parallel)
The circuit uses 2 x 12AY7 triodes in parallel per triode for low impedance output and drive capability. So a stereo build would use 4 tubes in total, 2 per side. However the circuit is flexible enough to experiment with other triodes, single and paralleled, such as Ecc82/83 and the larger double height triodes like E80CC for example. It also works well with miniature triodes, again parallel 2 of these.
I used 1 x Mullard E80CC per side. Since these are double height tubes they do not benefit from paralleling and the voltages are very close when using just the 1 E80CC triode per section rather than paralleling of 2 smaller 12AY7 or ECC83 for example. This is breaking away from the original design but it works even better and sounds fantastic. The key is experiment with tubes if you do not want to use parallel triodes. The Russian 6n6p should be a good alternative at a much lower cost than 4 x 12AY7 or 2 x E80CC which are now expensive to buy. Most Russian tubes are very under rated and much cheaper than NOS prices.
The circuit diagram only shows 1 triode at each stage and 1 audio channel. So if using E80CC tubes then its easy, just 1 tube per channel, the first half of the tube being the input triode stage and the second half of the tube the output triode stage. If using the smaller tubes then remember to parrallel each triode half of the tube. So then we have 1 full tube (parallelled triodes) for the input stage and 1 full tube (parallelled triodes) for the output stage. So 2 tubes for each channel, thus 4 in total for stereo.
I am using my original 12B4A pre-amp psu that outputs around 310V. By using 2K dropper resistors between cap PSU stages (like in the original m7 circuit) and 1 final 39K 3W dropper resistor at the HT output, this then gives around 250V.
This psu uses a 50VA 110V toroidal transformer from RS which is easy to buy plus a separate 50VA 2x6V transformer wired as 12V for the heaters. A voltage doubler is used to get (110V * 1.414 ) * 2 for the HT and heaters are regulated for low micro-phonics. The diode trick on the regulator ground is used to lift the 12V to 12.6V for the heaters. The regulator gets hot so use a good heatsink and insulate it from the metal case with a rubber or mica washer plus a plastic insert or plastic nut and bolt if using a metal case as the heatsink. Otherwise make sure the heatsink of the regulator does not short against the case if using a separate one and directly bolting it to the regulator.
Obviously if you have a 200 - 250V HT transformer then no need for the doubler circuit. Just use 2 diodes in full wave mode, a bridge rectifier or a tube rectifier for a soft start. You then adjust the resistor as shown in the PSU circuit diagram for the correct HT voltage. You may need to do some final adjusting when on load.
My PSU is a bit overkill with C-R-C-L-C-R-C and 1 final large film cap at the output. This makes the pre totally silent even into a solid state amp. I am sure you could get away with C-L-C if using a tube power amp but a good PSU seems to be a must in the build for high audio quality.
Output Caps and Optional Grid Stopper
You will need to double the size of the output cap if using other tubes then the paralleled 12AY7 and depending on the power amp input load. I used 2 x 2uF and there is no shortage of bass even into a solid state amp. I'd opt for at least 0.47uF cap to start with to be on the safe side to allow tube rolling if you are buying a set of caps. If bass is light you may need to increase its value.
You can add a 100R to 1K grid stopper resistor directly on each of the grid pins (input) of the tube(s) after the potentiometer output. Use the lowest value you can get away with so try 100R first and if you have any high frequency noise / oscillation that sounds like rustling and whistling then increase the value.
Cathode By-pass Caps
I don't really care for cathode by-pass caps since they can muddy the mid range and sound in general but in some cases it may be required to add a cathode by-pass cap on the input stage. This is only needed if you find the bass a bit lacking. It is a matter of connecting an electrolytic capacitor across the 2K resistor on each input tube. The - of the cap goes to ground and the + of the cap at the top of the resistor on the cathode (2.25V). Since the voltage is low a standard 25V electrolytic cap is fine. Experiment with values from 47uF to 1000uF, the lower the better for a cleaner sound.
Probably the best tube pre-amp I have used to date when using the Mullard E80CC tubes but I must stress that it needs a very good psu to sound the best. This was the last tube pre I built before moving back to playing with solid state builds for a change. I would certainly build another if I ever got back into tubes again but with the cost of NOS tubes I am sticking with class A solid state for now. Simply staggering sound with the right tubes.
** Main circuit and PSU design inside zip file download below **