Here's my 6EM7 SE amplifier experiments that modified from my 6EW7 kit PCB. Basically most of the schematic of basic 2 stages SE amplifier are identical, only values of parts that keep those tubes working in optimum condition. I'm building this just wanted to compare the sound quality of both 6EW7 and 6EM7.
The tube sockets for 6EM7 is octal 8 pin, and 6EW7 is 9 pin type. I observed the layout and found that if I solder the octal tube base on the component side, only 1 pin is not in position. I use a short wire for that pin and use hard copper wire for the rest of pins.
I started with the power stage and after looking at the curve of the 6EM7's power triode, I fixed with a cathode resistor that closed to optimum operating condition. After testing the operation of this power stage, I found that 6EM7 can't operate well at the point that I set according to what I calculated previously. The negative half wave looks clipping earlier than the positive half wave. I knew that the calculation is just a guide line, need to adjust to actual operating point by connect a wire wound variable resistor at cathode of the power triode and adjust it . I finalized with 560ohm of cathode resistor, which is the same as 6EW7.Coincident? or purposely made? I don't know.
So till now still nothing to modify. The driver stage has more gain, and also higher Rp, I choose 120Kohm for the Rp, that is the R6 and R7 on my 6EW7 kit's PCB. I keep the resistance to not more than 3 times of Rp, so that the clipping will smooth and cancelled with power stage, to maintain low distortion. Now another resistor need to re-adjust is cathode resistor for this driver stage. ( The R14 and R16 on my 6EW7 PCB) I adjusted to lowest distortion and finalized with 1.8Kohm.
Other than R6, R7, R14 and R16, others parts remains the same. The sound quality of 6EM7 is quite interesting, sweet and good sound stage, with a bit bass-shy than 6EW7, but natural sounding ,distortion on high frequency is slightly higher than 6EW7. Power output is nearly the same as 6EW7. I like sound quality of both 6EW7 and 6EM7, they have their own characteristics that really amused me.
If anyone of you interested in building one please download the schematic here.
I would like to thank Mr.Leonard from Honolulu, who visited me last month, give me some Salvania and RCA's 6EM7 for this project, several great sounding jazz CD's and highest grade YUNNAN tea .
