gofar99 wrote:
Hi, At the given voltages (180-225 B+) IMO there is no best value. There are several issues with ECC81s in this circuit. First the gain is way too much (instead of 7 it is 25). Too much for a line stage. Second the output impedance will be high....about 6.5K. It would be OK for a tube amp input of say 50K or more but might be unsuitable for a lot of solid state gear. All that said if you wish the best values I can determine are for both cathode resistors to be 100 ohms. This gives an idle current of 5.2 ma at 220volts. Output voltage into 50K is about +47/-31. Since you really need only about 1-3 volts it ought to be OK. The non-symmetrical range is likely to cause some harmonic distortion, but I have no way to tell short of building it. My guess would be that 1-3 volts output it should be in the sub 1% range. You can not bypass the lower cathode as the symmetry really gets bad there. I didn't check for higher B+ but it would seem to put the tube in a better range. Let me know what you have available and I re check it.
Good listening
Bruce
Hi Bruce
I modified the cathode resistor to 95 ohm and raised B + to 268 volts. I used ecc81 China, Chelmer, and Mullard cv4024. The best result is with cv4024. Distortion is not audible compared to ecc82. The sound is balanced, best low and high frequencies. Ecc82 has more mid frequencies.
Regards
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