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In Reply to: RE: should i change the operating point ? posted by DAK on April 19, 2017 at 17:56:19
Values around the 'SL7 are way off. More typical would be 100k anode resistor, 1.8K cathode resistor, 270k next stage grid resistor. Anode current should be a little under 1mA.
Buy Chinese. Bury freedom.
I know that is what the tube manual says but the cathode current is right around where it should be.
If you say the B+ is 220v and using a 10K plate load resistor, dropping Va to 180, your current is (220-180)/10000 = 0.004 or 4mA
0.004 * 180 = 0.72 watts which is pushing it with a 6SL7
But the bigger problem is that a 10K load on a 6SL7 is really inefficient. The 6SL7 is a low current voltage driver, and the 10K really handicaps the swing it can develop. The resistors look like the designer of your circuit was using a 6SN7 and you dropped a 6SL7 into it.
TKs suggestions will make your amp sound a lot better.
"But the bigger problem is that a 10K load on a 6SL7 is really inefficient. The 6SL7 is a low current voltage driver, and the 10K really handicaps the swing it can develop."
Not to mention the high levels of harmonic distortion due to the vertical load line.
Have Fun and Enjoy the Music
"Still Working the Problem"
Along with the 10k there is also a plate to plate fb resistor of 210k. It connects the plate of the 6sl7 with the plate of the 6l6.
I should probably try a 6sn7 as give it a listen, although, that should use a 100k plate to plate fb resistor.
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