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In Reply to: RE: Please help me troubleshoot my new SET posted by rebbi on January 27, 2015 at 15:23:48
OK, now that I looked at the schematic, the input stage comes before the volume control.
It is possible that the DAC is placing too much signal of the grid of the 6sn7 clipping it.
A "...CD player or a DAC with a adjustable [output]..." is a great idea.
Do you know the rated output voltage for the DAC and the phono pre?
I'll take a look at the bias point for the 6sn7 and that should tell me how much signal voltage it would take to clip it.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Follow Ups:
I wonder why they put the level control there?
That seems odd to me as well.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
As luck would have it, I already did my research on this. The DAC outputs 2.2 V. With my cartridge, the phono stage should be putting out about 550 mV.
2.2vac rms is 3.1 volts peak.
The 6sn7 has 4.75 volts bias. The 6sn7, in circuit, has a gain of about 15.35
3.1 times 15.25 is a peak voltage at the plate of 47.5 volts.
The 6sn7, in that circuit with those currents and voltages [7ma., 175 volts plate, 27333 ohms plate load] should output well beyond that without trouble.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Tre,
Thank you very, very much for taking the time to look all that up.
I have subsequently been in touch with Brian and he, like you, thinks that the 2.2 V output of the DAC should not be presenting any problem. At this point he thinks it is some kind of problem with the driver stage. Perhaps I reversed some resistor values or it may be that one of the three tubes mounted on the driver board are not good. In any event, he is going to help me troubleshoot this so I'm feeling better about where this is going. I will let you folks know what I find out, just for educational purposes. :-)
Thanks again!
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