Welcome! Need support, you got it. Or share you ideas and experiences.
Do it by parts, from input to output. The bits get more complex each time, so that's a reasonable sequence.
Do you "get" the input wiring and level control? :^) that's the simplest part. A CD player might generate an absolute maximum of 2vRMS sine wave, i.e. 2.8v peak, if you want to put some numbers on it.
Thanks for responding. I appreciate you putting little extra labels on the schematic by the way, it helps. I think i get the first stage, up to the first coupling cap. And I get the volume pot bit. I'm gonna need to revisit the data sheet i found on those 6DN7 tubes and my textbook, regardless.
The Rg and the lack of a resistor from the 2nd stage's cathode confuse me though. I don't think I'll go any further until I understand these parts. Could you shed some light on those two aspects for me?
OK, great - you understand the "normal" parts of the design, and have only stumbled at the weird part. This is an old Western Electric trick from the twenties. The power supply current is split, with the main circuit from ground to the positive, and a resistor Rbias from ground to the negative of the power supply. This creates a voltage below ground, which biases the grid of the output triode to a negative voltage.
Weird, I agree. The reason is more complicated. I wanted to connect the output transformer to the cathode to keep the signal current loop small - Cpf to OPT to the triode. No signal current flows through the other components, including the power supply (which has electrolytic capcitors which are not especially linear). But the OPT is wired as an autoformer, so if it is not grounded the output will not be grounded - and the output goes to headphones which really must be grounded for safety. So we needed a way to get a bias voltage so that the triode's cathode could be grounded and thus connect to the OPT.
Another complication - every triode is slightly different due to manufacturing variability. That's why cathode bias is used - the bias depends on the triode's current in a way that stabilizes the operating point for that particular triode through negative feedback. The feedback only operates at DC and frequencies below audibility. For this to work, each triode needs its own feedback, so there must be dual independent power supplies.
It's my goal to simplify this eventually by using transformers instead of autoformers, and that are rated to have some DC on the primary winding relative to ground. Then I can use a more conventional power supply and will need only one supply for both channels.
Hope that provides food for thought! Let me know if I've been unclear anywhere - web posting is not the most efficient communication method...
This is wonderful, and I've learned a lot already. I have a probably dumb question about the first part. when you said:
"...This creates a voltage below ground, which biases the grid of the output triode to a negative voltage."
Is this because the high negative voltage below Rg yields a positive voltage at the Cc-Rg-Rstop node which then gives you a negative bias at the second tube's grid because of Rstop?
This next bit I understand (and i like your thought process here, very logical):
"Weird, I agree..."
except does Cpf stand for parafeed capacitor?
And the last part is a little confusing to me, and I'm sure it's because I'm still getting my feet wet here:
"Another complication - every triode is slightly different due to
manufacturing variability. That's why cathode bias is used - the bias
depends on the triode's current in a way that stabilizes the operating
point for that particular triode through negative feedback. The feedback
only operates at DC and frequencies below audibility. For this to work,
each triode needs its own feedback, so there must be dual independent power supplies."
I guess I understood "cathode bias" to be a certain voltage applied to the cathode, but it is grounded, so maybe my understanding is wrong. Also, I guess I'm just lost when you start talking about negative feedback. I think I need to learn more about how the tube operates to get it, but could you explain where in the circuit the feedback comes from?
As always, I really appreciate you taking the time to explain these things to me, it's been very helpful, thanks!
You might not have seen it, but I posted a reply outside this thread so it won't get scrolled off the page so fast.
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