Indeed. 🙂 The 1/2 is a factor. Therefore if you have a tube with a Mu of 33 the answer would be
33/1 + 0,5 = 0,53 or practically a ratio of 50/50
another example Mu of 20
20/1 + 0,5 = 0,55
Now if the 300B is our first tube (of which we'll need two per channel) with a Mu of 3
1/3 + 0,5 = 0,83 in other words a significantly different ratio. This means you will have to go dig in your parts bin to find the correct ratio of resistors. (Roughly 80/20)
Hence John's suggestion that you can freely swap tubes (without having to change resistors) when the Mu is around 20 or >
bf, I think your math slipped a bit. Actually, not your math (the equation is correct), the arithmetic. You can't add something to 1/2 and end up with 0.5!
Bas has it right, at increasing mu, the ratio tends strongly to 0.5. An anal retentive might put a trimmer in there to null distortion for a particular tube, but that's even nuttier than I am.
Bas has it right, at increasing mu, the ratio tends strongly to 0.5. An anal retentive might put a trimmer in there to null distortion for a particular tube, but that's even nuttier than I am.
I assure you I am the opposite of an anal retentive...more an überslacker...but we added a trimmer in "our" aikido boards...such as Joe (burnedfinger) has..
Quote:
where did you get the value 8.25?
Darn, darn, darn, I can't get away with anything here. I guess I needed that third cup of coffee that I missed.
where did you get the value 8.25?
Darn, darn, darn, I can't get away with anything here. I guess I needed that third cup of coffee that I missed.
Let's use the GE data sheet for the 12BH7A.
We have a 300V B+, so figure you've got 150V across each tube. What current shall we choose? Look at the Average Transfer Characteristics. We can optimally run 15mA and be on the straighter portion of the 150V curve. Or we can trim back a bit, use 10mA, and rely on the distortion cancellation of the circuit. I don't like doing things that way, so we'll run it rich, 15mA.
From the Average Plate Characteristics, we see that we need about 5 volts of bias. So the cathode resistors are 5V/15mA = 0.33k, or 330 ohms.
We have a 300V B+, so figure you've got 150V across each tube. What current shall we choose? Look at the Average Transfer Characteristics. We can optimally run 15mA and be on the straighter portion of the 150V curve. Or we can trim back a bit, use 10mA, and rely on the distortion cancellation of the circuit. I don't like doing things that way, so we'll run it rich, 15mA.
From the Average Plate Characteristics, we see that we need about 5 volts of bias. So the cathode resistors are 5V/15mA = 0.33k, or 330 ohms.
Large and equal. Not terribly critical, but they should be reasonably well-matched. John suggested that they can be downsized in value to knock down the gain a bit, but I'd be leery of that.
burnedfingers said:
By looking at the schematic. (sfx: rim shot)
C1 must be large enough to pass AC at any noise frequencies that we care about. It's not critical in value.
R12 and 13 also just need to be large and equal. They're safety resistors.
This is a great thread for us that needs to be spoon fed... Thanks for taking the time to teach us!
I think I understand most of the calculations so far but it sounds like these are for only the second tube. Do they also apply to the first tube?
Next, if the ratios between the top and bottom are different, say 60/40, how do we calculate the resistors for that?
Lastly, what would be a good second tube to use if I wanted to have two line outputs (one to a sub and the other to my mains)? How would I do that? Do I just split the single out into two and terminate each with a 1M resistor?
TIA...
I think I understand most of the calculations so far but it sounds like these are for only the second tube. Do they also apply to the first tube?
Next, if the ratios between the top and bottom are different, say 60/40, how do we calculate the resistors for that?
Lastly, what would be a good second tube to use if I wanted to have two line outputs (one to a sub and the other to my mains)? How would I do that? Do I just split the single out into two and terminate each with a 1M resistor?
TIA...
Well, the 12BH7 calcs are for the tube in the first slot. But really, you calculate cathode resistor values the same way, regardless of where the tube is.
To get more than one output, simply add another pair of jacks in parallel with the first pair. No need for extra components.
I don't understand the ratio question. Ratio of what?
To get more than one output, simply add another pair of jacks in parallel with the first pair. No need for extra components.
I don't understand the ratio question. Ratio of what?
Sy, this was in post #5:
What is this ratio you mentioned here? I'm also not too sure where the 200k string resistance comes from.
What is this ratio you mentioned here? I'm also not too sure where the 200k string resistance comes from.
That's the proportion of the bottom resistor in the divider string for the output stage (R10 and 11 in Bas's schematic). The formula comes right out of John Broskie's writeup on Tubecad.
As I said, the 200K number is pretty arbitrary- we don't want it too low because that will load things down. We don't want it too high because stray capacitance and leakage can work their nasty charms. So 100K, 200K, 500K, anything like that will do just fine.
As I said, the 200K number is pretty arbitrary- we don't want it too low because that will load things down. We don't want it too high because stray capacitance and leakage can work their nasty charms. So 100K, 200K, 500K, anything like that will do just fine.
I had better let someone else do the math, LOL. Wait! You were going to do a 12AT7 first and we haven't even touched on the single tube front ends yet. I have more questions that need answers and a fresh pot of coffee to drink.
Using the schematic I built a 6au6 as my first tube and 5687 as my second tube. The sound is great and there is something to be said about the 6au6. The gain is a tad too much when using a CD as a source. Has slight hum in both channels when using international brand 6au6's. Has slight hum in one channel only and this follows the tube when using a different brand. Unfortunately I only have 4 tubes in which to try but I am sure I could find a set of quiet tubes.
Based on my limited exposure to the single tube front end I believe it too needs to be explored in more detail because I don't understand the noise injection as it used a single 560K resistor.
Using the schematic I built a 6au6 as my first tube and 5687 as my second tube. The sound is great and there is something to be said about the 6au6. The gain is a tad too much when using a CD as a source. Has slight hum in both channels when using international brand 6au6's. Has slight hum in one channel only and this follows the tube when using a different brand. Unfortunately I only have 4 tubes in which to try but I am sure I could find a set of quiet tubes.
Based on my limited exposure to the single tube front end I believe it too needs to be explored in more detail because I don't understand the noise injection as it used a single 560K resistor.
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