Total newbie!
I've taken on an interesting project as I've always had an interest in electronics, and felt this would be a great way to learn. This is my first day delving.
I'm designing and building a small battery-powered, tube-amplified musical instrument preamp. The signal from which will eventually be used to power a small external 12vdc automobile power amplifier and out to a pair of 4" speakers. The whole thing will be built into a suitcase.
The reason I chose to use a tube as an amplification medium as opposed to a FET, JFET, MOSFET or whatever else relates to both sound and selling power. Tubes sell in this industry, and they do tend to sound better, due to their clipping mechanics and the way it produces harmonics.
I have picked up a book entitled "Designing Tube Preamps for Guitar and Bass" by Merlin Blencowe. Excellent book, but I am attempting to adapt what is given to work for my application. I am sure I will have more questions but right now these are my primary obstacles;
I have selected for this project a 12AJ6 tube, and a 12vdc / 6.8a LiPo battery pack. The 12AJ6 is designed to run at 12vdc. Minimum current to function correctly is about 4a. Now, according to the book, a load line is a requirement on a circuit like this, which runs in series with the tube from the HT or battery. I'm still not entirely sure how a load line works, but working with the equation given, I believe what I am trying to do is keep the tube from seeing more current than it can handle? (That was question one)
If that is the point of the load line and I am on the right track, according to the equation, if I wanted to have 4a of maximum current at 12v, I would want a .003kohm resistor. 12/.003=4,000ma or 4a. (Note: I think I actually want a 5a or higher max current, but I'm just running numbers) V/R=I
Right now I am seeing an odd relationship with the numbers that I don't fully understand, and am patently convinced I am doing something wrong. According to this, if I where to run a .001k resistor (assuming there is such a thing), I would see 12a in current? Would that mean I would be forcing/allowing the batteries to discharge faster than is safe or would it mean that the .001k resistor would essentially do nothing, as it wouldn't "clip" the current until 12a, and my HT/B+ is only capable of 6.8a? (That was question 2)
My final question is; if all of this is correct and I really do need a .003k resistor on my load line to reach a 4a current, in my shopping for resistors I have also realized that I am apparently running way more wattage from the battery than any .003k resistor can handle? As 12v x 4a = 48 watts? The highest handling .003k resistor I can find is about a watt. (That was question 3)
As a final note, I found a schematic of something somewhat similar to what I am designing. Powering a similar tube with a 12vdc battery/supply, although in this case powering an entire amp. 2 Preamp tubes / 2 Power tubes. According to the schematic, it calls for a 1M resistor on the load line between HT/B+ and the tube. Why? Whats the math here? Why am I so far off? (Question 4)
Thanks in advance for any and all responses.
I've taken on an interesting project as I've always had an interest in electronics, and felt this would be a great way to learn. This is my first day delving.
I'm designing and building a small battery-powered, tube-amplified musical instrument preamp. The signal from which will eventually be used to power a small external 12vdc automobile power amplifier and out to a pair of 4" speakers. The whole thing will be built into a suitcase.
The reason I chose to use a tube as an amplification medium as opposed to a FET, JFET, MOSFET or whatever else relates to both sound and selling power. Tubes sell in this industry, and they do tend to sound better, due to their clipping mechanics and the way it produces harmonics.
I have picked up a book entitled "Designing Tube Preamps for Guitar and Bass" by Merlin Blencowe. Excellent book, but I am attempting to adapt what is given to work for my application. I am sure I will have more questions but right now these are my primary obstacles;
I have selected for this project a 12AJ6 tube, and a 12vdc / 6.8a LiPo battery pack. The 12AJ6 is designed to run at 12vdc. Minimum current to function correctly is about 4a. Now, according to the book, a load line is a requirement on a circuit like this, which runs in series with the tube from the HT or battery. I'm still not entirely sure how a load line works, but working with the equation given, I believe what I am trying to do is keep the tube from seeing more current than it can handle? (That was question one)
If that is the point of the load line and I am on the right track, according to the equation, if I wanted to have 4a of maximum current at 12v, I would want a .003kohm resistor. 12/.003=4,000ma or 4a. (Note: I think I actually want a 5a or higher max current, but I'm just running numbers) V/R=I
Right now I am seeing an odd relationship with the numbers that I don't fully understand, and am patently convinced I am doing something wrong. According to this, if I where to run a .001k resistor (assuming there is such a thing), I would see 12a in current? Would that mean I would be forcing/allowing the batteries to discharge faster than is safe or would it mean that the .001k resistor would essentially do nothing, as it wouldn't "clip" the current until 12a, and my HT/B+ is only capable of 6.8a? (That was question 2)
My final question is; if all of this is correct and I really do need a .003k resistor on my load line to reach a 4a current, in my shopping for resistors I have also realized that I am apparently running way more wattage from the battery than any .003k resistor can handle? As 12v x 4a = 48 watts? The highest handling .003k resistor I can find is about a watt. (That was question 3)
As a final note, I found a schematic of something somewhat similar to what I am designing. Powering a similar tube with a 12vdc battery/supply, although in this case powering an entire amp. 2 Preamp tubes / 2 Power tubes. According to the schematic, it calls for a 1M resistor on the load line between HT/B+ and the tube. Why? Whats the math here? Why am I so far off? (Question 4)
Thanks in advance for any and all responses.
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