Yet another load-line question
- Thread starter eljefe3126
- Start date
eljefe3126
New Member
I've decided that if I press forward with this, I'm probably going to roll my own OPT. And I'm not necessaarily trying to clone a Major, but rather seeing if there's part of the KT-88 design envelope that nobody else has exploited. I find that everytime I learn a new governing equation, new possibilities open up.
Things To Do at this point:
1. Take best guess at "off the charts" screen current characteristics, and develop a table of output power vs. Ea and RL(a-a), given the constraint that no part of the "Class B" load line has an instant plate dissipation of more than 96 watts. I already know that more power comes from higher Ea and relatively high RL(a-a) (compared to say, a Major), but in that region, screen dissipation is unacceptably high.
2. Do the same thing for a triode connection and compare. This might be a short exercise.
3. Same for ultra-linear connection. I may ask a few questions here to nail down my understanding of screen operation under load in UL.
4. Same, but investigating Class AB2 operation as a means of holding down screen dissipation. I suspect that the Fender 400 PS and 300 PS did this.
5. Investigate the "high impedance triode" circuit on the tube data sheet for the KT-88. I think that if I can understand UL operation, I'll be prepared for this.
One thing I've learned is that you can't trust the left side of the Ra curve in the interactive tube data sheet that was linked a few posts above. The curve in the printed data sheets isn't quite as steep as that one, so I'm using the printed version in my spreadsheets.
You're wise to be cautious with the screen grid, as that is where so many tubes get into trouble..........
Dave
Dave
cozido512
Member
This is very easy to do with circuit simulators.
All these modes of operation are well covered in existing texts.
This mode has practically no coverage in existing texts, and may be your best chance to "do something new".
Good luck with your project and keep us posted...
eljefe3126
New Member
Well, 1-3 above are done, and 4 and 5 seem to complement each other.
My big takeaway was that tetrode connection, triode connection, and ultralinear connection are the same internal to the tube. Oh, of course they are very different with regards to how the circuit sees the tube, but inside the tube, it neither knows nor cares if the reason that there is 200 V on the screens is because the screens are tied to a 200 V rail, or if the screen is tied to a plate that just happens to be at 200 V, or if the 40% tap on the transformer just happens to be at 200 V at that moment. Either way, the screen voltage and correspondingly, the plate current at a given plate voltage, will be the same.
I also realized that the left hand side of the Ra curve, maybe I should call it the "diode line" or something, is pretty much a constant and a given, and nothing you do with either the other grids and screens nor with the external circuit is going to get you an operating point to the left of that line. I may have been laboring under the misconception that Class AB2 operation could do that.
For my purposes, I found it useful to assume infinite current capacity to the right of that line and to plot load lines that didn't exceed some value of plate dissipation rather than trying to hit the knee of some curve. The concept was that if I found a load line that I liked, I could come back and see if there was any way to get the current needed for that particular load line. I found that as I rotated the load line around a given plate dissipation curve that I could get more output power by raising Ea, right up to the 800 V maximum for a KT-88. I could rotate the load line by dropping both Ea and RL(a-a), and as Ea moved left, the other end of the load line slid up the "diode line", and Pout slowly dropped.
The fly in the ointment was screen dissipation. No matter what I did, it seemed to want to go over 20 W, and I need to keep it at 8-ish or less. And I think that's where Class AB2 operation and positive grid voltages come in.
As I understand it, you can raise the "top", "flat" part of the Ra curve by either raising screen voltage, or by taking the grid positive. Looking at 6L6GC curves, it seems that screen current rises monotonically with either a rise in sceen voltage, or in grid voltage above 0 volts. But screen dissipation is the product of screen current and screen voltage, and I hope to get "something for nothing" by holding screen voltage low, and getting the plate current I need by raising grid voltage above 0 V and going into Class AB2 operation. From the 6L6GC data it appears that each mA of plate current "bought" with positive grid voltage "costs" about the same in terms of screen current as a mA "bought" with more screen voltage, but I can't say that with certainty. But unless screen current rises with the square of plate current as grid voltage rises, I think I'm still ahead in the game by using grid voltage instead of higher screen voltage.
And I think that takes me back to the "high impedance triode" Class B circuit in the GEC manual. Do I understand correctly that it really sends 180+180 V rms to the grids and screens as an input signal? That would be 255 V peak-to-peak, meaning grid voltage would go as high as 255 V on each side, while the screen voltage would never go higher than 255 V. In terms of getting the necessary plate current through keeping screen voltage low and grid voltage high, that seems like an extreme case to me. Am I really seeing what I think I'm seeing there? I wonder if I could accomplish the same thing by tying the screens to 255 V and just letting the grids cycle as per the schematic.
More importantly, I don't see where the 10 mA bias current comes from. Is that just what happens when you set Va(o) at 850 V, Vg1 at 0, and Vg2 at 0, respectively?
My big takeaway was that tetrode connection, triode connection, and ultralinear connection are the same internal to the tube. Oh, of course they are very different with regards to how the circuit sees the tube, but inside the tube, it neither knows nor cares if the reason that there is 200 V on the screens is because the screens are tied to a 200 V rail, or if the screen is tied to a plate that just happens to be at 200 V, or if the 40% tap on the transformer just happens to be at 200 V at that moment. Either way, the screen voltage and correspondingly, the plate current at a given plate voltage, will be the same.
I also realized that the left hand side of the Ra curve, maybe I should call it the "diode line" or something, is pretty much a constant and a given, and nothing you do with either the other grids and screens nor with the external circuit is going to get you an operating point to the left of that line. I may have been laboring under the misconception that Class AB2 operation could do that.
For my purposes, I found it useful to assume infinite current capacity to the right of that line and to plot load lines that didn't exceed some value of plate dissipation rather than trying to hit the knee of some curve. The concept was that if I found a load line that I liked, I could come back and see if there was any way to get the current needed for that particular load line. I found that as I rotated the load line around a given plate dissipation curve that I could get more output power by raising Ea, right up to the 800 V maximum for a KT-88. I could rotate the load line by dropping both Ea and RL(a-a), and as Ea moved left, the other end of the load line slid up the "diode line", and Pout slowly dropped.
The fly in the ointment was screen dissipation. No matter what I did, it seemed to want to go over 20 W, and I need to keep it at 8-ish or less. And I think that's where Class AB2 operation and positive grid voltages come in.
As I understand it, you can raise the "top", "flat" part of the Ra curve by either raising screen voltage, or by taking the grid positive. Looking at 6L6GC curves, it seems that screen current rises monotonically with either a rise in sceen voltage, or in grid voltage above 0 volts. But screen dissipation is the product of screen current and screen voltage, and I hope to get "something for nothing" by holding screen voltage low, and getting the plate current I need by raising grid voltage above 0 V and going into Class AB2 operation. From the 6L6GC data it appears that each mA of plate current "bought" with positive grid voltage "costs" about the same in terms of screen current as a mA "bought" with more screen voltage, but I can't say that with certainty. But unless screen current rises with the square of plate current as grid voltage rises, I think I'm still ahead in the game by using grid voltage instead of higher screen voltage.
And I think that takes me back to the "high impedance triode" Class B circuit in the GEC manual. Do I understand correctly that it really sends 180+180 V rms to the grids and screens as an input signal? That would be 255 V peak-to-peak, meaning grid voltage would go as high as 255 V on each side, while the screen voltage would never go higher than 255 V. In terms of getting the necessary plate current through keeping screen voltage low and grid voltage high, that seems like an extreme case to me. Am I really seeing what I think I'm seeing there? I wonder if I could accomplish the same thing by tying the screens to 255 V and just letting the grids cycle as per the schematic.
More importantly, I don't see where the 10 mA bias current comes from. Is that just what happens when you set Va(o) at 850 V, Vg1 at 0, and Vg2 at 0, respectively?
eljefe3126
New Member
So the grid resistor and the grid form a voltage divider pair? If so, how do I determine either grid current or grid voltage?
The schematic suggests anything from a 22k to a 100k resistor. That seems like a fivefold range of acceptable currents and voltages.
The schematic suggests anything from a 22k to a 100k resistor. That seems like a fivefold range of acceptable currents and voltages.
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