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SET Asylum Single Ended Triodes (SETs), the ultimate tube lovers dream. |
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In Reply to: And now for the math... posted by uW on August 30, 2001 at 11:14:23:
I think Steve's examples were using a 6SN7 tube.This is kind of hard to do in an e-mail... guess we need an interactive internet white board or something. But I'll give it a shot.
One caveat - I haven't put a lot of thought into this, nor run any simulations or actually built the circuit.
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The overall B+ voltage is limited on the low side by the "voltage compliance" of the current source. Simply put, the minimum voltage that must be maintained across the current source for it to still try to regulate current. For a tube current source, you can imagine as the voltage across the tube drops, the bias voltage also drops, until the point that there is no bias and grid current starts to flow. At this point the tube can no longer conduct the required current.
On the high side, the B+ voltage (again for a tube current source) will be limited by either the maximum plate dissipation or the maximum plate voltage rating of the current source tube.
Note that you need to look at these constraints not just at the nomial bias point of the amp, but also at the extremes of the swings of the plate voltage of your amplifier tube at the maximum output voltage.
Taking my circuit idea as an example, first I set the operating point for the amplifier tube (right off the Amperex data sheet for EL-34 in triode mode) of B+ = 375V, Ip = 70mA. They also spec a cathode bias resistor of 370 ohms, though you can figure this out by looking at the plate curves (this corresponds to a bias of 26V, so a plate-to-cathode voltage of 349V).
So, the bottom of the current source will be at 375V, connected to the plate of the amplifier tube. This voltage will swing about +/- 190V at an output power of 6W into a reflected load impedance of 3000 ohms, which (again according to the datasheet) should be the max output of the amp. (This comes from Prms = ((Vpeak * .707)^2) / R). This means that the bottom of the current source will swing from 375V +/-190V, or a minimum of 185V and a maximum of 565V. Our current source must still be in compliance with 565V on the bottom, and not fry itself with 185V on it. From this you can see that even with a perfect current source you would need a B+ of 565V minimum. In reality, a tube current source will probably need at least about 100V to be in compliance, so my first guess for B+ is around 660V.
Now, I'm going to make an educated guess at an operating point for the upper EL34, the current source. There are curves in the data sheet at a screen voltage of 250V. If I want 70mA, and (arbitrarily) use a 250V screen voltage, the curves show that I can get there with a bias voltage of 16V at a plate voltage around 260V.
To get the 16V bias, at 70mA, this takes about a 230 ohm resistor (16V / .07A = 228 ohms). So this is what I'll use for the resistor between the cathode of the current source and the plate of the aplifier tube. The grid of the current source is then connected to the lower end of this resistor (the plate of the amp tube).
The screen grid needs to have a constant 250V (relative to the cathode) to work like a pentode. So, the screen is connected to B+ with a 1K resistor (somewhat arbitrary - small enough so it doesn't waste too much voltage, but big enough so the screen capacitor doesn't have to be huge), and bypassed to the cathode with a cap big enough to hold the voltage constant - say, 10uF for 1K (again, relative to the cathode, not to ground). Again from the curves, it looks like this operating poiint will have about 10mA of screen current, so the screen resistor will drop 10V.
To get the B+ for this whole thing, if you add together the 375V (on the amp tube), plus the 16V to bias the current source, plus the 250V screen voltage, plus the 10V drop on the screen resistor, you wind up with 650V.
Looking at the swing shown above, with a B+ of 650V, the current source will see 275V nominal, a minimum of 85V, and a maximum of 465V.
If you look at the curves again for the EL34 in pentode mode, it looks like with a screen voltage of 250V (remember the cap is holding the screen up at 250V above the cathode, even when the cathode has swung to 565V), it looks like the EL-34 will still be able to supply 70mA with as little as 20-30V on the plate... so it looks like there's enough compliance at this operating point.
On the high side, there shouldn't be any problem with either power (70mA * 260V = 18W, the tube is rated for 24) nor voltage (800V).
So, I think this operating point should work. You can probably go lower than 650V, and you could certainly go higher.
Hopefully this helps???
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Follow Ups
- I'll try and descibe the circuit... (long) - Pete Millett 08/30/0114:01:06 08/30/01 (2)
- Good grief! - uW 14:16:09 08/30/01 (1)
- Looks like I have a project for the weekend.... - Pete Millett 15:51:52 08/30/01 (0)
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