Let's first make an initial estimate for the plate supply voltage required to produce 25 watts of output power (or more).
One of the most important parameters in power tube design is a parameter called
"Imax". "I" is the symbol universally used to represent "current". Imax, defined here, is the amount of plate current measured for a vacuum tube operating at the following conditions:
Control grid 1 voltage (Ec1) is 0 volts ("E" universally denotes voltage) Plate voltage (Eb) at 60% of normal or expected plate operating voltage
(In Chapter 17.0, we'll discuss in more detail the implications of Imax for beam power tubes, how the parameter affects circuit details and how it can be changed when a performance improvement or cost advantage is indicated.)
Let's use these terms to make some approximations of performance:
Pout = 0.32 x Imax x Eb or Imax x Eb = 3.125 x Pout
Pout is the required output power in watts, Imax and Eb are as described above.
We can simplify and re-arrange the formula, replacing Pout with 25 watts to give us:
25 = 0.32 x Imax x Eb rearranging Imax x Eb = 78.125
Now, refer to the data sheet "plate characteristics" curves (shown below), find the graph that represents "plate current" in the vertical (Y) axis and "plate voltage" in the horizontal (X) axis. Look at the various curves in that graph and find the curve that is labeled "Ib @ Ec1 = 0", which just means that the curve represents the plate current when the control grid voltage is 0 volts and at the screen grid voltage noted on the data sheet. The symbols used by the manufacturer who prepared the data sheet may vary slightly but you should be able to figure things out.
Examining the curve, you'll see that the variation in plate current is fairly small for large variations of plate voltage. Find the approximate center of the curve (from the "knee" of the curve to the end of the curve) and note the plate current value there … for our 6JN6 tube, around 380 milliamps or 0.38 amperes.
Now we can re-arrange/simplify our formula again, writing in .38 for "Imax":
0.38 x Eb = 78.125 so Eb = 78.125 / 0.38 and Eb = 206 volts
Trial and error approaches work well for vacuum tube designs, one needn't follow the above procedure to get from "A" to "B". The classical vacuum tube design procedures were usually graphical (and therefore intuitive). Reading through this discussion will hopefully offer some understanding of the techniques and allow one to develop a successful approach that is not necessarily identical to the one that I've taken.
An interesting fact that I've noted in reading old literature regarding tube design, is that the engineers rarely used RMS voltage or current terms; they preferred the use of average voltage and current. There's not a lot of difference between the two distinctions but there are places in this discussion where we will use both terms, so let's understand the difference. Consider a signal (alternating) voltage with an amplitude of + and - 1 volt. We would refer to the amplitude of the signal as 1 volt peak, or more commonly, 2 volts peak-to-peak.
0.000ms 0.500ms 1.000ms 1.500ms 2.000ms 2.500ms 3.000ms 3.500ms 4.000ms 4.500ms 5.000ms
1.000 V
0.750 V
0.500 V
0.250 V
0.000 V
-0.250 V
-0.500 V
-0.750 V
-1.000 V A: v1_1
If we want to convert a peak-to-peak voltage (or a peak voltage) to either RMS or average, the conversions are as follows:
V peak = V rms / (2)0.5
= .707 x V rms V peak-peak = V rms / [2 x (2) 0.5] = .354 x V rms
V peak = 2 x V avg / p = .637 x V avg
V peak-peak = V avg / p = .318 x V avg
So the practical difference between the two terms is some 10% or so. Let's also point out that it's necessary to keep the various units of measurement consistent when making any computations. As an example, the following units are
consistent:
Volts, amperes, watts
Millivolts, milliamperes, milliwatts Microvolts, microamperes, microwatts
The following units are inconsistent and will result in computational error:
Volts, milliamperes, watts Millivolts, milliwatts, amperes Volts, amperes, microwatts
While we are examining the plate characteristics, there a few other pieces of information that we should note. In the legend for the plate graphs, there are two provisions, one reads as follows:
Ec2 = 150 volts Ec2 is the technical abbreviation for screen grid (or grid 2) voltage.
The curves from which we extracted several items of data were measured with the screen grid biased at 150 volts, it follows that if the screen grid is NOT biased at 150 volts, then the information is invalid. Add the screen bias voltage to the information that we're accumulating regarding the design of this stage.
The other note in the plate characteristics legend reads: Grid 3 tied to cathode.
What this means is that the "repellor" (grid 3) must be electrically connected to the cathode to obtain performance similar to the data measured . Let's add that fact to our design information.
Now let's be clear on what the 206 volts represents. It's not actually the plate supply voltage, as one might logically infer, the 206 volts is actually the voltage swing at the plate. If our vacuum tube were perfect, 206 volts would be the amount of voltage deviation between plate and cathode. But if we spend a
moment looking at the plate curves, we see that they are linear over most of the plate voltage range but definitely NOT the entire range.
When the control grid voltage, Ic1, is 0, the plate curve starts to deviate from linearity at about 80 volts. The plate voltage, under maximum drive conditions, can't swing below 80 volts without severe distortion. So to insure linear
operation, we should set the plate voltage to swing 206 volts above 80 volts. The plate voltage simply becomes
Eb = 206 + 80 = 286 volts
This is a very conservative operating point - perhaps a greater change than actually required but let's use it for now. We can always reiterate the estimates based on a lower value of Eb if necessary.
At this point, a brief iteration is required to refine our estimate of Imax. Recall that Imax is the plate current for 0 volts grid bias and at 60% of plate voltage, so we the new value of Imax needs to be extracted from the plate curve at 60% or 286 volts or 172 volts. Referring to the plate curve, Imax = 370 mA.