The grounded grid configuration is perhaps the easiest to build, and operationally most stable RF power amplifier design available to the home builder.
Among it's attributes are the following:
Comparatively low (60-150 ohm) input impedance.
Input signal is 180 degrees out of phase with the output signal, virtually eliminating the potential for self-oscillation and tube take-off.
May be build around zero bias triodes and tetrodes, eliminating the necessity to design and construct a separate grid bias power supply.
Operationally pass the power in excess of the drive requirement to the output circuit.
Extremely easy to construct and operate.
|The operation of many power triodes and tetrodes in grounded grid is relatively simple to understand. Like the grid driven configuration, B+ is applied
to the plate and B- to the cathode. Unlike the grid driven amplifier, however, both the grid and cathode are at DC ground potential (which also means that the B- terminal of the supply is attached to
ground). To get a better idea of what this looks like consider the diagram on the left.
The tube illustrated is a tetrode. What's most odd is that both the control and screen grid are strapped directly to ground. In a conventional amplifier circuit, the screen grid aids in maintaining high current levels, despite fluctuations in the plate voltage, when a constant voltage is applied to it. In grounded grid, this function is no longer necessary. Consequently, it is simply tied to the control grid, which raises an interesting question. If we don't need it, then why use a tube with a screen grid in the first place? And the answer - because that is what we have available. True, it would be preferable to employ say a 3-500Z, which in many respects is an equivalent triode. But for now just think of it this way - we went to the junk box to see what we could find. There weren't any 3-500Zs, but there was a 4-400C - so we used that instead. Actually, a section is devoted to the selection of tubes, as well as the uncanny nature of supply and demand.
Turning back to the diagram, note that the cathode is common with the heater - in other words, the filament acts as both heater and cathode. The cathode/filament is heated by application of 5 VAC from the Filament Transformer. At the same time, the operational cathode is at DC ground potential via the centertap of the filament transformer. This is not to say, however, that it is at ground potential for RF - the bi-filar choke between the transformer and the tube raise it above ground for RF, but permits it to remain at ground potential with respect to DC.
In the case of a 4-400c, with no signal applied to the cathode, an idling current of approximately 35 ma will flow between the cathode and plate. When a signal is applied, peak current will flow from the cathode to plate during the negative half of the input cycle, and will reach its lowest level at the plate during the positive half of the input cycle. This places the input and output signal 180 degrees out of phase and prevents coupling between the output and input stages, thereby eliminating much of the potential for self oscillation.
When selecting a tube for grounded grid service, it is important to opt for what are known as high mu tubes. The term mu refers to the interelectrode relationships that determine its ability to amplify a signal. A high mu tube will generally have a high amplification factor and a low mu tube an insufficient amplification factor for grounded grid service. Although a further explanation of mu might be interesting, it would carry us far beyond the goal of simply laying out the basics necessary to design and build a reliable RF "work horse". Suffice to say that a high mu tube, or a tube with a high amplification factor will be required. Further, all of the triodes and tetrodes detailed in the tube selection section of this site provide the degree of amplification necessary for grounded grid.
For our purposes here, I have selected a single 4-400c as our tube of choice. From the tube selection guide, the filament requires 5.0 VAC at 14.5 amps. The maximum plate voltage is 4,000 volts DC, and with typical operation in grounded grid at around 3,000. The maximum plate dissipation listed is 400 watts. Since we will be running Class Ab, the plate will be required to dissipate approximately 35 percent of the power consumed in the process of amplification. To arrive at the amount of plate current we can expect, we must first arrive at the total power input to the tube. We can do this by 400/35*100, or just around 1,150 watts in. By dividing our power in by the plate voltage, we arrive at a current of .350 amps, or 350 ma. It is important to understand at this point that with a given plate voltage, the load we place on the tube will determine the amount of current it draws, and as such the amount of power that will be dissipated by the plate and the amount of power that will be transferred to the antenna. Our goal is to maximize the amount of power we are able to deliver to the antenna without melting down the tube in the process. Consequently, our design load becomes relatively important.
Single sideband and CW service are both intermittent - in the case of SSB syllabic in nature. By this I mean that neither requires a solid,
uninterrupted rf carrier for any prolonged period of time. There is an average that takes place - half above the tubes maximum rating and half below. Consequently, we can load the tube to deliver
twice the amount of current at signal peaks, understanding that the lulls will fall far below the tubes dissipation ratings. In this way, the tube will the average amount of power the tube will be
called upon to dissipate will not exceed its ratings. Therefore, the formula for computing the best load for our amplifier is as follows:
In the case of our 4-400c, our best load - that which will transfer the maximum amount of available power to our antenna - is 3,000 volts divided by .7 amps (2X350ma), or approximately 4,300 ohms.
Armed with the RF plate load resistance for our particular tube, it is now possible to refer to the pi or pi-l network chart to determine the values we
will need for C4, C5, and L1 (and L2 for pi-l) for each given band we intend to operate on.
From the pi net table in the Output Network section of the site, the values for C4, C5, and L1, for operation on 80 meters are 100 uuf, 680 uuf, and 42
uh, respectively. Since all the other bands we intend to operate on are above 80 meters, these represent the maximum values that will be required to construct the output network. A transmitting type
variable with greater plate spacing should be employed for C4, whereas, a receiving type variable capacitor will be sufficient in most instances for C5.
A pi-l net, such as that above, may also be selected. The values for each component are available in the Output Network section of the site, as well. The advantages of a pi-l net are its ability to provide a greater transformation ratio (feed line impedance), and provide better harmonic suppression. Keep in mind, however, the voltage handling capacity of C5 will be somewhat higher with the addition of L2. Consequently, a transmitting type variable should be used for C5.
Now for a little cleanup - referring to the diagram below, note that the bifilar choke isolates the cathode/filament from the filament transformer, as
well as aids in preventing RF from entering the DC circuit from its feed point at C1. Note, also, that Cbp between each side of the bifilar choke and ground, on the choke's transformer end, are
intended to bypass any RF that appears on that end of the choke to ground. A nominal value for capacitors Cbp is .01 mmf, and may be disc ceramic. C1 is the cathode capacitor, and C2 serves to
equalize both sides of the filament with respect to RF. The nominal value of these capacitors is .01, although silver micas are preferred. C3, the plate blocking capacitor, should be in the
neighborhood of 500 uuf, rated at 2.5 times the plate voltage.