Grounded grid 813 amplifier grounded
Over the past several months we have had numerous requests for information on a grounded-grid amplifier using the popular type We are pleased to present this nicely turned out version by W6FLT. The amplifier was designed to be used with any transmitter of the watt output class serving as the driver. In my own case, the driver happens to be the Heath Apache, thus the similarity in panels. The amplifier operates at mA and volt.
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Category: 813 hf amplifier schematic
I've constructed RF power amplifiers since I started in amateur radio nearly 50 years ago, my first transmitters and receivers were all homebrew. My first actual designs were in a technical college electronics lab in the late 60's, where as a lab project associated with engineering courses I designed a grid driven neutralized PA stage. The was quite a large tube for me back then, and it was interesting learning how to use information in data sheets to calculate and estimate component values.
Over the years, especially with the recent decline in vacuum tube quality, I've come to firmly believe the only place to connect the grid in a grounded grid amplifier is directly to ground!
The grid not only shields the input from direct RF feedback from the anode, it is also a good shield to prevent or minimize the anode voltage that might appear on the cathode during tube arcs. Floating the grid above ground is bad for RF, and bad for arc protection. The only thing preventing full anode voltage from appearing on the cathode of the Z is the ground on the control grid. The control grid ground is also the single most critical connection for stability.
The grid connection to ground should always be as wide and short as possible, and use as many pins as possible. An amplifier with the input applied between the cathode and grid and the output between the anode and grid is called a grounded-grid amplifier.
This is true even when the amplifier does not have a directly grounded grid. The grid RF reference point, which is the chassis ground, is the common reference for both input and output power. The input and output circuit of a grounded grid amplifier are connected in series through the tube. Plate current is common through both cathode and anode, and only dc plate voltage is not.
Back when we measured power as plate input power and not RF output power, the FCC even had a rule similar to this. The FCC wanted driver plate input power to be included as a full part of power amplifier plate input power.
Thus kilowatt grounded-grid amplifiers, like the Heathkit SB, when driven by watt exciters could only run watts input if the operator wished to comply with FCC rules. Probably based more on FCC conservatism than actual operation, a few widely accepted handbooks and authorities claim driver power adds to output power via feedthrough and is not accounted for in the metering system. Thinking unclearly, these books propose full driver power be deducted from output power when calculating efficiency.
In actual operation, all extra current contributed by the exciter is fully accounted for in the plate current metering. The only thing not accounted for is a portion of the average cathode-to-grid voltage, which directly adds to the anode-cathode voltage during negative cathode swings.
During positive cathode voltage excursions the gird is more negative compared to the cathode, so the tube cuts off. Since the tube is just "coasting", the positive cathode swing does not detract from effective operating anode voltage.
This asymmetrical tube conduction causes the RF voltage between cathode and grid to contribute to amplifier output by adding effective anode-to-cathode voltage without the additional voltage showing on meters.
If we look at this circuit we can see what that happens, because the anode and cathode are indeed in series! As in all series circuits, current is the same at all points in a mesh or loop. We only have to insert an anode current meter in the anode or in the negative rail of the HV supply to measure the full effect of drive power on plate input power. The metering shortfall is confined to measuring effective anode-to-cathode voltage, since the meter connects from HV to ground to the grid, not to the cathode.
The meter also cannot read the time-varying cathode voltage accurately because it is the wrong type of meter it can't read average voltage and it is connected across the wrong two points. HV is read from grid to anode, while the signal source is in series with the HV supply, adding grid-cathode RF voltage to effective high voltage on negative swings of the cathode. Since the tube conducts heavily during negative cathode swings, that is also when the extra voltage provides the largest contribution to output power.
With the output and input in series, a grounded grid amplifier has large amounts of negative feedback. This negative feedback reduces distortion and stage gain.
The larger the ratio of Zin to Zout and the higher the mu, the greater the stage gain. This means two things influence gain of a grounded grid stage:. The ratio of R2 output load impedance at the anode plus input resistance to input resistance driving impedance of the tube.
The lower cathode driving impedance is, for a given anode operating impedance, the higher the stage gain. The 3CXA7 would fit this category and is a fairly high gain tube with relatively modest anode voltages.
The 3CX series and electrically similar Z are lower gain tubes, even when operating at more than twice the anode voltage of a 3CX The YC 3CPXA7 has a particularly low cathode drive impedance, and thus has very high gain in grounded grid at high anode voltages. Cathode voltage swing depends on the grid to cathode voltage required to move the tube through the conduction desired or required.
Let's assume we have a tube operating in AB1 with a grid-to-cathode bias voltage of volts. As long as the grid voltage is not very much positive with respect to the cathode, the grid-cathode path will not show grid current.
Sine we know the negative grid to cathode voltage is 50 volts, we know the negative cathode swing cannot exceed around 50 volts on peaks. Otherwise the control grid will start looking positive with respect to the cathode. This means the cathode could not have much more than 50 volts negative on peaks. With a sine wave, this is volts peak-to-peak voltage at the cathode. This would take the grid right up to the point of zero current, where any additional voltage would cause grid current because the grid would go positive.
So the cathode voltage would be about 35 volts RMS, or 50 volts peak, or volts peak to peak. When A is positive with respect to B, the grid is more negative with respect to the cathode. This is because Eb adds in series aiding with the voltage from A to B. It is just like two batteries in series aiding at that instant of time. This cuts the tube plate current off, because bias in now volts. When the drive source voltage swings negative, A is negative with respect to B.
This is now like two batteries in series, the voltage across L1 and the voltage across Eb, but opposing each other. At the driver positive sine wave crest, these voltages subtract to zero volts. This is not enough to cause grid current grid does not go positive with respect to cathode but it does cause very high plate current. Note that the more positive direction cathode voltage swing reduces effective screen to cathode and anode to cathode voltages, while a negative swing the same direction that tends to decrease negative control grid bias increases effective screen and anode voltages to cathode!
This works in the same direction as control grid to cathode voltage, also tending to increase or decrease anode current in step with the effects on the control grid. This could be considered positive feedback, although the amount is slight for higher anode and screen voltage tubes higher compared to grid bias.
All plate and screen current flows through L1 in the cathode system. If we have an effective RMS time-varying signal anode current of 1 ampere, the generator source would have to force 1 amp of RMS current to counter act that current.
This is the current varying throughout the RF cycle, not the steady or average indicated dc plate current. If you pay attention to polarities, you will probably be able to see how this works. The anode path RF current is out-of-phase with what grid excitation requires to create that RF anode current. Since the control grid draws no current, we are left only with the out-of-phase anode current and a negligible amount of screen current.
This system requires around 35 volts RMS into the cathode to swing the grid-cathode voltage to the edge of grid current AB2. Thus we would have about 35 volts at 1 amp RMS, or 35 ohms driving impedance. Driving impedance in this case would be 35 ohms, which is 1 amp RMS, 35 volts RMS, and of course 35 watts average power.
From this you can see how the grid bias point necessary for proper bias ties into gain and drive power. If the tube required volts or so bias in AB1, and had the same anode effective RMS current, drive power would double. We would need The RMS voltage into the tank would be the effective one ampere RMS signal current times anode impedance presented to the anode by the tank system and load.
Let's assume that impedance presented to the anode is ohms. From this, we can see load power is watts. It also follows gain is 3dB less if anode current is the same and we double bias, because we have twice the required grid voltage cathode voltage swing for the same anode current when the tube requires double bias voltage. I've neglected minor things like screen current and amplification factor to make this simple.
From this we can seen why gain of any cathode driven amplifier varies greatly with the ratio of anode load impedance and cathode driving impedance. As an interesting side note, this is why the gain of most cathode-driven tetrode amplifiers does not change greatly with or without use screen and grid bias voltages. I learned this lesson when I had a pair of A tubes cathode driven.
They were difficult to drive to full output because cathode voltage swing required to move the tubes through the normal operating load line was so great. In grounded grid without screen voltage, the driving impedance per tube was just over ohms.
In an attempt to improve linearity and reduce drive power requirements, I built a screen and bias supply. These tubes required about volts of control grid bias when screen voltage was applied.
After designing and constructing a very good bias and screen supply, vacuum tube regulated with regulator tubes, drive requirements barely changed. Of course IMD products were reduced greatly because the tubes had much better ratios of screen to control grid current control grid current went to zero mA. The only way to increase gain was to move the high voltage up over volts, which of course would increase anode impedance for the same RF power level.
Grounded grid tetrodes are much cleaner than grid driven tetrodes. This he negative feedback in the anode current flowing through the input system. This is also why we want a tuned input right at the cathode, not three feet or 15 feet away like many assume is OK.
A tuned input a long distance away can look good for SWR and drive power, but it can increase IMD and can put a big bite on efficiency. People who never measure anything but SWR will pipe up and say "I use an external tuner and it works fine" or they don't use a tuned input at all, because they only measure SWR.
When we don't measure or attempt to observe problems, we assume there are no problems.
A low cost 600 watt ultra-linear amplifier
Here's a peek at the "business end" of a pair of beam-power tetrodes, operating as grounded-grid triodes in a Class AB2 linear amplifier at AI2Q. This rig is usually operated on either M or M. A solid-state power supply is built-in. This rig is usually operated on M CW. Output power is W.
Category: 813 amplifier schematic
Sample email asking for donations for death QRZ Forums. With all the emphases on commercially manufactured gear. I was wondering how many people are running homebrew 's. With all the Technician upgrading and considering how cheap these tubes are you would think there would be a lot of interest. If I remember correctly a single is good for about PEP. I guess I will be the first to confess, I have a nice homebrew amp running two 's. I was even thinking of building another one. I guess it's time to do something with all those tubes, I have a drawer full of 's. Ran them at volts and mA. I only built the tank to handle 20 meters.
Metering of a Grounded Grid Amplifier
Post by Don Haworth » Tue Nov 04, pm. Post by philwashere » Tue Nov 04, pm. Post by Rune » Tue Nov 04, pm. Post by Curt Reed » Tue Nov 04, pm. Post by Curt Reed » Wed Nov 05, pm.
Oh no, there's been an error
While the usual approach over the past twenty years or so in the design of a linear amplifier is to opt for a triode [ or triodes ] running class AB 2 in grounded grid configuration, I chose to revamp a design rarely seen these days. This is the once well known G2DAF circuit. It was quite popular with British amateurs, and to a lesser extent, with USA amateurs in the days of rigs with a vacuum tube output stage. For those not familiar with the configuration, a resistor or resistive network of - W replaces the tuned circuit of a grid fed amplifier. Tetrodes or pentodes are used because they have a very low value of plate to control grid capacitance.
813 Linear Amplifier
I am n ot confused. I thought I read that the builder did not want to fool with bias supplies so he is running it as a triode. By the way, I have a 30S1. The 4CXA in that 2m amp is used with 1. However, one problem with these tetrodes is that if both grids have he same applied voltage the control grid will dissipate more power than the screen grid rather than the other way around. Since there is no way to tell how much current each is drawing you can't tell when you are getting into trouble. However, it does look like the amp builder will be operating at a lower power level.
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It is only 11" wide, 4" high, and 9" deep. The amplifier features fast warm up and Watt RF output with Watts of drive. The unit is considerably over designed as can be seen in the pictures documenting the construction [ Hits: Votes: 11 Rating: 6. This, very compact, dual version will deliver about watts output. Covers all bands including WARC bands.
The output stage is an tube, connected as a triode. This amplifier without modifications will allow to use the GM70 and the FU output tubes. The is a general-purpose transmitting beam power tube with. With a thoriated tungsten filament, the was introduced by RCA in News of the new transmitting tube first reached the amateur community in the November. Amplifier Input Transformer: The input impedance of a grounded grid amplifier is typically several hundred ohms.
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