| Amplifier Rear Panel: The rear panel of the amplifier contains connections for the power supply, meter, and control circuits. The connector at the upper left supplies 117 VAC for the filament transformer. The BIAS adjustment just to the right sets the operating bias. The two SO-239 connectors in the middle of the rear panel are for RF INput and OUTput. To the right of them are the High Voltage (HV) and ground (GRND) connections for the 2000 volt plate supply, which is fed to the amplifier through a coaxial cable. The connector at the upper right (MTR) is for the meter panel, and the connector at the lower left (CTRL) provides connections for the bias supply and control circuits. The antenna changeover relay is visible on the top of the chassis at right |
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| Amplifier Power Supply: The power supply for the amplifier is a real beast. It is a beautiful example of engineering to make use of what you have. The plate supply consists of three full wave, choke input power supplies run in series to obtain the 2000 volt plate potential needed for the 813. One supply has an output of approximately 450 volts, while the other two full wave supplies each have an output of 800 volts. The two 800 volt supplies have dual primaries which can be run in series or parallel, as determined by a front panel switch. This permits operation at reduced plate voltage for tune up purposes. The 450 volt supply can also be used as a screen grid supply, though in this amplifier the 813 screen grid is connected in parallel with the control grid and the tube is run as a high-u triode. A regulated bias supply is also included. This is only used to cut off the tube in standby mode, since in operate mode the 813 is run with zero bias. |
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| Amplifier Power Supply Underside: Since the amplifier power supply is actually four power supplies on one chassis, it gets quite crowded underneath. The 450 volt supply is at the upper left, and the bias supply is at the lower left. The two 800 volt supplies occupy the right side of the chassis. The 450 volt supply and the two 800 volt supplies are connected in series to proved the 2000+ volts needed for the 813 plate. White teflon insulated wire is used throughout the power supply. The circuit board at left is the rectifier stack for the 450 volt supply, and the two circuit boards at right are the rectifier stacks for the two 800 volt supplies. All of the power supplies, except for the bias supply, use choke input filtering, which provides much better voltage regulation than capacitor input filtering. |
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| Amplifier Power Supply Filter Capacitor and Rectifier Stack Close-up: The four 800 volt supply filter capacitors (blue) and their associated bleeder/equalizing resistors (two capacitor/resistor combinations for each supply) are visible at the left in this photo. The two double-sided circuit boards at the right are the rectifier stacks for each supply. For each 800 volt supply, 14 silicon diodes (seven for each side of the full wave center-tapped circuit) are strung in series, each shunted by a 240k resistor to equalize the reverse voltage drops. The original diodes were surplus, unmarked, "top hat" style diodes, but several of these failed after a short time. They were all replaced with modern 1N4006 diodes. The newer diodes have much larger PIV ratings and much larger peak current capabilities. |
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| Amplifier Underside: Compared to the power supply, the amplifier is very sparse underneath. In this photo the rear of the amplifier is to the left. The bottom of the antenna changeover/bias relay is visible at the upper left, and the bottom of the filament transformer can be seen at the lower left. The filament RF choke is the long coil at the bottom of the photo. The mode switch, which is mounted on the front panel, is visible at right, and just below it are the green and red standby/operate indicator lamps, also mounted on the front panel. Output from the plate tank circuit is taken through a feed-through bushing visible just behind the mode switch, and a coaxial cable carries the output to the changeover relay. |
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| Amplifier Tube Socket Close-up: Circuit wiring must be done carefully in amplifiers to avoid parasitic oscillations. Although the tube in this amplifier operates between 7 and 20 MHz, it can (and will) oscillate on spurious frequencies if given the opportunity to do so. In this view of the tube socket, notice how the grid bypass capacitors (the two, blue, dipped capacitors) are connected with very short leads to the tube socket pins and then grounded directly at the tube socket mounting lugs. Also notice how the G3/BF pin (for the beam forming plates) is grounded directly to the tube socket mounting lug. Such connections keep stray inductance to a minimum and help to avoid parasitic oscillations. The large brown capacitors on the left side of the tube socket connect the RF input to the cathode/filament of the tube. The large filament RF choke at bottom permits the filament voltage to reach the cathode/filament, while preventing the RF from flowing back to the filament supply. |
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2E26 Exciter
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