We have all heard the comment that life is too short for QRP. Well, if you are one of those guys with that view, then here is something more than QRP. This was built by me about twenty years ago. It will crank out 1500 watts key down without catching on fire.
Behold a homebrew 20M Legal Limit Linear RF Amp from N6QW. This was a project that had always been a dream of mine and uses a 3CX1500A7. This amp was as much a Mechanical Design as it was an Electronic Design. There is far more involved than glue down pads, ugly construction or an Analog VFO.
First, I should mention it has two PIC16F84 Micro-controllers that provide timing and power control functions. One PIC16F84 is programmed as a 3 Minute Time Delay before you can apply High Voltage to the Plate. Just like Mary Jo -- you got to get it warmed up first.
Main Control Box ~PIC16F84 bottom left.
There is also a step start circuitry on the power supply as there are eight 560Uf @450VDC electrolytic caps on the output of the 10.5HY Filter Choke for an effective capacitance of 70Uf at 3600 volts DC. There is very little voltage sag when you hit voice peaks. If you did not provide a step start at turn on, that bank of caps looks like a dead short. The bias is provided via a low value high wattage Zener Diode (upper left-hand corner).
As the Cap Charges, power resistors are in line so only 1/2 the supply voltage is seen by the plate transformer. Upon being charged the Contactor (120VDC Field Coil) closes, shorting the resistors and full voltage supplied. Step Start!
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| No sign of glue down pads or ugly construction! |
The base of the power supply is 17X17 inches by 1/4-inch steel plate with all of the holes tapped for 6, 8 or 10-32 bolts. This was a complex wiring job as this is High Voltage and all the red wires are 50KV rated. This amp will not work on 110 VAC. It is designed for operation on 220VAC. When I was selling automated industrial controls, I sure learned a lot from the Panel Builders.
The Power Supply delivers 3000 VDC at 1-amp CCS. The power supply alone weighs about 170 pounds.
The 2nd PIC16F84 provides a step start for the filaments so that they are slowly brought up to voltage with about a 5 second delay.
Protective circuitry includes grid protection, so you don't overdrive the amp. If the rated grid current is exceeded it trips offline. Worst case a fuse on the control box blows. About 40 watts of drive produces 1500 watts. There is also a micro-switch on the main amp case lid. If the lid is off the case, the power supply gets turned off.
The output tank is Pi L for increased attenuation of out of band signals. Those copper coils were hand wound by me as was the plate circuit RF Choke. WYKSYCDS.
Red Wires are rated for 50KV. Pure Elegance
Plan view of the amp. Plate cap is 7.5KV rated. Glitch Resistor.
The bottom Plenum Chamber for tube cooling. Upper LH corner has a tuned input. Not well seen at the bottom of the photo is a 1 inch in diameter pipe that passes through the chassis to carry control wiring from the control box on the back apron to the front panel.
Rounding out the features are Vacuum Relays for TR. All Coax runs are outside the Amp box. If you look at the control box along the right-hand side are five 2200uf electrolytic caps that are a part of keeping the blower on for about 20 seconds after power off to clear the amp box of any heat.
Yes, I know that is one fine looking amp build! In case you are wondering -- about $3K in parts -- about $2 a watt.
A lot of thought and planning went into this build and most of it dealt with safety. There is even a glitch resistor, a low value high wattage unit in the B+ supply to the plate. It is mounted on insulated standoffs. and can be seen in the photo taken from above of the main amp hardware. Look along the inside back panel at about midpoint and you will the resistor. Its function should there be an internal short in the tube -- it gets smoked, and the voltage is cut to the tube.
The 3CX1500A7 was purchased as a pre-owned tube but guaranteed for 30 days. I paid about $400 for the tube. That same pre-owned tube today is $800, and a NIB is $2000.
Imagine a homebrew SDR running full legal limit on a homebrew amp! I guess those guys on 40M that were taking about amps in an earlier post would have trouble believing I was running a high power SDR station that was hand built. This is so much fun and homebrew can be so much more than an MC1496 Direct Conversion Receiver!
73's
Pete N6QW