~ Updated 1/23/2024: Under Construction…some thoughts about Linear Amplifiers ~
Many articles about RF amplifiers have been published and it’s impossible to list them all here, so I’m going to pick and choose some I like. Mostly, I gravitate to what can be built from the junkbox or scrapyard and I tend to concentrate on that. In other words, I’m cheap.
Most information here will concern Grounded Grid (GG) amplifiers, due to their simple design, ease of construction and because most of us have an 80 – 100 watt transceiver with which to drive a GG unit. Properly driven, a GG linear amp amplifies the input power by a factor of 10, which means your signal is theoretically twice as loud.
If you are a QRP operator and have only a QRP unit to use as a driver, you probably don’t want a big linear amp anyway, but if you do, the old 813 can be driven with only about 5 watts – but not in GG. Search the web for inspiration – there’s lots of it.
It does not matter how the watt is radiated; from a $25 813 tube or a $2,000 solid-state class D amp – the watts sound the same on the receiving end. Amplifiers aren’t so much the great equalizers – antennas and operating skill are.
My favorite tube: The venerable 813 tube was first developed in 1936 and was so robust and useful that it is still in production, 87 years later, in China and Russia. They are inexpensive; I have two NOS American ones with graphite plates bought for $25 each and one NOS with a steel plate given to me. Compare those prices to 572’s or other tubes! Use the graphite-plate ones rather than the cheaper steel plate tube. The graphite ones are more robust.
Previously, I had put together a 2 x 813 linear but it contained power supply and filament xfmrs and it was too heavy for me to easily move it around (I’m almost 83). So I’ve taken it apart and will rebuild it into two separate ventilated HeathKit cabinets (P.S. in one and RF deck in the other). I’ll put some photos of that here “soon.”
One 813 can easily provide 500 watts input and grounded-grid circuits provide a simple, stable design. Here’s a tube data sheet, courtesy of Dr. Greg Latta, AA8V.
813’s are rated for a 2500 volt plate supply but units have been run for years with 3000 volts or more. There is no discernable difference on the receiving end as the result of using more voltage so just go with 2500 – 2800 volts and the tubes will last 20 or 30 years. I doubt that any difference can be noticed on the receiving end, whether 2200 or 2800 volts is used.
A Grounded Grid linear amplifier means ground the grids! Use the shortest, widest leads possible to ground them. Don’t strap them to ground through an 0.01 capacitor. W8JI presents excellent information about GG amplifiers and it’s well worth studying.
Here’s an example of what is probably the simplest dual-tube, 1000 watt design, using the 813 tube as an example. If you want only 500 watts input, just use a single tube and adjust the filament supply if needed. The “Z” in the signal input line is just the usual resistor/coil arrangement commonly found in the plate lead. If the unit is built properly, a single Z in the input will suppress transients, but feel free to put one in each plate lead if you prefer. 813’s are quite stable in GG circuits.
Here’s an example of a power supply to use with the above 2 x 813, 1 x 813 or the Lew McCoy design (below). It uses two discarded microwave oven transformers (both of approximately equal size & ohms in the secondaries) to provide HV. You could use only one but it will work harder. For the 500 watt version, one would certainly be enough.
The Lew McCoy single-813 design can be found here. Note that you can ground ALL the grids directly and skip the fancy metering scheme in this design. For plate volts, measure across the final resistor in the equalizing string as he has done. For plate MA, put the meter in the center tap of the filament transformer. This way you can have two meters or, if only, one, use a switch to change it from Volts to MA. Or use his scheme.
Here’s another, nice clean design.
Here is a link to an alternative design that eliminates the need for a filament choke and provides some input tuning capability (see below for more about filament chokes).
Here’s an excellent and extensive article about pi networks for RF amplifiers. Even though written in 1972, the simple math and the tables provided in this Sept 1972 Ham Radio issue are as valid today as they were then.
If space is critical, look at this very detailed article about using a T200-2 iron powder core toroid for the pi network coil.
An inexpensive power supply for a grounded-grid amplifier can use two repurposed microwave amplifier transformers (MOTs) for the HV supply. A revised circuit, showing how to use half the B+ voltage for tuneup purposes, will be added soon.
A single MOT can also be used. Be careful, as the xfmr laminations may be at a high voltage. Put a thermistor or a soft-start device in the primary winding circuit or the power supply might blow a breaker at turn-on, if it turns on at exactly the wrong point in the 120 VAC phase.
The 813 requires 10 volts at 5 amps (50 watts) for its filament and a way to provide this must be found. A 10 volt transformer can be purchased or a microwave oven transformer can be rewound. If you rewind one, put a thermistor in its primary as with the HV one.
The transformer from a discarded UPS unit could be used (run backwards) with slight modification or with a suitable dropping resistor or a ‘bucking’ transformer in the primary as these would provide about 12 volts or a little more, under load.
Filament Chokes: The old B&W FC30 or FC30a filament chokes are excellent, if an affordable one can be found. The Heath SB-200 used a 10 uh choke wound on a ferrite rod but this ran out of steam on 80 meters. More uh is better; say 12 -14 uh, or even 20.
A classic homebrew solution is to wind a bifilar choke on a ferrite antenna rod. A good one would be 2 x 26 turns on a 9.5mm diameter rod, with the winding using about 100 mm of the rod. There are many Google references to this.
If space is tight in the amplifier or if a more modern approach is desired, use a toroid. An FT-240-61 would do nicely. Use the “FT” calculator in the next paragraph to calculate the number of bifilar turns needed (hint: 10 or 12 would do; 14 is even better).
Or, if space is tight, refer to the T200-2 article listed above.
Calculator to find the inductance or calculate the number of required turns on a ferrite core FT-NNN-NN toroid.
Calculator to find the inductance or calculate the number of required turns on an Iron Powder toroid.