HF Amplifier-Primary


Helpful hints on purchasing an amplifier.

  Buying an HF amplifier can be very confusing.
  The following article will give you enough information so you can choose an HF amplifier that fits your needs. 



10 to 160 meter 3CX1000A7 Home Brew GG Amplifier
(1500 watts output power, all modes)

KU3X-3000


Pictured is my home brew amplifier that uses a single 3CX1000A7 tube in grounded grid. This tube is no longer made but is one of the smallest tubes that can be rebuilt. My low voltage power supply runs 2800 volts key down. At 100 watts of drive power the amp will easily produce 1500 watts out. My higher voltage power supply (shown below) runs about 3300 volts key down and the amp will easily supply 1500 watts out with much less drive power. I tried to keep the Q of the Pi-L tank system around 12. The amp will cover 10 to 160 meters and has tuned inputs. It will cover the WARC bands by putting the tuned input switch in the bypass position and using the tuner in the radio to couple the amp. The plate dissipation of the tube is rated at 1000 watts. With extra cooling the plate dissipation can be as high as 1500 watts, as per Eimac specs. 

The 3CX1000A7 is an old but interesting tube. It’s kind of a cross between an 8877 and a 3-1000Z tube. It was developed before the 8877. It looks something like an 8877 but uses a thoriated tungsten filament like the 3-1000Z uses.  It is a directly heated cathode, like the 3-1000Z so the tube is instant on, no waiting for the filament to heat up like the 8877.

If cooled properly the tube can easily deliver over 2000 watts in grounded grid class AB2. I run my amp around 1200 to 1300 watts out. You’ll never see the difference between 1200 watts and 1500 watts, so why bother? At that power level the tube will last longer and everything runs cooler.




Not shown in the picture, to the left, are the tuned inputs. They were added at a later date. 

The amplifier will also work on 12 and 17 meters but I did not provide tuned inputs for those bands. I can put the tuned inputs in the bypass position and use the tuner in the radio to provide a low SWR between the amp and the radio. I may add tuned inputs for those two bands down the road?......or not.

The tube is biased with a 12 volt 50 watt zener diode shunted with a 500 ohm 10 watt safety resistor. 

The grids are above ground with 6 each 1000 pf door knob capacitors shunted with a 1 mh choke. This type of disign was used by Collins for many year and supplies some possitive feedback. 








The Pi Section of the amp uses a 1250 pf at 1.5KV air variable for a load capacitor and a 1000 pf at 5 kv vac vari for the tune capacitor.

The plate choke is home grown. I used #24 enamel wire, 182 close wound turns on a 1 inch ceramic form. A .01 at 5 kv capacitor is shunted to ground at the base of the plate choke. A small VHF choke was placed in series and also is shunted to ground with a .002 capacitor. There are no self resonant frequencies in any of the ham bands on this plate choke. This amp can also be used on the WARC bands but I did not provide tuned inputs for those bands. The tuned inputs are bypassed when the amp is used on those bands. 

Four each 500 pf at 20 kv capacitors were used as DC blocking capacitors on top of the plate choke.

On top of the tube is an aluminum plate cap. This is not needed for cooling but it made for a nice electrical attachment point. 







I wanted a healthy band switch and I found one. These type of parts are getting harder to find at hamfests.

The Pi section takes the plate impedance from 2000 ohms down to 200 ohms. Not shown in this picture is the 4 to 1 unun that will take the 200 ohms dowwn to 50 ohms. Some people call this Pi-L and some do not? Command, Alpha and numberous other commercial amplifiers use this same idea. One thing this type of arrangement does is make tuning 80 and 160 meters much better. It also makes for a cleaner output. 

The door knob capacitors shown on the bottom left hand side of the picture are load padding capacitors for 80 and 160 meters. I wasn't sure if I would need all of them so I made it that I can easily put them in and out of service with the use of a long screwdriver. It ended up that I only needed one per band but decided to leave them in place. 







The power supply uses a 1 amp swinging choke. It provides 3600 volts to the amplifier. The transformer is rated around 3.6 kva. No load to full load voltage drop is only 150 volts. 

Powering up the supply is done through a 3 second start relay package. Charging those capacitors looks like a dead short from zero volts. A 3 second step start is more than enough time to keep from tripping the breaker or pitting the power relays.

Full wave bridge rectifiers are K2AW 1 amp 14 PIV.

A very small sprite fan moves air across the bleeder resistors. The bleeder resistors load the power supply down to 10% of the full load. This helps with the regulation of the power supply. 

All leads entering or leaving the power supply are filter by either capacitors and / or ferrite beads.

The power suppy is 140 pounds, it's on casters.

All of the high voltage power supplies that I have ever made use the same plugs for the interconnecting cables and are interchangable from one amp to another. This makes it nice if a power supply would ever fail. None have evern failed, but I thought it was a good idea.

Subpages (1): 4-1000 Backup amp