This page documents Jim's homebrew linear amplifier. The amplifier utilizes (2) Eimac 3CX800A7 tubes. It covers 160 trough 10 meters. The amplifier is all self contained in a desktop package. The amplifier is used on a daily basis and is used on SSB, RTTY and CW and puts out 1500 watts with little effort even during RTTY contesting. The smooth and stable operation of the unit makes it a joy to operate and the best part; it's homebrew!
The cabinet is from Byers Custom Cabinets. The front panel was manufactured by Jim and all of the cutting was performed by hand. There was no template or pattern used. The meter cutouts were cut and filed by hand. The meters are from a Command Technologies Commander amplifier. They were relatively inexpensive and because the Commander uses the same tube makeup, they were just the ticket! The tune and load knobs are 6:1 ball drive type and were purchaced from a Ten Tec. They make for very smooth and repeatable tuning of the amplifier.
The reset switch if for resetting a fault condition that has occurred during operation of the amplifier. Fault and overdrive protection is handled by a Triode Board from G3SEK and is available from: www.ifwtech.co.uk/g3sek This board is just exactly what the doctor ordered for a home brew amplifier such as this that uses expensive triode tubes. I'm confident that the protection offered with this board will provide years of reliability for the valuable 3CX800A7 tubes.
This picture shows the power supply an overall view of the amplifier. The high voltage transformer is a Peter Dahl unit. As you can see, there are just 7 filter capacitors as the plate voltage is approximately 2700 VDC. You can see the high voltage step start relays and associated resistors mounted on the inside of the rear pane. The circuit board in the upper right corner of the power supply is a Triode Board by G3SKA. The Triode board encompasses the following:
The small transformers mounted near the filter capacitor bank are for filament (lower transformer). This is a Radio Shack 12.6 volt 3 amp transformer that has had some turns added to bring the voltage up to 13.6 Volts for the 3CX800A7 tubes. The upper transformer is a 24 volt unit for control circuitry on the Triode Board.
The small "PERF" board that is near the plate transformer contains the metering resistors for the plate voltage meter as well as a high voltage divider to signal the Triode Board that the plate voltage is present. The small pot on this board is to calibrate the high voltage meter.
Shown is the RF side of the amplifier. The Pi coils, band switch, L network torrid and the tuned input board are from a Ten Tec Titan 425 amplifier. The tune capacitor is actually two separate capacitors. The small unit is used on 10, 15, and 20 meters. The larger tune capacitor is switched into the circuit on 40, 80 and 160 meters. With the small amount of tune capacitance on the higher MHz bands, tuning is VERY smooth and easy. The load capacitor is below the tune capacitor and is hardly visible.
The two Eimac 3CX800A7 tubes with their associated chimneys are hard to miss with the bright blue color. The chimneys are made out of silicone based turbo charger hose that was purchased at a local hose and fitting distributor. The hose comes in many different sizes and is easy to work with. Itís also very rigid. The hose makes a nice tight air seal on the underside of the top cover.
The antenna change over relays are located in the far left of the picture. The relay enclosed in the black box is a T/R relay from a 100 watt VHF Motorola Micor mobile radio. The relay is actually a reed type relay and is used for the input side of the circuit. This relay is VERY fast switching. The black box is the coil that slides over the housing of the relay. The coil voltage is 12 volts DC. A 150 Ohm dropping resistor is in series with the coil as the relay control voltage is 28 volts. The other relay with the blue band around the base is a Kilovac HC1A vacuum relay with a 28 volt coil. This vacuum relay is used for high power output power switching.
Some detail on the cooling system:
All of this air is drawn into the blower, through the anode cooler fins on the tubes and then exhausted out the top of the amplifier. You can put your hand on top of the amplifier cabinet when running 1500 watts on during a RTTY contest and it's very cool. The heat does poor out of the exhaust ports over the tubes.
There are no special parasitic suppression measures taken in this amplifier. The supressers consist of (2) 100 Ohm carbon resistors in parallel across a piece of 3/8" silver plated copper braid. The braid then is secured onto the anode of each tube with a small stainless steal hose clamp. The orange plate blocking capactitors are 500pF rated at 20Kv each. I have not observed any excessive heating during prolonged transmit even running 1500 watts on RTTY.