4-1000 Home Brew Amp Grounded Grid AB2
(1500 Watts output power, all modes)
This amplifier was put into service around 1988 and was replaced by the 3CX1000A7 amplifier in 2010.
It covers 10 to 160 meters including the WARC bands. It has tuned inputs for the normal bands but the tuned input switch
can be placed in the bypass position when using the amp on the WARC bands. The tuner inside of the radio is used to couple
the amplifier for the WARC bands.
A 750 PF vacuum variable is used for the tuning capacitor and an air variable is used for the loading capacitor. Using a vacuum
variable capacitor for the the loading capacitor may make the amp look nice but it just makes changing bands much slower.
A Pi network with a Q of 12 was used for this amplifier.
The cabinet size is 17" wide, 17" deep and 14" high. It is built on a 17" by 17" by 4" Bud chassis.
To the right of the main tank coil is a small trim capacitor mounted on the inside wall of the cabinet.
This capacitor is used to tune out the stray reactance from the antenna changeover relay.
The tube is cooled by a Dayton Blower mounted on the rear of the cabinet.
The plate choke is home made and does a great job on all of the HF ham bands.
The 4-1000 tube is place at DC ground with copper straps.
The antenna and bias relay is enclosed in a small Bud Box seen on the left side of the amplifier.
The filament choke was made by Barker and Williamson. I now make my own that offers more choking reactance on 160
meters but there would be no reason to swap these two chokes. The amp runs smoothly and I have no stray RF issues.
This power supply uses a power transformer that has four taps to set the output voltage.
Output voltages can be set from 3500 volts to 4400 volts with capacitor input. The transformer is oil filled with a 3.25 KVA rating. I used a full wave center tap with 20 mf of capacitance.
The supply voltage is set for 4400 volts no load and drops to 4000 volts key down. With 70 watts of drive from the exciter the amp can easily produce 1500 watts out on all bands. A step start relay will close after 2 seconds.
This is now a backup power supply for the 3CX1000A7 amplifier. To use this supply on the 3CX1000A7 amp the supply
voltage is lowered to the 3500 volt tap.
All high voltage power supplies that I have ever made use the same plugs for interconnecting the power supply to the amplifier and are interchangeable from one amp to another. This makes it nice if a power supply would ever fail. Not that one has ever failed, but I thought
it was a good idea.