4-1000 Backup amp

4-1000 Home Brew Amp Grounded Grid AB2

(1500  Watts output power, all modes)

This amplifier was put into service around 1992 and was replaced by the 3CX1000A7 amplifier in 2010.

After I had this amp in retirement for 9 years, I put it back in service on a second station. It still works as it did from day one. It's really a work horse and was always a very dependable amp. 

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, including the WARC bands.

The 4-1000 tube is placed 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 small black heat sink shown in the top center of the deck has a 5 volt zener diode mounted on it for biasing the tube. I found it was not needed so it was later removed. At 4500 volts on the plate, the amp keyed, no RF drive, resting plate current is only 100 ma. You will find that even up to 5 KV on the plate, no biasing is needed. I have a spectrum analyzer and check the IMD. This amp is well within FCC requirements for both IMD and harmonic radiation, with room to spare.

The filament choke was made by Barker and Williamson. I now make my own that offers more choking resistance on 160 meters but there would be no reason to swap these  chokes. The amp runs smoothly and I have no stray RF issues.

I love the smell of burning electrons in the morning !

This power supply uses a power transformer that has four taps to set the output voltage.

Output voltages can be set from 3800 volts to 4500 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 4500 volts no load and drops to 4000 volts key down. With 100 watts of drive from the exciter the amp can produce 1500 watts out on all bands. A step start relay will close after 2 seconds.


All high voltage power supplies that I have ever made use the same type 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.