AM-6154-small.JPGNote/update: with a solid mains power supply, undistorted 700W can be achieved!

In 1992 I bought a surplus AM-6154 amplifier from departing US-forces in Darmstadt (Germany).
The amplifier was intented to serve as a backup during 144 MHz IARU-contests of PE0MAR/p, now PA6NL.
And, it did for one contest. The second contest the amp delivered only 200 Watts and died quietly, and was subsequently stored for almost 15 years with the mark: “To be repaired”.

My intention was to revitalize the amplifier. Not because of the power (only about 400 Watts output if I remember it correctly) and/or urgent needs. No, it is because of the fact that having 25 kg of worthless iron scrap is of no use at all. This year I found the time and patience to investigate the AM-6154 amplifier and got it running again.

This blog entry is not supposed to be an extensive story about all the modifications I made on this amplifier. However, I will try to describe some experiences during the repairement of this nice (and heavy!) amplifier.

Normally AM-6154 amplifiers are equipped with an Amperex DX393, an equivalent of the Eimac 8930, which is a ruggedized version of the well known 4CX250B. After several ‘reconditioning’ experiments I had to conclude that my DX393 was really beyond reconditioning and had to be discarded.

It was my intention to use the Russian GS36B tetrode as an alternative for the DX393. Although publications on the net suggest that a 1 on 1 replacement is possible, this cannot be true due to the physical dimensions and electrical characteristics of the tube. The anode diameter of the GS36B is ca. 1 mm smaller than that of the DX393.
I compensated this with several layers of copper foil around the GS36B anode until the tube fitted well into the silver fingerstock anode ring of the AM-6154 cavity.

The control grid voltage (Vg1) had to be modified. I replaced the original 100V/1W zener diode VR1 with two 39V/1W zeners in parallel. I determined from the datasheet that with Va=2000V and Vg2=390V, the GS36B needs about -45V to draw 50 mA of anode current.

The GS36B tube draws more screen (G2) current than the DX393 (which may have non linear negative screen currents at certain stages). The Vg2 dropped significantly (390 -> 350V (!)) at larger output levels. I shortened one of the 100K/100W resistors in the HV power supply to increase the zener current to ca. 16 mA (2000 - 390 / 100000 = 16 mA). This eliminated the Vg2 dropping almost completely and increased the output considerably.

I considered the G1 input circuit far to complicated and modifications, as proposed on the internet, too ’sloppy’ and chose for a brute force approach. I substituted the original 1/4 wave inductor for a 1/2 wave contrapsion.
For further information look here.

With ca. 3.5 Watts of drive the amplifier draws about 450 mA anode current (idling current = 50 mA) and produces solid >550W output, as measured with a Bird 43 and both a 1000C and 1000D slug. Here you can see and hear me tuning the amplifier (.MOV, 19.2 MB).

Download the manual of this AM-6154 amplifier.