Jurgen PE1LWT (see right, click on picture to enlarge), bought a second AM6154 for me in Friedrichshaven last June. It looked like a virgin and the first challenge was to power it up. The first result was that we did not experience a big bang with corresponding flashes, also not after powering up the 2.4 kV plate voltage. Because this amplifier was originally designed for aeronautical AM (amplitude modulation), the amplifier was immediately set in ‘idle’ mode.
While approaching the amplifier, we saw that the plate current was ‘ticking’ as well as the screen voltage. We first thought it was the electrolytic capacitor in the screen power supply. It turned out that the screen (G2) glass overvoltage protector was arcing. My philosophy is, remove as much protective devices as possible because they cause and create more damage than they protect!
So we did.The amplifier was left into idle mode during a coffee break and my cat jumped on it, almost got electrocuted because his tail was hanging in the high voltage power supply. Besides the activity of the cat, we did not hear strange noises, nor could we smell strange odours from the amplifier.
So, the next step would be to apply some drive. We applied a little drive to the amplifier, tuned it and soon measured 100W output.
After tweaking and tuning we reached 300W with almost 12W (!) of drive (see right, click on picture to enlarge). We expected a bit more output, but, we left the amplifier ‘key down’ at 300W during the evening. This resulted in a VERY HOT Spinner dummyload. But also it demonstrated that the amplifier was stable with this output and… it is a quick way to recondition a tube!
My other amplifier, with a GS-36B inserted, delivers almost 3 dB of output more. So, it was time to modify the amplifier and make it ‘contest ready’.
The input circuit was completely modified according to the FYM-tetrode match. Perhaps for reasons of (in)stability, but I cannot understand why the standard input circuit of the AM6154 is designed so inefficiently. After this and some other modifications, the amplifier delivered a stable 500W with 2.2W of drive power. A good backup amplifier for the contest!!