[Amps] new 2200m/630m amplifier - distorted waveform output

[Roger Graves]

I would like advice on a new 2200/630m amplifier that I have built. It works but exhibits two problems.

It uses a pair of APT5023 mosfets, 48Vdc supply, output transformer 3T:6T on a pair of 26mm type 75 tube cores to make a big binocular core, DC feed via a separate center tapped choke, biased for AB (or maybe closer to B as the bias is only 0.3A). On 2200 and 630 my SoftRock RXTX with 1W output provides sufficient drive to drive it to the start of flat-topping at about 200W output. I have been running it at about 100W output.

I have two questions that I would like advice on.

1) When PTT is asserted (with no RF drive), which results in a relay applying the gate bias and doing the antenna change over, there is a turn on transient visible on an oscilloscope. There are two short bursts of oscillation (at max power) each at about 100 kHz. This is over by about 1 msec after which it seems stable. 

Should I worry about this? If so, can you suggest a cure?

2) The output waveform is close to a sinewave (with no LPF) when driving a 50 Ohm dummy load. However, when driving my antenna, especially on 137 kHz, the waveform is distorted. On 137 kHz the antenna load is 50 Ohm and close to resistive (at 137 kHz) as best as I can measure. I tried inserting a 1:1 balun at the output of the output transformer, which had no effect. A LPF cleans up what comes out of the filter nicely. However, the input to the filter (amp output) still looks distorted in the same way. I am using a T format LPF which I believe is the correct configuration for the amp output stage which is (correct me if I am wrong) voltage feed b/c of the center tapped choke DC connection. The input drive to the amplifier looks like a nice sinewave in all conditions.

Do you see a problem operating the amp into a T (inductor input and output) LPF with the distorted output (voltage) waveform? It seems to be running reasonably cool so I don’t see an overheating problem. I would prefer to see a clean output from the amplifier and would like to operate with linear modes occasionally so the distorted waveform leads me worry about IMD (I have not run IMD tests).

Oscilloscope screenshot attached. 

BTW, I tried using a center tap on the output transformer instead of the separate choke feed. The result was a badly distorted waveform into the 50 Ohm dummy load on all bands. (It works on 160 also, but the Softrock can only drive it to 50W.) I don’t understand why the center tap method resulted in such a different output.

Thanks for your help.

73,
Roger

Output waveform when connected to antenna matched to 50 Ohm at 137.4 kHz.