Hi Chris,

At PS side of the choke, no RF behaving or misbehaving.

At output transformer side of the choke, mainly negative-going
pulses which become distorted similarly to the output when misbehaving.

Drain waveform is like a sine wave with regular shaped positive
half, negative half is double peak. When misbehaving, same basic
shape but blurry just as the RF output is... looks as if amplitude
constantly changes from cycle to cycle, as though modulated by a
lower frequency.

Gate waveform almost a pure sine wave (remember this is a class
AB/B/C amp), develops amplitude blur or modulation when misbehaving.

My "gut" tells me this amp is oscillating at a frequency well below
the band (maybe at VLF) but only when feeding the antenna. Perhaps
the fact the antenna is a very reactive load at VLF sets it off? I
don't know why it would happen at different RF power levels with 13V
/ 19V PSU unless it is proportional to current somewhere...

I'm about ready to take this amp down to the train track and strap
it on a rail... might be rather satisfying. ;-)

Paul N1BUG



On 12/29/2017 06:26 AM, Chris Wilson wrote:
>
>
> Hello N1BUG,
>
>
> What does the scope show if you look at each side of the choke in the
> PA? I did some tests ages ago using the stacked chip caps on the PA
> side of the choke compared to conventional leaded caps and the
> difference was marked. I would certainly try a proper dust iron core
> for the choke, ferrite can't be right... I don't use a LPF after the
> U3S but my amp is "Class D" and is happy with being driven with square
> waves.
>
> Friday, December 29, 2017, 11:13:03 AM, you wrote:
>
>> One final thing.
>
>> Thought maybe something in the antenna was breaking down (arcing or
>> corona), so made another check:
>
>> I now have the capability to run the PA on 13V or 19V. At 19V I can
>> make more power into the antenna before the signal goes dirty,
>> compared to 13V. RF power is RF power. If it was something failing
>> in the antenna system I think it should happen at the same RF power,
>> no matter what DC supply voltage I use on the PA...
>
>> Paul