http://elecraft.85.s1.nabble.com/QRO-KPA500-Failure-tp7658244p7658344.html
One point to consider is the output network that is used to match the output device impedance to 50 ohms. It is a fixed value or ratio. Thus if designed for rated power then at 1/2 or 1/4 power the value would not seem to provide a proper match for effective transfer of energy. This can attribute to overall efficiency.
> On Feb 10, 2020, at 9:38 AM, Doug Turnbull <
[hidden email]> wrote:
>
> Hello Andy,
>
> I do not have the numbers and also would like them. I understand
> that efficiency decreases as one derates from the maximum of 1500W output.
> It surprises me that the power dissipated would peak at half power output or
> 750 Watts. This may be so. If so then almost certainly the Mean Time
> Between Failures or MTBF is worsened. I am not in a position to
> determine any of this but maybe some guidance from Elecraft itself would be
> helpful for both KPA 500 and 1500.
>
> 73 Doug EI2CN
>
>
>
> From: Andy Durbin <
[hidden email]>
> Sent: Monday 10 February 2020 14:04
> To: Doug Turnbull <
[hidden email]>;
[hidden email]
> Subject: Re: [Elecraft] QRO - KPA500 Failure
>
>
>
> Doug,
>
>
>
> "I would like to know if running 2/3, 1/2 or even 1/4 maximum power output
> increases reliability and MTBF."
>
>
>
> I have no experience of amplifier design but I would expect reliability of
> the finals to depend on temperature. Finals temperature will depend on the
> power dissipated in the finals (PA dissipation) and the effectiveness of the
> cooling design.
>
>
>
> To understands finals dissipation one would need to produce one's own test
> data or rely on the work of others. My test data for the KPA500 shows PA
> dissipation peaks at about half power output. So far no one has disputed
> the validity of my test data.
>
>
>
> It is my understanding that running an amplifier at reduced power only
> reduces PA dissipation if the finals voltage is adjusted give a good load
> match for that reduced power condition. The KPA500 has no provision for
> adjusting PA voltage for the reduced power case. For reduced power the PA
> voltage is actually higher than for full power because the high voltage
> supply is not regulated.
>
>
>
> As I said, I have no experience in amplifier design and would appreciate
> comments from those who do.
>
>
>
> 73,
>
> Andy, k3wyc
>
>
>
>
>
>
>
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