[Elecraft] Upper limits of KPA500?

[Dave Cole]

Thank you for a very thorough, and cogent, explanation of that.

73, and thanks,
Dave (NK7Z)
https://www.nk7z.net
ARRL Volunteer Examiner
ARRL Technical Specialist
ARRL Asst. Director, NW Division, Technical Resources

On 5/7/20 7:12 AM, Ignacy wrote:
> NK7Z wrote
>>
>> Initial testing here indicates no power level is safe...  At 150 watts
>> the KPA500 reaches 70C pretty fast, in under two minutes, and 70C is my
>> upper limit for testing...  At 500 watts it reaches 70C in about the
>> same time...
>>
>> Can I assume the amp is not the most efficient thing in the world at 200
>> watts?
> 
> Correct!
> 
> Up to a certain limit, the output power increases with PA current
> quadratically. So the maximum dissipation may be below the max power, and
> decreasing power does not reduce dissipation much.
> 
> Assume that at 600W KPA500 operates at 60% efficiency. 1000W total, 400W
> dissipation.
> Now reduced the power to 1/4 or 150W. The input current is reduced 50% to
> 500W. The dissipation is 350W, not much below 400W at full power. With
> KAP500, the dissipation at low power can actually get worse than predicted
> since the power supply is unregulated and has higher voltage at lower
> current.
> 
> Reduction of dissipation at low power can be done at least 2 ways. One is
> reduction of voltage as done in SPE amps. The other one is changing the the
> transformation ratio of output amp, e.g., in KX3; hard to do at high power.
> 
> One can have the action of a different transformation ratio with a manual
> tuner. Once my 20A power supply died but I had a 5A supply available. But
> the max power was only 5W before the power supply switched off. 1/4 curent =
> 1/16 power. By adjusting a manual tuner for max output I got 20W. But KPA
> may not let this trick go because of SWR. protection.
> 
> Ignacy, NO9E
> 
> 
> 
> 
> 
> --
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