[Elecraft] KPA1500 issues/blown finals

[dyno lab]

An independent engineering study has identified internal electrical stress conditions within the Elecraft KPA1500 HF power amplifier that occur during normal operation at 1500 W output. The analysis, based on oscilloscope measurements, spectrum analysis, LTspice modeling, and PCB layout review, shows that the PA Output Board experiences elevated drain voltage peaks and significant RF ripple on the 50 V supply rail.
Testing into a 50 Ω dummy load indicated drain voltage peaks exceeding 150 V and RF ripple greater than 40 V peak to peak on the supply rail. These effects are attributed to an inadequate FET drain choke, insufficient center tap bypassing, and inductive PCB traces on the PA Output Board layout.
The high drain voltage peaks far exceed the BLF188 maximum drain voltage rating of 135 volts which can cause "Hot Carrier Injection" and FET failure under normal operating conditions. And the very high RF ripple on the supply rail causes extreme drain voltage distortion resulting in high harmonic output and intermodulation distortion.
The internal stress mechanisms may therefore affect long term device reliability when operated continuously at full power. The study recommends everyday operation in the 800–1000 W range, with full 1500 W output reserved for short SSB bursts. High duty cycle digital modes such as FT8 and RTTY are best operated at 600–800 W.
Test data available upon request.
Hal
W7YNC

> On 02/15/2026 9:32 AM PST dlrwild1--- via Elecraft <elecraft at mailman.qth.net> wrote:
> 
>  
> Does anyone have knowledge of what is causing 15+ repair returns of the $7K KPA1500 amp?
> Bob WildermanK3SRO
> 
> 
> Sent from the all new AOL app for iOS
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