[Elecraft] Question Regarding KPA-500 on 117 VAC

[David Olean]

Hello Bernie

I have a KPA500 and run it on solar power with a 120 volt AC inverter.  
I have never seen the issues that you are recounting. The first thing to 
do is check the voltage going in to the KPA500 with a good voltmeter.  
The next thing is something in the KPA500 ac input circuit that might be 
mis behaving. Put an AC voltmeter on the transformer primary and monitor 
that. Those points should be pretty stiff.  Bad circuit breakers could 
be a problem. You might also have a bad circuit connection in  an AC box 
or even an outlet. I had such a situation and it caused poor regulation 
and a slight terrible smell as the plastic was baking.

I ran my KPA500 only on 28 and 50 MHz, but had no problem getting gobs 
of output on each band. VSWR was under 1.5:1. As I recaqll drive was 
19-20 watts and output would easily hit 500 or even more with the 117 
vac input. My inverter is a sine wave type by Xantrex, fed with a 500 AH 
AGM battery. Lately I got a smaller 150AH lithium battery and the amp 
still works just fine with 117 volts.

Dave K1WHS

On 3/7/2022 1:54 AM, Bernie and Cheryl wrote:
> Dear Folks:
>
> I have been using my KPA-500 (driven by the K3s and transmitting 
> through the KAT-500) for about five years now.  I have long noticed 
> that, when first using it, or if it has sat idling for a while, the 
> amp's supply voltage will be low on transmit and won't go higher until 
> the amp has been used for a couple of minutes. This becomes critical 
> in some instances since, particularly on 20 meters, the supply voltage 
> will be below 60 volts on transmit for a while and won't increase 
> unless I reduce the drive power by a few watts.  I do always reduce 
> the drive power to make sure the supply voltage stays at 60 or above 
> (I have been told that this keeps the amp operating within normal 
> limits and the math indicates that this will make sure the current 
> doesn't get too high).  Once I've been transmitting for a minute or 
> two (I almost always use CW), the amp appears to warm up and I can 
> bump up the drive power a few watts for higher power output and still 
> keep the supply voltage at 60 or above.
>
> For the record, the amp is plugged into a normal wall outlet supplying 
> 117VAC (or whatever the house is supplying at that moment), though it 
> is not using any of the same outlets that the rest of the shack uses.  
> I realize it is highly likely that all the outlets in the shack are on 
> the same breaker, but my prior tube-type amp used to dim the lights a 
> bit when I keyed it and had it plugged into the same outlets as the 
> rest of the shack.  That stopped happening once I switched the outlet 
> for the tube amp and now I'm using that same outlet solely for the 
> KPA-500.
>
> My first question is whether any of you with a KPA-500 experience the 
> same thing, i.e., supply voltage is a little low at the beginning and 
> increases as the amp warms up?
>
> My second question concerns the fact that, even with the amp warmed 
> up, I can seldom (if ever) hit 500 watts out on 20 meters, though I 
> can do it easily on all the other bands (I recognize that this is 
> likely caused by my antenna's characteristics on that band, though SWR 
> doesn't exceed 1.7 at any frequency that I use, even without the 
> tuner).  I simply can't get the drive up high enough to get there, 
> without the supply voltage dipping below 60. Usually, I can get 
> between 350 to 425 watts out.  Do you think this would be different if 
> I had a 220 volt outlet installed in the shack?  For the record, I 
> recognize that the 100 watt or so decrease in power out doesn't make 
> that much difference.  I am asking because I wonder if it would be 
> easier on the amp to run it off of a 220 volt outlet.
>
> As always, thanks for any help and advice.
>
> 73 de Bernie, KF0QS
>
>
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