[Elecraft] KPA500 Power Output

[Bob Wolbert, K6XX]

I believe the most significant contributor to key-down power sag is the
increase in MOSFET channel resistance with temperature. As the MOSFETs heat,
their ON resistance increases slightly, which drops output power slightly.

Fortunately this is a small effect, especially when measured in dB.

73 de Bob, K6XX
  

> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net 
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Ron 
> D'Eau Claire
> Sent: Friday, January 27, 2012 10:18 AM
> To: N4CW at aol.com
> Cc: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] KPA500 Power Output
> 
> A quick way to check Jack's suggestion that the mains source 
> voltage may be
> drooping is to monitor the HV on the KPA500 while 
> transmitting to see if it
> decreases over time. 
> 
> Ron AC7AC
> 
> -----Original Message-----
> From: elecraft-bounces at mailman.qth.net
> [mailto:elecraft-bounces at mailman.qth.net] On Behalf Of Jack Brindle
> Sent: Friday, January 27, 2012 9:55 AM
> To: N4CW at aol.com
> Cc: elecraft at mailman.qth.net
> Subject: Re: [Elecraft] KPA500 Power Output
> 
> Are you sure it is the KPA500 that is causing the drop off? 
> There are  
> a lot of variables here. The exciter could be scaling back, 
> the power  
> source (110/220) could be limiting you or something else may be  
> happening. As power cords heat, they tend to restrict the electron  
> flow. Especially if you are powered from 110V, that could be the  
> source of several problems. Be sure not to replace the 
> supplied power  
> cord with a smaller one - this is a recipe for reduced output 
> in itself.
> 
> We have also seen transmitters decrease their output power 
> when keyed  
> down for a long time. This might be due to heating in the TX, ALC  
> action, or other things specific to the radio.
> 
> I would suggest adding a wattmeter between the exciter and 
> the KPA500  
> so you can monitor power between the devices. Also check the power  
> source wiring to make sure there are no limitations there. In 
> general,  
> look to see if there are other factors external to the KPA causing  
> power reduction.
> 
> Jack B, W6FB
> 
> 
> On Jan 27, 2012, at 8:57 AM, N4CW at aol.com wrote:
> 
> > Sorry if this has been covered before, but I can't recall that it  
> > has. Is
> > there any reason why power output
> > falls off over time? Here's what I did: using MMTTY, I "diddled"  
> > into a
> > 50-Ohm dummy load for one minute.
> > I initially had a power output of 525 Watts which went to 500 W  
> > shortly and
> > went down to about 465 W
> > when MMTTY "diddling" timed out. I repeated that process later,  
> > monitoring
> > current and noted that it also
> > went down over time...makes sense!
> > But my question is WHY? I was under the impression that I 
> needed to  
> > run the
> > amp at 500 W on RTTY for
> > optimum efficiency because in an earlier posting I noted 
> that at 150W
> > output (in BPSK31), the amp was very inefficient and the fan was  
> > pretty loud. If
> > I can't derate the output and I can't run full output, what 
> do I do  
> > with
> > the  KPA500 in the digital modes?
> >
> > Bert, N4CW
> >
> >
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