[Elecraft] Re: Setting KPA100 Bias?

[Tom Hammond NØSS]

GM John:

>How fast is "when you first enter TUNE mode"? Given 532 mA, my target is
>932 mA. When I hit TUNE in CW TEST, it looks like the current is following
>an exponential curve, the rate of increase slowing to a slow change after
>about 25 seconds. If I set R6 for the target current in a few seconds, the
>plateau is about 1.2A or higher and I wouldn't say it is slowly drifting
>higher from the 932 mA. Is the resistance in my jury-rigged meter cabling
>between the power supply and the K2 cable causing an unusually long
>time-constant?

If I recall, and I don't think it's menthioned in the manual, Wayne kinda 
intended for the bias to be set when the heatsink was cool, and without the 
fan running (due to heatsink temp, NOT because it's been turned off). The 
bias WILL change as a function of heatsink temperature. This is possibly 
why you will see the slow increase over time. I believe a slow run-up of 
the bias current is probably a fact of life, but sould not be a significant 
hinderance in setting the bias. I wouldn't expect it should take more than 
5 seconds to actually MAKE the bias setting, maybe less. This shouldn't 
contribute terribly to heatsink heating. If you DO need to repeat the step, 
wait a bit, to give any 'localized' heatsink heating a chance to dissipate 
before you hit it again. And use an analog meter if one is available.

The bias CAN be set +/- a few mA... maybe +/- 20mA, but I'd want to get a 
reading from either Wayne, Eric, or Gary on this before I'd ask you to take 
it as gospel.

>Is N5WU's method, posted by Tom, of measuring the voltage change across R7
>likely a better method in my case?

There's probably not significant difference in the two methods... esp. with 
regard to you seeing the bias slowly increase.

>Or am I about to enter the Trouble Shooting mode in the upcoming Transmit
>Tests?

I doubt it... sounds like you'll be OK.

73,

Tom