Driving KPA500 with 2W for digital work?

Rob Fissel

So I've had a KPA500 up and running for a few weeks now. I do a fair amount of digital work, and was thinking to myself...

Instead of bypassing the amp, and using the Flex to run, say, PSK @ 30 watts, why not excite the amp with 2W to get about 30W out? Frequently, I'll get into rag chews where the rig is keyed down for upwards of 4-5 minuets, and the rig will start to get pretty warm. I figure, why not let the amp do the heavy lifting, and save a little life on the Flex's transistors. 

Is this ok? I have read that IMD starts to suffer at low drive levels, but I haven't found much out there regarding this phenomenon with solid state amps. 

Suggestions?

Norm - W7CK

I do it all the time.  I have never had a problem with distortion or anything like that due to low drive levels.  Although I've run my drive level down to 2 watts or so, most of the time I am driving it will a bit more than that.  I usually run about 50 watts out of my amp when running digital.  This is because of the poor antenna I'm using.  I guess you could call Elecraft if you have doubts about driving with only 2 watts.

k3Tim

Whereas PSK is a constant envelope mode, amp nonlinearity shouldn't be a concern, correct? IMD is concerns are for SSB. You should be able to use a class C amp for PSK. I believe this is correct but someone knows for certain, PSE jump in

Doug Hall

No, PSK31 actually isn't a constant envelope mode, although it sounds like one. Unlike RTTY it does not have a crest factor of 1. Data is encoded by shifting the phase of the signal 180 degrees (for a zero bit) or not shifting it (for a one bit) and subsequently the signal is filtered to smooth rise and fall times. This eliminates discontinuities (clicks) and minimizes occupied bandwidth. A linear amplifier is necessary to get the benefit of this filtering.
73,
Doug K4DSP

Norm - W7CK

I put mine up on 20m PSK this evening and ran about 2 watts into it without any problems at all. 

Rob Fissel

Got a quick response from Elecraft, for anyone that is interested:

You can drive the KPA with 2 watts as you suggest. We have customers driving the KPA with their KX3's and it works fine. You will not have a IMD issue driving the amplifier with a low drive power.

Dave - W6OVP

Part of the magic of the digital modes is they force us to use low power because our transmitters literally cannot "take the heat". This limitation has created a new low power paradise which makes it possible for QRP (for various reasons) hams to compete, and the virtues of digital to be demonstrated.

I shudder to watch the power slowly creeping up from a few watts to more, and dread the day when amplifiers that can run "1500W, 100% duty cycle, no time limitation, key down forever" find a home here. Not that I would personally mind having a little more power on JT65 of course (cough cough) ... <g>.

Rob Fissel

I'm with you, Dave.

FWIW, the Alpha amps are actually already rated at 100% key down full duty continuous @ 1.5kW. Imagine a JT9 signal at a full gallon and a half. 

For the most part, I run PSK anywhere from 15-30 watts. I figure I might as well offload that work onto a beefier PA. 

JT modes I rarely ever go above 10W, and that's after a few attempts at lower power to work another station. When calling CQ, I almost always run Joe Taylor modes at 500mW, WSPR included.