K3s driving a KPA1500  how much drive power is nominal for 1500 watts? #kpa1500
John Canfield
Hey guys,
Thursday the Brown Santa (UPS) delivered my birthday present for the next ten years  a KPA1500 amp. I'm slowly learning how to use it and I'm not sure if things are acting right. Going directly into a dummy from the amp, it only takes about 28 watts drive from the K3s for a full 1500 watts output. I can reduce the drive to five watts or lower and drop the amp's output to ~150200 watts. Does this sound correct? Thanks for any advice. John, WB5THT


Adrian Fewster
I would expect it designed that way for best IMD results and cleanest signal quality. The K3 specs would be best at that low wattage. I use to run mine at 15v under load for best results. Adrian , vk4tux
On 3/2/20 12:53 am, John Canfield
wrote:
Hey guys,


John Canfield
Thanks Adrian!


Rob Sherwood
Yes the K3 is cleanest around 35 watts. No I never tested it at 30 watts vs. 35 watts, etc. I doubt you could tell the difference. Rob, NC0B
From: ElecraftK3@groups.io [mailto:ElecraftK3@groups.io]
On Behalf Of Adrian Fewster
Sent: Sunday, February 02, 2020 8:03 AM To: ElecraftK3@groups.io Subject: Re: [ElecraftK3] K3s driving a KPA1500  how much drive power is nominal for 1500 watts? #KPA1500
I would expect it designed that way for best IMD results and cleanest signal quality. The K3 specs would be best at that low wattage. I use to run mine at 15v under load for best results. Adrian , vk4tux On 3/2/20 12:53 am, John Canfield wrote:


Bob McGraw  K4TAX
In doing the math, 28 watts to 1500 watts is 17.3 dB gain, while
5 watts to 150 watts is 14.8 dB gain, and 5 watts to 200 watts is
16.0 dB gain. The point is the amp appears to be operating
normally with your numbers. Ideally, the gain of a linear
amplifier is constant up to the point of compression nearing
maximum output. To better understand this, more accurate numbers
would be needed. And it is not at all unusual for an amp to have
different gain values on different bands. Simple calculations support your observations: dB =10log(P1/P2) 73 Bob, K4TAX
On 2/2/2020 8:53 AM, John Canfield
wrote:
Hey guys,


John Canfield
Thanks again guys and thanks Bob for the numbers. I wasn't making notes when tracking drive vs. output power  just a fairly gross report of what I was seeing. The entire reason for my question is my Acom 1000 amp (4CX800) would need about 55 watts drive to produce ~950 watts, I was very surprised the KPA1500 needed so little drive power.
That Acom 1000 was a very good amp and practically idiot proof which I needed with wire antennas, now I have a DB18E and a 75M inverted V dipole so we're always under 2:1. (The Acom was sold 45 minutes after I listed it on QRZ!) 73 de John WB5THT


Christopher Scibelli
Good. 35 watts of drive from my K3 drives my Alpha 8410 to legal limit power. Chris NU1O
Original Message
From: Rob Sherwood <rob@...> To: ElecraftK3@groups.io <ElecraftK3@groups.io> Sent: Sun, Feb 2, 2020 11:00 am Subject: Re: [ElecraftK3] K3s driving a KPA1500  how much drive power is nominal for 1500 watts? #KPA1500 Yes the K3 is cleanest around 35 watts. No I never tested it at 30 watts vs. 35 watts, etc. I doubt you could tell the difference. Rob, NC0B
From: ElecraftK3@groups.io [mailto:ElecraftK3@groups.io] On Behalf Of Adrian Fewster
I would expect it designed that way for best IMD results and cleanest signal quality. The K3 specs would be best at that low wattage. I use to run mine at 15v under load for best results. Adrian , vk4tux On 3/2/20 12:53 am, John Canfield wrote:


Clay Autery
Sounds friggin' awesome! ______________________ Clay Autery, KY5G (318) 5181389 On 02/02/20 08:53, John Canfield wrote:
Hey guys,


Jim Denneny
About 25  30 watts for 1500 watts input.
Jim K7EG


Adrian Fewster
Jim, Can you clarify what you mean by 1500 watts input ? I thought that 1500 watts was the output figure on the amp with that drive, and that the input value would be much higher allowing for efficiency?
Or do you mean to the the antenna input ?
Adrian , vk4tux
On 4/2/20 5:42 am, Jim Denneny wrote:
About 25  30 watts for 1500 watts input.


John Canfield
I think that was a typo  I'm sure he meant 1500 watts out.


Ray
FCC max gain for amps is 15 dB.
Ray W8LYJ


Jim Denneny
Adrian, I mean 1500W as indicated on the led power bar whatever that means  input / output.
73, Jim K7EG


Bob McGraw  K4TAX
That display should be indicating OUTPUT power. I suggest you use an external meter of known accuracy and a 50 ohm dummy load of known accuracy. Measure both the input to the amp from the transceiver and the output of the amp as indicated on the external meter. From your earlier numbers, they appear to be of some noted inaccuracy. Also note that just because a dummy load says 50 ohms on the
label is no assurance it is actually 50 ohms. I have 2 of which
neither are actually 50 ohms. One is 13% high measuring 56.5
ohms. Since most ham power meters are measuring voltage
referenced 50 ohms, any error in the 50 ohm load will likewise
show as an error in power measurement. Based on ~15 dB amp gain and 1500 watts output into 50 ohms
Resistive, the drive power should be 47.5 watts for a gain of
14.99 dB. 73 Bob, K4TAX
On 2/4/2020 10:34 AM, Jim Denneny
wrote:
Adrian, I mean 1500W as indicated on the led power bar whatever that means  input / output.


Mike VE3YF
I have only received my KPA1500 recently, But I decided to put it through the paces with the K3S being the exciter.
I found that into a Cantenna Dummy Load that has not been tested for 50 Ohm impedance that below is what I get from the KPA1500 Display and a LP100A meter, but never really used the LED's as the indication of power. 20m  10m approx 30w output on K3S yield approx 1425 1475w Output 40m  approx 27w output on K3S yield approx 1450w Output 80m  approx 25w output on K3S yield approx 1450w Output 30m and 160m antennae are only rated for 1kw, so didn't do the test on those bands. Mileage will vary from station to station, and as Bob mentions the Dummy Loads actual impedance is a factor, but so far I am happy with the KPA1500 performance, and see no need to go out and get a lab spec Dummy Load for something I rarely use. 73 De Mike VE3YF


Bob McGraw  K4TAX
Re dummy load. First measure it with an ohm meter. It should measure 50 ohms +/ 5% or better. Then measure it with your antenna bridge from the lowest to highest frequency you intend to use. An MFJ259 or other known accurate bridge is suitable. Make sure you use a known good, short length, low loss coax as a jumper. The bridge should show 50 ohms and an SWR value of less than 1.2:1. Higher values indicated some value of reactance. A good dummy load will be resistive at all frequencies and show no reactance. The R value is resistance and the reactance is the “j” part of the impedance. Ideally it should be zero as measured on a VNA.
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I’ve found that both the air cooled loads and the “gallon of oil” types will change values as they heat. Heat one with some RF and measure it again while hot. If you find it to be different when hot than when cold, just be forewarned what you have to consider when making power measurements. It doesn’t take an expensive or precision load. It just takes knowledge and knowing what you are dealing with. Bob, K4TAX
On Feb 5, 2020, at 9:26 PM, Mike VE3YF <mike@...> wrote:

