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KPA-1500 on digital modes

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Wes Stewart-2

Re: KPA-1500 on digital modes

Is this a typo?

Wes N7WS

On 8/31/2019 7:45 PM, Jim Brown wrote:
>
> K6XX, one of the Elecraft engineers who works on their power amps, gave a talk
> to our contest club stressing the importance of providing a matched load to
> the output devices, and to the output itself. He observed distortion
> *increases* significantly when that condition is satisfied, whether it is an
> antenna tuner for a solid state output stage or the Pi or Pi-L in a tube amp.

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Jim Brown-10

Re: KPA-1500 on digital modes

Yes. :) The missing word is "NOT" satisfied.

73, Jim

On 9/1/2019 5:14 AM, Wes wrote:

> Is this a typo?
>
> Wes N7WS
>
> On 8/31/2019 7:45 PM, Jim Brown wrote:
>>
>> K6XX, one of the Elecraft engineers who works on their power amps,
>> gave a talk to our contest club stressing the importance of providing
>> a matched load to the output devices, and to the output itself. He
>> observed distortion *increases* significantly when that condition is
>> satisfied, whether it is an antenna tuner for a solid state output
>> stage or the Pi or Pi-L in a tube amp.
>
>
> ______________________________________________________________
> Elecraft mailing list
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K9MA

Re: KPA-1500 on digital modes

In reply to this post by Jim Brown-10

> On 8/31/2019 6:02 PM, Roger D Johnson wrote:
>
>>
>> Isn't this exactly the way some of the first ham linears were designed
>> for an input of 2000 watts in SSB and 1000 watts in CW? There
>> was a switch on the front panel for SSB/CW mode which dropped
>> the plate voltage for CW mode so that the tubes saw the same
>> load impedance they did in SSB mode thereby keeping the efficiency
>> high and not requiring retuning between modes.

I think it was more to make it harder to exceed 1 kW input on CW. It is
almost always possible to load an amplifier (with the usual PI network)
for efficient operation at less than maximum output. It just has to be
loaded lightly and not driven too hard. That conflicts, however, with
the usual procedure of loading for maximum output. Also, with two plate
voltage settings you could usually tune for maximum output at the low
setting at about 1 kW input, and it would be tuned about right for 2 kW
at the high setting. This reduced the stress during tuning, and probably
was the most common strictly legal way to tune for 2 kW PEP input in
those days.

Actually, legally tuning for 2 kW input in those days was tricky.
Collins used a bridge which compared RF input and output voltages and
allowed tuning at low power. I used that circuit in an amplifier I built
about 50 years ago, and it worked pretty well.

A PI network in a tube amplifier allows properly loading the amplifier
into a range of load impedances, but the operator or autotuner is
looking at grid current, plate current, input/output voltage ratios,
etc. to get the tuning right. This sets the tuning for efficient, linear
operation. A solid state amplifier and ATU work very differently. The
amplifier is designed to operate properly into a 50 Ohm resistive load,
and the job of the ATU is to make the actual load as close to that as
possible. One result of that is that a rather small mismatch can have a
large effect on efficiency and/or linearity.

73,

Scott K9MA

--
Scott K9MA

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N4ZR

Re: KPA-1500 on digital modes

FWIW, one of the things I like best about my KPA-1500 is that you can
use its antenna tuner with the amp on Standby

73, Pete N4ZR
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On 9/1/2019 4:02 PM, K9MA wrote:

>
>> On 8/31/2019 6:02 PM, Roger D Johnson wrote:
>>
>>>
>>> Isn't this exactly the way some of the first ham linears were designed
>>> for an input of 2000 watts in SSB and 1000 watts in CW? There
>>> was a switch on the front panel for SSB/CW mode which dropped
>>> the plate voltage for CW mode so that the tubes saw the same
>>> load impedance they did in SSB mode thereby keeping the efficiency
>>> high and not requiring retuning between modes.
>
> I think it was more to make it harder to exceed 1 kW input on CW. It
> is almost always possible to load an amplifier (with the usual PI
> network) for efficient operation at less than maximum output. It just
> has to be loaded lightly and not driven too hard. That conflicts,
> however, with the usual procedure of loading for maximum output. Also,
> with two plate voltage settings you could usually tune for maximum
> output at the low setting at about 1 kW input, and it would be tuned
> about right for 2 kW at the high setting. This reduced the stress
> during tuning, and probably was the most common strictly legal way to
> tune for 2 kW PEP input in those days.
>
> Actually, legally tuning for 2 kW input in those days was tricky.
> Collins used a bridge which compared RF input and output voltages and
> allowed tuning at low power. I used that circuit in an amplifier I
> built about 50 years ago, and it worked pretty well.
>
> A PI network in a tube amplifier allows properly loading the amplifier
> into a range of load impedances, but the operator or autotuner is
> looking at grid current, plate current, input/output voltage ratios,
> etc. to get the tuning right. This sets the tuning for efficient,
> linear operation. A solid state amplifier and ATU work very
> differently. The amplifier is designed to operate properly into a 50
> Ohm resistive load, and the job of the ATU is to make the actual load
> as close to that as possible. One result of that is that a rather
> small mismatch can have a large effect on efficiency and/or linearity.
>
>
> 73,
>
> Scott K9MA
>

Edward R Cole

Re: KPA-1500 on digital modes

In reply to this post by N4ZR

Never realized that, but I checked dc input power vs output RF power
on my LDMOS amps and that appears to be true.

I had intended to run my 2m 1500w amp at 600w for SSB (to be kinder
and more linear), while running JT65 at 1300w which is where
saturation begins.

1385w RF: 37A x 47.2V = 1746w (disp = 361w)
600w RF: 25A x 48.5V = 1212w (disp = 612w)
http://www.kl7uw.com/test2.pdf

MRF-1K50N disp = 2941w at 25c (derate above 25c)
https://www.nxp.com/docs/en/data-sheet/MRF1K50N.pdf

But average current draw in SSB is less than JT65 (which is keydown
carrier). So I guess it depends on how long-winded your SSB talking is.
Typically voice duty cycle is 20% at max whereas JT65 is 50/120 seconds = 42%

Fortunately, I have a thermistor on the RF deck to monitor
temperature with a digital voltmeter. Control board illuminates
"overtemp" lamp at 43c and 50v is shutdown at 55c. Extended running
of JT65 shows heatsink is a shade under 43c (lots of fans cooling things).
http://www.kl7uw.com/144_1500W_PA.htm

73, Ed - KL7UW

Date: Sat, 31 Aug 2019 17:04:38 -0500
From: K9MA <[hidden email]>
To: [hidden email]
Subject: Re: [Elecraft] KPA-1500 on digital modes

This is nothing unique to the KPA1500: ALL class AB amplifiers behave
this way. Even an "ideal" class AB amplifier has a maximum dissipation
at around half of full output.

One thing running at lower output probably will buy you is lower IMD.

73,
Scott K9MA

73, Ed - KL7UW
http://www.kl7uw.com
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