[Elecraft] KPA-1500 on digital modes

[N4ZR]

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
>