[Elecraft] KPA-1500 on digital modes

[K9MA]

> 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

k9ma at sdellington.us