[AMPS] Tuning question

[2]

>
>Rich said:
>
>>Amplifiers need to be tuned up at 100% of the drive that will actually be 
>>applied.  
>
>Conventional amps should be tuned at 100%, as Rich says. However, this doesn't
>necessarily apply if the amp has circuitry that allows correct tuning and
>loading to be achieved at lower power levels. Some do - not all. 

//   Tuning up an amplifier is basically about adjusting the tank to 
match RL to R-load - usually 50-ohms.   The sticky wicket is that RL is a 
function of E/I.  Maintaining this ratio is important in acheiving an 
accurate tuneup.   With a triode, reducing anode voltage also reduces 
anode current, so it should be possible to accomplish an accurate, 
less-stressful tuneup by  reducing anode voltage.  Many amplifiers have a 
CW-Tune position that provides lower anode voltage.  In theory, it should 
work.  However, in practice, an optimal tuneup on CW/Tune does not 
produce the same E/I ratio on the higher-V SSB position.  The result is 
ok but not wonderful.  With tetrode and pentodes, decreasing anode 
voltage has little effect on anode current, so this method is not going 
to fly.   

>The auto 
>tune amps tend to. If you have detectors  on the input electrode and the 
plate 
>so the ratio of the RF volts on these two electrodes can be monitored, you 
can set
>the loading correctly at low power. There was a 3-1000Z GG amp in the 1970 
ARRL
>Handbook which did this. In a grounded cathode amp, if you monitor the 
>relative phase of anode and grid volts and maintain the TUNE control for them 
to be 
>180 degrees out of phase, you can tune the tank without a plate current dip. 
>It's a bit more difficult in GG because of fedthrough power. 
>
>Most HF tube amps will be about 50% efficient, give or take, when tuned 
>up.

//  ... 60% is fairly common in Class AB_ with unpusillanimous anode 
potentials.  

> If you reduce the drive, the efficiency will drop off.  In the limit, even 
>with no RF input, you still have the standing current. At very low output 
powers, the
>efficiency in terms of DC in to RF out is extremely poor, increasing as the
>drive level is increased.
>
//   The critical moment in an amplifier is that fleeting instant when 
cathode current peaks and anode potential is minimal.  This is where the 
match needs to be optimal.  If it's right there, it will be a delight 
elsewhere, and one's signal won't be abominal.. 

//  Free, unsolicited advise:  Unless an amplifier has a 2500W anode 
supply, and the operator has a free supply of tubes, tuning up with a 30% 
- 50% duty-cycle pulse is not a bad way to go.  [a tuning pulser road-map 
is available on my Web site]  

cheers, Pete

-  R. L. Measures, 805.386.3734,AG6K, www.vcnet.com/measures.  
end


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