Tom asked:
> >Could you describe the symptoms that indicated an "instabilty
> >problem"?
km1h@juno.com wrote:
> Typical symptoms with an AL-82, etc are a shorted tube, blown plate
> choke, blown band switch.
That's what I was afraid of. None of these things are traceable to
oscillations. Look at this with what we all know about theory:
1.) The impedance of the tuning capacitor is very low (less than r=2
x=20 ohms) from 120 MHz to 1200 MHz (the range of my network analyzer).
The bandswitch is downstream of this shunt reactance, so we know a VHF
oscillation of any practical magnitude can't drive the bandswitch with
enough voltage to produce an arc.
2.) The plate choke fails from operation at series resonant frequencies.
That has been a problem, especially if the PA is ever operated on 27 MHz
(one of the intentional series resonant points).
3.) A shorted tube is no more likely caused by "oscillation" than any
other form of excessive drive. By far the most common cause of arcing
tubes is gas. Gas is more common with graphite tube, because the anodes
"trap" gas in the graphite. The gas is released later, and causes an
arc.
> Many failures can be traced to a poor
> understanding on how to even tune the damn thing. I cant believe how many
> hams havent a clue as to what they are doing.
Bingo. That's why manuals tell customers to overcouple the PA, by
loading it with MORE than the expected drive.
When a PA is undercoupled, tank voltage soars. Only when something
absorbs the energy that never made it to the load, will the voltage
increase stop. As Bill Turner pointed out, and you confirmed, the worse
thing in the world is to undercouple the PA.
It's not only bad for spatter, it damages components from tubes to
bandswitches.
The question is, how to say it in a way the customer understands. Any
suggestions?
73 Tom
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