[Amps] exciter overshoot

[R L Measures]

On Jun 9, 2006, at 7:37 AM, Gudguyham at aol.com wrote:

> I have noticed that some amplifier manufactuers discuss "exciter  
> overshoot"
> in their operating manuals.  Apparently some exciters at the moment of
> initial RF keydown will for a split fraction of a second put out  
> full power
> regardless of the RF setting on the exciter, and even a split  
> second burst of  power
> beyond 100 watts.

I've measured as much as 170w initial surge from an adjusted to 100w  
radio.

> It is a known fact that a bandswitch or a  tuning capacitor
> arc can occur if too much drive is applied with too light  loading  
> to the
> amplifier.

However, tubes are emission limited, so there's a practical iimit.   
For example 3-500s are linear up to c. 1.2a-pk and they can be driven  
to c. 1.6a-pk, but that's about the end of the road since at that  
point they are pretty much saturated.

> I know that Alpha amps respond very quickly to  sudden spikes in
> overdrive.  One ham I know had a major problem with his  Alpha amp  
> using an Icom
> exciter.  Could some bandswitch and tuning  capacitor arcs be  
> attributed to
> exciter overshoot?

Surely, but since the amount of overshoot is emission-limited by the  
electron tube's cathode, the Tune-C would have to be pretty marginal  
to start with in order to arc.

> I know from personal  experince that some of the arcs I have
> seen occured when the initial drive from  the exciter was applied. THE
> QUESTION IS..............
>
> Has anyone measured exciter overshoot?  It would have to be done  
> with  a
> scope I am sure since a meter would never react that fast to see it.

correct
>
> Lou W1QJ
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R L MEASURES, AG6K. 805-386-3734
r at somis.org