[AMPS] Re: Parasitics

[Tom Rauch]

> Date:          Wed, 13 May 1998 01:22:15 -0800
> From:          Rich Measures <measures@vc.net>

> I agree on the tank inductor point, Mr. Rauch.  However, as I recall you 
> measured numerous VHF resonances in someTune caps., which means that 
> C-Tune is not necessarily a "very low impedance".  

You recall incorrectly. The only resonance I found below several 
hundreds of MHz was a series resonance, where impedance went almost 
to zero. 

If the capacitor had  a parallel resonance, harmonic suppression 
would be severely compromised. That capacitor wouldn't  be used in 
type accepted devices requiring 45 dB of harmonic suppression.

> >The load, however, can upset the cart at HF near and below the 
> >operating frequency and lower all the way to VLF.
> >
> At VLF, a series1000pF HV-DC blocking capacitor is a high Z.  At VLF, a 
> 50uH shunt HV-RFC is a low Z.  It looks to me like VLF would be virtually 
> blocked from the HF tank.   

Not true, it depends on load reactance at VLF. 

If the output port is effectively shorted for VLF, the parallel 
resonant circuit of the anode blocking cap and choke is 712 KHz.  But 
if series inductance is added in that path at VLF, the blocking 
capacitor effectively has less capacitive reactance at a given 
frequency and the resonance can shift quite a bit lower, well into 
the VLF range.

> The bandswitch is interesting to me because the arc was so hot that it 
> caused the  (alumina) ceramic to heat so rapidly that it fractured, and, 
> it caused shiny spots on the ceramic surface due to melting.  (switch 
> courtesy of Randy Boyle, KT2D)

Nonsense Rich. You only "claim" the bandswitch was damaged by VHF. 
You have nothing more than a guess or personal opinion what happened.

In a less than 360 degree conduction angle PA, if plate load 
impedance is raised with a high Q tank, the anode and tank voltage 
can soar.

The peak anode voltage can swing below the zero rail of the 
chassis (negative), as well as greatly exceed twice the dc HV.

It is possible, by unloading a tank in a conventional PA, to generate 
voltages of many times normal operating voltages. Underloading of a 
tank is the single largest cause of tank circuit arcs, and is a 
repeatable and measurable cause source of damage.   

> The measured breakdown potential in a typical SB-220, TL-922, AL-80, 
> AL-82, on and on,  bandswitch is about 6000v.  
 
So? Under load-fault conditions voltage can easily reach that 
value, unless something else breaks down.

73, Tom W8JI
w8ji.tom@MCIONE.com

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