[AMPS] Re: Parasitics

[Rich Measures]

>> 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. 

What is the impedance of a series resonance, slightly off-resonance.  
>
>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 mysterious VLF signal is essentially blocked from entering the 
tank by the high reactance of the 1000pF DC blocking capacitor and the 
low reactance of the 50uH HV-RFC, the mysterious VLF signal does not 
reach the load.  
>
>If the output port is effectively shorted for VLF, the parallel 
>resonant circuit of the anode blocking cap and choke is 712 KHz.  

They are undoubtedly not in parallel, Mr. Rauch. 

>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.

I don't think so.  
>
>> 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.

In a SB-220, a max. mistune potential of 3600pk-v was measured.  
>
>The peak anode voltage can swing below the zero rail of the 
>chassis (negative), as well as greatly exceed twice the dc HV.

not according to the above measurement
>
>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.   

Underloading will not cause a Tune-C arc in my SB-220 or TL-922.  The 
breakdown potential of a SB-220 Tune-C is c. 3600v, and for the TL-922,  
c. 5900v.  
>
>> 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.
>
The measured potentials seem to indicate otherwise.   


Rich...

R. L. Measures, 805-386-3734, AG6K   


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