[AMPS] Power Handling of Resistors

[Tom Rauch]

Hi Dave,

> Using the SCIENTIFIC METHOD  I learned in CollegePhysics, I OBSERVED the
> metal fragments and powder residue of the bandswitch which resulted from
> the BIG, LOUD, LIGHT EMITTING BANG. 

We can determine if it was a VHF signal by measuring the 
impedances in the tank. I've NEVER found a tank that had 
impedance capable of passing VHF energy, unless they were VHF 
tanks or something built on plywood with six inch ground leads.

If a conventional tank circuit suppresses VHF harmonics any 
reasonable amount at all, it won't pass enough energy to allow a 
VHF arc across the switch. One doesn't have to be highly 
educated or experienced to understand such a simple system.    
 
> After doing so, I concluded that the source of the energy which caused the
> vaporization was EITHER my 5KW power supply OR a bolt of lightning from
> outer space.  

That's right. Either is a logical source.

To develop enough voltage across bandswitch contacts that 
connect from one end of the tank coil to the other to cause them to 
fail, there must be a reasonably high impedance at the frequency 
causing the problem.between those points and a reasonably high 
impedance to ground at the tank input at that frequency.

That condition is met ONLY at or near the operating frequency.

Of course lightning can cause the problem also, since it has so 
much energy that even a small sample of the broad energy 
spectrum can cause damage.

My point is only that a sudden burst of a millisecond or so of 
energy, or a VHF signal, does not fit the results.  
   
> After employing the Scientific Method once again, I observed that my roof
> was still intact, so I reasoned that the energy OBVIOUSLY MUST have come
> from MY power supply (or maybe a photon event).

Since we both know (if you have the background you imply) a 
photon event does not have enough energy to do anything to the 
tube, we can rule out that nonsense.

We are left with either an oscillation or actual RF excitation at the 
resonant frequency of the tank as the source of the voltage that 
caused the arc.

Of course the energy can come from the supply, and at the 
resonant frequency of the tank it doesn't need to be a large amount 
of energy. It simply has to have enough voltage to start and arc, 
and enough current to sustain the arc until the contacts are 
melted. As little as 10 watts of power can totally destroy a small 
bandswitch contact while it is arcing, because a small contact isn't 
even capable of dissipating as much energy as a two watt resistor.

None of this stuff is all that difficult to understand, it is easy to 
develop several times the dc anode voltage in a high-Q tank circuit 
if the load is removed and the tube is pulling on the tank and 
supply at the same frequency where the tank is resonant.

High-vacuum gas arcs or whiskers inside the tube aren't hard to 
understand either. 

The only thing that makes this stuff difficult is trying to fit parasitics 
into every failure, and trying to fit one cure into every stability issue. 
That is where the very poor science comes in, and where the fights 
start.

In those discussions, the group always splits into two camps. One 
camp says "here are the rules the circuits must follow, it can't 
happen that way" and the other camp says "we can't explain how it 
breaks the rules, but we see the results".

Well, it doesn't have to break ANY of the rules and the failures 
described can happen, they just generally can't happen the "VHF 
parasitic way". Failures certainly can't be induced because of 
photons, that would be laughable if it wasn't such a sad 
demonstration of how little people know about the world we live in.


73, Tom W8JI
w8ji@contesting.com

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