>
>I asked:
>> >Please Rich, enlighten me on how the shiny surface means it was
>> >a parasitic, and the bubbled surface means it was HF heating.
>
>Rich replied:
>> I did not say that a shiny surface means it was a parasitic oscillation.
>> A shiny surface indicates that the outside of the resistor did not get
>> hot. A bubbled surface indicates that the outside of the resistor was hot
>> for an extended period. A >3x increase in R indicates that the insides
>> were severely damaged Add a shiny case, and the >3x change suggest that
>> the change probably happened quickly. . .
>
>OK.
>
>You say a sharp pulse of VHF current heats the resistor from the
>inside out,
This is my guess as to what took place.
>while a longer HF current heats the resistor and
>severely damages the outside.
>
The outside is not severely damaged because it is still intact and doing
its job of supporting the wire leads and carbon element.
>That seems to disagree with what you say about other conductors.
>
>For example you claim gold coatings on grids can only be
>damaged by VHF parasitics, and not by dc or lower frequency
>currents.
Correct. Boiling gold requires a higher temperature than is needed to
melt iron. For example, to increase the temperature of the entire >50g.
grid in an 8877 to the boiling point of gold would require an amount of
energy that is far beyond the capability of the typical anode supply.
However, thanks to skin-effect, the energy available from the anode
supply is able to remove thin layers of gold. According to Eimac's Mr.
Willis B. Foote, during the development of the 8877, the design team
discovered that grids could be gold-sputtered in thin layers by an
"oscillation condition". //For a photograph of a gold-sputtered 8877
grid see "Parasitics Revisited" in the September and October 1990 issues
of *QST*. For a copy of the Foote letter, see my Web site.// For a
sample of a gold-sputtered 8877, try contacting a local AL-1500 owner.
>
>How are grids damaged from the outside in (as you have claimed
>many times) while resistors are damaged from the inside out by
>short bursts of VHF energy?
RF travels on the surface of conductors. Gold is a conductor. The
phenolic case of a carbon-comp resistor is not a conductor.
>How can the resistor get that hot
>inside when the saturated anode current is only about ten
>amperes,
P = I^2 x R, and I = 9 or 10 amperes, P could be 81 or so x 50 to 100
(ohms) watts. Approx. 4kW is seeming quite a bit for a 2w-rated
resistor.
> the duty cycle is in nanoseconds,
This is not known.
>:and the resistance is
>under 100 ohms?
>
>This almost sounds like your theory that photons arriving from outer
>space can make amplifiers on standby explode because the
>photons hit the amplifier so hard they make the standby relay arc,
>and the arcing relay starts a parasitic in what is an otherwise
>stable amplifier that is just sitting there on standby!
Borrow a geiger counter, Mr. Rauch, and tune in on what's happening on
the upper frequencies. Be not surprised if you occasionally encounter
some humungous signals.
>
>I think the photon theory outdoes the reverse skin effect theory
>shiny resistor theory.
>
Many years ago there was a widely-ridiculed theory that supernovas emit
neutrinos. During the last supernova, 19 neutrinos were detected deep in
gold mines. . At one time, Einstein's theory that gravity waves bend
the paths of (massless) photons was thought to be ridiculous by some.
Now we know it is true.
>
cheers, Tom
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