Warning: This is a long reply. Rich unfans are strongly advised to
click Delete at this point.
...snip...
>> Unless showered with spilt coffee, the AL-80 and the SB-1000
>>bandswitches can withstand over 5500v.
>
>I believe you are wrong about the switch specs Rich....
I did not say that these were the switch specs. This is the measured
withstanding voltage for a clean, dry switch.
When I receive your toasted AL-82 bandswitch, Carl, I will measure the
voltage withstanding ability of the remaining good contacts, and post the
result. .
....snip....
>In a mistuning test, the
>>measured piv increased a bit under 30% maximum. However, the settings
>>to
>>produce this condition were far, far removed from the settings that
>>produced max. RF to the antenna, which is how most folks tune an
>>amplifier.
>
>That statement only holds true...with reservations....when you are
>dumping into a purely resistive load. Reactances as are typical with some
>ham antennas can transform that voltage to very high levels....if the
>load is not absorbing power the tank voltage can approach that calculated
>in the UNLOADED Q of the circuit...not the magic Q of 12.
10 to 15 is close enough, IMO.
> You were close
>the other day when you mentioned the Tesla Coil effect. The actual
>voltage can vary between both extremes.
Surely, but what percentage of amplifier operators tune up with the
output coax disconnected? When the antenna has a low SWR and the
bandswitch mysteriously crispy-critters after the amp. has been tuned up,
I know to check the value of the VHF suppressor R. I got into the same
argument with Mr. Rauch. He presumes that bandswitches in Ameritron
amplifiers only arc because of operator error (a euphemism for
drooling-idiot), 'cheap coax' and 'bad antennas'. However, I have spoken
with a number of people who have had such bandswitches fail from arcing,
and I am convinced that Mr. Rauch is using a Rauschian Fact-Filter. (sic)
>>--.Carl: Have you measured the VHF suppressor resistance in an
>>amplifier
>>that came in for repair with a crispy-crittered bandswitch?
>
>No, the old suppressor usually gets trashed anyway and I use my own
>version for a replacement.
If the VHF suppressor is being trashed, one snip, clip on an ohm meter,
and you have a clue as to whether VHF oscillation was present. It takes
less than 15 seconds.
>Time is money and my customers bitch when I do
>R&D on their nickle. After 5,10,20 years any carbon comp resistor is
>going to change value when subjected to even its rated dissapation at DC.
> Severe changes in value I have found, and mentioned here months ago with
>nary a response, ...
I missed the post.
>...are usually indicitave of high levels of RF above
>50MHz....parasitics, harmonics or whatever. Those 2W carbons just do not
>like VHF heat period.
Agreed. Knowing this as you do, why presume that a bandswitch was
toasted by a drooling-idiot operator? If you don't ohm, I don't see how
you could have known.
Rich---
R. L. Measures, 805-386-3734, AG6K
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