>
>Rich-
>
>Stick the right frequency of AC across that choke and it's
>gonna burn in almost the same place every time.
With 1mH, >15A of grid-current is not unlikely. The damage to
grid-current meters is from DC current because the needle is bent.
>This would support your suggestion ofVHF parasitics but
>does not explain how AC at VHF could bend the heater
>with no involvement from the plate supply, which if you
>measure as suggested will tel you if you have enough
>current to bend your heater or grid.
>For AC to bend the thing, it would have to be mechanically
>resonant at the 'parasitic' frequency. VLF parasitic
>maybe but VHF, you're gonna need a pretty low mass
>if you expect to excite physically or you're gonna have to have
>you AC wavelength close to the size of the parallel elements
>and in just the right phase if it's gonna squeeze them together.
>
>How high can the RF voltage get in a high Q tank?
>Higher than 10kV?
In a SB-220, the tank's Tune-C limits the potential to c. 3600V peak by
breaking down. Normal HF operating potential is c. 2700V.
>High enough to initiate a plate supply short through the tube?
If the anode-grid path shorted, there would be an arc-mark on the grid.
I have not seen one in a grid-fil shorted tube -- nor have I found a
shorted tube that was gassy. I doubt that Mr. Rauch's disappearing gas
theory is possible without direct intervention from the Fairy Godmother.
>How's that for heater bending voodoo :-)
>
My guess is that a single DC event bends the hot filament.
cheers, Bob.
>On Mon, 10 Feb 2003, 2 wrote:
>
>>
>>
>> >It was proposed that the change in geometry was a result of
>> >'excessive' DC current. If you take the tube out of the socket,
>> >create a short, and look at the current during this experiment,
>> >you have a better handle on it's possible contribution to the
>> >observed physical changes in your tube.
>>
>> The grid-fil current is virtually zero. A grid-fil short does not burn
>> out the choke.
>> >
>> >If the chokes are all burnt in the same spot it might be AC...
>>
>> The chokes are 1mH.
>>
>> >of the same frequency in every case...
>> >then again, the wire might be thin in that spot on all the chokes :-)
>> >
>> >
>> >-bob
>> >ah7i
>> >
>> >
>> >2 wrote:
>> >
>> >>
>> >>
>> >>>Classic divide and conquer analysis...
>> >>>
>> >>>You want an upper bound on the current.
>> >>>Take the effing tube out of the circuit and
>> >>>measure the peak current that pops your choke.
>> >>>
>> >>>Do this by simulating the 'event' (short in tube)
>> >>>with a relay.
>> >>>
>> >>>
>> >>
>> >>^^ A short in the tube does not blow the choke. I simulated a
>> >>grid-filament 3-500Z short with a clip lead, and the result was a
>> >>unspectacular rise in DC anode current. The choke did not implode or
>> >>burn out.
>> >>
>> >>
>> >>>This will tell you if all the other speculation
>> >>>is within the realm of reason or Bull Shit.
>> >>>
>> >>>Depending on what you measure, you might
>> >>>end up with a clue as to eggs before chickens
>> >>>or the other way around...
>> >>>
>> >>>-Bob
>> >>>ah7i
>> >>>(It is eggs before chickens, from an evolutionary POV)
>> >>>
>> >>>
>> >>>
>> >>>On Sun, 9 Feb 2003, 2 wrote:
>> >>>
>> >>>
>> >>>
>> >>>>The trick is to have the scope connected when an event takes place --
>> >>>>which could be once in maybe a year or eight.
>> >>>>
>> >>>>
>> >>>>
>> >>>>>Why speculate? If you've a storage scope or a
>> >>>>>scope with triggered sweep amd a camera
>> >>>>>it's easy to measure the destruction of that
>> >>>>>28 guage choke and get a handle on the
>> >>>>>upper limit of that current.
>> >>>>>
>> >>>>>-bob
>> >>>>>ah7i
>> >>>>>
>> >>>>>
>> >>>>>2 wrote:
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>Jeffrey Madore wrote:
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>As I alluded to, I think we can leave the terrestrial magnetic field
>> >>>>>out of the equation.
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>That is clear from Eric's calculation.
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>A length of power cable in free air exhibits the property of inductive
>> >>>>>reactance when there is AC current flow. This XL results when the
>> >>>>>cables own magnetic field expands and collapses about the conductor
>> >>>>>itself. The result is a CEMF which produces current that is out of
>> >>>>>phase with the source current and thus bucks or impedes the flow of
>> >>>>>current from the source. Hence, we have these two current components
>> >>>>>which are out of phase, producing magnetic reaction and mechanical
>> >>>>>
>> >>>>>
>> >>force.
>> >>
>> >>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>If you can describe the scenario, you can predict the force. Please may
>> >>>>>we see a calculation?
>> >>>>>...
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>
>> >>>>>** not until someone measures the currents present during the event
>that
>> >>>>>preceeds a 3-500Z fil/grid short. So far, the only quantity we can
>guess
>> >>>>>at is the >15a it takes to melt #28 Cu in the grid's DC return path.
>> >>>>>
>> >>>>>- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
>> >>>>>www.vcnet.com/measures.
>> >>>>>end
>> >>>>>
...
>> >>>>>
...
- R. L. Measures, a.k.a. Rich..., 805.386.3734, AG6K,
www.vcnet.com/measures.
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