[AMPS] Another arc question

[Rich Measures]

>
>On Wed, 6 May 98 07:40:26 -0800 Rich Measures <measures@vc.net> writes:
>SNIP
>>
>>The problem at hand is that gas is Not being detected in 3-500Z tubes 
>>with bent filament helices.  
>>
>>Until recently, I owned some new 4E27 pentodes that were manufactured in 
>>the 1940s.  Some of them still exhibited good vacuums, however other 
>>4E27s  from the same lot were quite gassy, presumably due to imperfect 
>>metal/glass seals.  
>
>Are you now confusing or mixing up "gas"  Rich.  Gas due to imperfect
>seals is not the same as gas released internally from the materials. 

The point is that gas is not gradually absorbed by a getter -- i.e., the 
rauchian disappearing gas theory does not wash.  However, the basic 
problem is that gas is seldom found in tubes with bent filament helices / 
a filament-grid short.  

>I also suspect that your microamp hipot tester will not be capable of
>creating the conditions necessary to recreate a gas discharge event...

It matters not whether the high potential supply being used to measure 
gas leakage is capable of 1mA or 1000A.  

>but I will leave that to Arlen and others with a formal education to comment 
on.
>
>
>Again, it may sound like I am asking old or dumb questions, but I am 
>>
>>>>relatively new to this group (couple months) and haven't heard all the 
>>>>answers before.  And as I pointed out before, I am not an expert on tubes.
>>>
>>>Being new to the group may explain lack of expertise on tubes, but it 
>>>does not explain absence of knowledge of undergraduate physics.
>
>AMEN!
>
>>>
>>>Sorry to be so rough, Jon, 
>
>
>Sometimes it takes a 2x4 approach to get a persons attention Arlen. I
>think your message was well put and I know that I learned from it.
>........
>>
>>I concur that Jon erred on the carbonization of arc-marks matter,  Arlen, 
>>however, perhaps You would like to proffer a scientific explanation to 
>>the [AMPS] group on how/why the filament helices in a 3-500Z can be 
>>suddenly bent by a "gas discharge" or "barnacles" -- even though the grid 
>>is grounded -- even though no arc-marks can be found at autopsy -- 
>>even though the VHF suppressor resistor has more than doubled in resistance 
>>without exibiting cosmetic damage?   
>
>As has been mentioned here numerous times...how do you know the condition
>of the resistor minutes, days or weeks before the arc?  Unless you can
>recreate the event in a controlled experiment I fail to see why you
>continue to harp on the resistor.

because the suppressor resistor is virtually shorted out by  <1-milliohm 
of copper buswire that typically has 0.08 uH of L, which should protect 
it from DC arcs in the tube, but which would not protect it from bursts 
of vhf energy.  .  

>In the case of the SB-220 would not the almost instantaneous discharge of
>the 3200VDC ( in standby which is where most arcs appear to occur) not
>cause damage to the resistor? 

no, no

>Would not the field developed in the
>suppressor L be sufficient to "bend" the carbon in the resistor causing
>the case to crack?  If one were to believe that the magnetic field caused
>the filament to bend then the same reasoning would hold for the
>resistor...at least it makes sense to this uneducated dummy.
>
not to yours truly

>Rich has also failed to ever explain how an amp in standby, biased well
>beyond cut-off can suddenly have a parasitic event.  Particularly in an
>amp that is completely stable when keyed with no drive and the Tune and
>Load caps are varied in an attempt to force a parasitic.
>Not to be confused by external arcs in tank circuit components while the
>amp is in a RF amplifying state.
>
Only 5% or so of the parasitic oscillations I hear about fall into this 
category, however, the open contact gap in a typical cathode bias relay 
is only 25 thousandths, which is easily broken down if the tube conducts 
for a microsecond or so for who knows whatever reason.  .  

cheers
Rich...

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


--
FAQ on WWW:               http://www.contesting.com/ampfaq.html
Submissions:              amps@contesting.com
Administrative requests:  amps-REQUEST@contesting.com
Problems:                 owner-amps@contesting.com
Search:                   http://www.contesting.com/km9p/search.htm