[AMPS] Another arc question

[Jon Ogden]

>>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.
>>
HUH?  How can a magnetic field bend carbon?  Last time I checked carbon 
was not a magnetic material.  I think Carl here is just trying to throw 
stuff out here to confuse the issue.  Junk science to coin his own words. 
 It is a TOTALLY different thing for a magnetic field to be able to move 
metal.  Metal is a magnetic material (ok, ok..there are varying magnetic 
properties..I know that).  ARRGH...Such hogwash.

>
>>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.


OK wise guy...answer this:

Last night I had an event of some sort shortly after powering up my 
4-1000A.  Just with the tube idling, no drive and no messing with caps, I 
blew out a 1/2 Ohm parallel resistor combination that was rated at 
probably at least 7 watts.  Yes, I was running with Rich's supressors and 
experimenting with how they work in my amp compared to my original 
supressor.  But what supressor works better is not my point here.  This 
resistor combo had only 50 mA of DC current flowing through it when it 
popped.  Now the combo is basically an open circuit.  These were metal 
oxide film resistors and not carbon ones.  The anode current meter only 
read about 50 mA at the time of the event.

Rich's circuit has this resistor combo in series with the Plate RFC prior 
to hooking into the anode output line.  So theoretically it should only 
carry the plate current.

Now Carl, please explain how this resistor blew.  If you need more 
circuit details I will give them to you in great detail.  In order to 
blow this up with DC current I would need probably 3 to 4 amps running 
through it!  

In my mind, some sort of oscillatory condition caused this to happen.  
It's the ONLY conclusion.  No tube arcs, no nothing.  Just a popped 
resistor.

Obviously my circuit is not unconditionally stable.  Perhaps some of the 
thermal noise from the tube got this oscillation going.

So parasitics can be silent killers and happen without the amp being 
keyed or having signal pumped through it. Also it matters not wether an 
amp is biased into cutoff.  Having the tube in the active region sure 
helps, but it is not necessary, IMHO.  Being in cutoff does not guarantee 
stability.  Otherwise, my resistor would not have gone *POOF*.

So if that wasn't a parasite that happened....What was it?  Perhaps with 
your exceeding great knowledge and experience you can elmer me here.

73,

Jon
KE9NA

--------------------------------------------------------------------------
Jon Ogden

jono@webspun.com
www.qsl.net/ke9na

"A life lived in fear is a life half lived."


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