[AMPS] Another arc question

[Geurts, Gerard]

Jon Ogden wrote:
> 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*.
> 
Your amp was not in cutoff. Unless you call 50 mA plate current
'cutoff'. When this event happened your amp was biased in the 'active
region'. What are you trying to proof?

> 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
> 
I thought the whole idea of using Rich's suppressor is to stop
parasitics from happening in the first place. Now you are telling us
that what happened in your amp has to be caused by parasitics. While
using Rich's suppressors? Apparently they are not as good as some people
think. Or your amplifier is so badly constructed that even Rich's
suppressors won't help anymore. 

Which one is it, Jon?

Cheers

Gerard

AA3ES, MoAIU

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