[sdiy] Tube amp cap discharge tool?

[Bob Weigel]

Sure but that's continuous duty. Where heat has time to build up and 
hurt things.  Let me give you an example. Ordered from Parts express 
WHEN I WAS YOUNG AND NAIVE..ahem ....:-) (yes as to indicate hopefully I 
dont' do things this bonehead anymore..) to build my first mains which I 
still use btw. Sound like a set of tight bookshelf speakers 500 yards 
away. Really love those things.  But tougher to lift every time somehow 
:-))   SOME...capacitors to build crossovers for them.  The 'tech' they 
had assured me that the stock electrolytics should be finnnnneeee for 
any speaker because "no current goes through them".  I TRIED to explain 
to the guy that functionally..current DOES go through them but he 
couldn't get the concept of current travelling from lead onto the plate 
and off again as an AC current..continuously.
      Well..after the smoke cleared I ordered some of the higher end 
ones and things went better let's just say. :-)   I sent the guy the 
documented proof that he was ignorant....and that my suspicions were 
correct.  That little cap could not handle the amount of current I was 
going to run at 400Wrms/channel and whatever that wound up being on that 
particular part current wise.  It melted them nicely.
       Now while we are talking much much more current here, we're 
talking a much much shorter time.  And what killed that cap was the 
INABILITY TO DISSIPATE THE HEAT that was produced over a long period of 
time.  In a dischage situation we are talking about a millisecond of 
action or more depending on how much the integrated impedence the plasma 
interface left out in Harry's equation presents us.  (Yes when that 
first tiny piece of metal touches, it is ENSHROUDED in what looks like a 
CATACLYSMIC EXPLOSION if you were actually...some little creature 
watching the event.  Instantly the surface that touches it becomes 
oxidized and pieces are spattered off the anode and this wild thing 
happens that we view as a 'spark'. )  UNLESS of course you do something 
crazy like coat your screwdriver with dielectric grease and slam it 
against a very good matching flat surface.  I've never calculated what 
could happen in a situation like that but intuitively I doubt you'd even 
hurt anything there.   Anyway..I've spent way too long on this but I 
kind of LOVE the topic because it digs into some very groundwork areas 
that should inspire deeper understandings of some kind of important 
things in electronics.  Many get the practical knowledge that says 'do 
this, don't do that'..but very few in this field really dig into the low 
level picture of what is going and AND ..it is from this kind of stuff 
that the most revolutionary thoughts in new design spawn.    People 
often get caught in a rut of using the same parts and doing things the 
same way. Hey..let's design some new ways of doing things.  That's where 
it's really it it would seem to me. And to do that..well you have to get 
back to the nuts and bolts of it all and start thinking about what 
electrons do when they interact with all kinds of natural 'situations'.  
-Bob

The Peasant wrote:

>>Never seen direct discharging a typical tube amp capacitor cause a 
>>problem.  
>>    
>>
>
>Most DC filtering capacitors are only rated for a small amount of power 
>dissipation and, for example, cannot be used on an AC line like a motor 
>starting capacitor (these have much higher power ratings). Shorting a capacitor 
>with a significant charge in it will exceed this power rating and will very 
>likely damage the capacitor.
>
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>>You've got to remember I've been 
>>doing this stuff since I was a boy and boys often test things.  
>>Sometimes foolishly, but in the process..hopefully they survive and 
>>learn lessons from those things and begin to comprehend limits.
>>    
>>
>
>Please realise that many of us here are in the same position as you. I 
>personally have been working with electronics for 35 years, ever since I could 
>say "resistor", and I know that I am not the most experienced or knowledgeable 
>one here by far. I still believe that shorting out capacitors with screwdrivers 
>is a very unwise thing to do. For the small amount of time that it would take 
>to wire up a clip lead with a discharging resistor, it really makes sense to 
>play it safe. I certainly wouldn't want anyone working on my equipment to start 
>shorting out power supply capacitors.
>
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>>We live in a very fear filled society today 
>>    
>>
>--SNIP-- 
>I warned him not to go there with it but 
>  
>
>>he had to please his lady (who later messed around on him..  oh well. ) 
>>    
>>
>
>Thanks for the story, but I don't really see much relevance here. Would you not 
>wear a seat belt because "I have driven without one for many years, and never 
>had a problem"? Personal experience is important, but if a bunch of other 
>experienced fellows told me something that I was not aware of, I would tend to 
>listen to them.
>
>Take care,
>Doug 
>______________________
>The Electronic Peasant
>
>www.electronicpeasant.com
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