[sdiy] Tube amp cap discharge tool?

[Bob Weigel]

Never seen direct discharging a typical tube amp capacitor cause a 
problem.  Knowing the physics involved fairly well, I'd guess not but 
I've never actually done the calculations.  I'm in a great position to 
learn things because I'm the only guy in the area doing this.  I tend to 
have a VERY high return rate if problems recurr or like that.  I've done 
a fairly vast number of tube amps since I've been here; it's the most 
common thing I see probably if you break things down into simple classes 
like that.   I've replace a LOT of capacitors that were simply old shelf 
life exceeded and often blistering electrolytics.    In the process I've 
also left some that seemed good on esr and value still at customer's 
request.  Many of those I discharged directly.  I have seen zero 
reported failues of capacitors in amps where this was potentially done 
by me in the last 7 years. 
      The physics involved...basically you have a fairly large surface 
area of electrolyte sandwiched between conductive foil.  The fact that 
you are SEEING a spark...is a good thing.  That's where a large 
percentage of the energy is being absorbed.  If a cap is already 
internally damaged then you might develop a hot spot that will become 
brittle and degrade in the conductive layer to wire interface.  However, 
given the amount of area that is usually involved there due to the 
construction technique inside these things, that isn't really very likely. 
      The electrolyte itself meanwhile is a material which contains a 
lot of easily reconfigured electric dipoles.  When you discharge, these 
all move as quickly as allowed by their molecular structural 
interactions.  But remember, were it not for the dielectric constant of 
that material...the capacitor would be holding a relatively feeble 
amount of charge at the voltage it was at before the discharge event.   
Basically...they respond when they feel like it therefore.  That is why 
they have limited HF response and an "esr" (equivelent series 
resistance) spec.  Discharging them rapidly is just the inverse of 
hitting them with a big square wave.   And ALL capacitors have some kind 
of rating as to how many of those kinds of events they can take without 
being damaged if the manufacterer has bothered to do the testing.  
But..like I say in my testing...all the ones I've seen can take it once 
with a good long rest afterwards :-).  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.
       We live in a very fear filled society today compared to when I 
was growing up.  People have a harder time learning I believe because of 
this.  eg.   driving.   I enjoyed driving until a few years back when I 
sold my Nova.  Hehe.  Before I hit ice on a night where I perceived it 
was 45ºF, I drove a 350 vega I had built from garbage and a motor my dad 
and I put together before he died in '82.  I tried a lot of things that 
were..well quite unconventional.  As a result I broke the rear end 
brackets about 3x before I finally devised a design that could take the 
torque.  I guessed w/out making calculations that some used nova springs 
might stiffen up the rear end just right...and..I turned the machine 
into something that could act like a jackhammer on wheels :-)  PUnch it 
and it would resonate on and off the ground in the rear at 10-15hz 
probably.  Hehe.  What a ride!
        And many other things I learned in experimenting with my own 
ignition system design and so on.  I spent a couple years messing with 
that thing on and off.  But when I got done hey...I knew how to built a 
car with stock parts that went 0-60 in arond 5 seconds and got 27mpg on 
the road!  Not bad for a holly 600.   The...windshield wipers started by 
themselves at 130mph and I decided that was enough for the speed test. 
:-)  I've always been more into controlled accelerations and maneuvre's 
than speed.  Too many bad things can happen even with a sway bar 
installed that just degenerate into an immediate death situation at high 
speeds.
       But hey people travel at those speeds in Europe routinely...not 
in vega's all that often but still...somebody had to design something 
and know the limits of it and test it in a way they saw as reasonable.  
Each person is responsible for doing that and hey...with a zero accident 
(save the unexpected ice which had me going off the road as I hit it 
passing a guy after a truck had already done so in front of me who was 
going 20mph...) record...and the extensive amount of driving I've done I 
can pretty much tell critics of my style in that area to get a life :-). 
        It's a complex universe and there's a balance.  THIS GROUP IS 
ABOUT CREATIVITY.  I always try to post caution, as in this case in 
trying to qualify that I'm only talking about capacitors in normal 
guitar tube amps and not some monster PA one which might come up from 
time to time.  But too much caution can lead to a fear that will stifle 
creativity.  START SMALL is my recommendation for anyone in this group 
not very experienced in electronics.  For instance..line up some 
capacitors of various values.  Charge them up to 10V and discharge them 
from the smallest value starting at say 10uF to the largest you feel 
comfortable doing.  (ie. whent it starts to leave a sign of carbon 
desposit on the screwdriver tip..that's probably far enough.)   Then 
step it up to 20 Volts on a few of them to compare how that amount of 
energy looks.  Then if they are rated for it 30V and all this time being 
SUPER careful not to hold the leads or any such thing per chance you 
have a cut in your finger at these voltages and zap the energy though 
you somehow.
       Anyway, very much like I recommend people who drive cars to get 
out on an icy day and carefully make their way to an abandoned parking 
lot.  Again..start slow and being pulling maneuvre's until you have a 
grasp of the amount of control you really have on a sheet of ice.  Then 
pray you never have the kind of horrible luck I have where I am already 
headed towards the ditch as I hit the ice ;-). But....THE GREATEST 
DANGER is to be CLUELESS because at some point reality just might 
intrude on you.  And you might be like the more typical guy who I was 
trying to pass who went "Oh no honey bear...ICE!!!" as he and his wife 
slid off the side of the road for absolutely NO reason.  Nit wit.  Of 
course the clueless guy had no damage to his car though...mine happens 
to go off a 4 or 5' drop into a field; does a nose roll and lands on the 
side.  Sold it to a kid who worked it back into some shape then hit a 
stump with it in the cascades.  I warned him not to go there with it but 
he had to please his lady (who later messed around on him..  oh well. ) 
  -Bob

The Peasant wrote:

>>Harry...the context was "how do we do this in a tube amp".  With a well 
>>insulated handle...how is the energy stored going to hurt anyone? 
>>(assuming this isn't some collasal amp that I've never witnessed..I 
>>mean..use some common sense.. if you see one..be sceeerd..be very 
>>scared).  I routinely have 'let 'er rip' for the last many amps I've 
>>worked on and while I recommend not sticking your face right in the arc 
>>zone..nobody gets hurt by the 100uF 450V caps that are commonly the max 
>>section found in a tube amp.    -Bob
>>    
>>
> 
>While you may have the sense not to try this with a larger capacitor/voltage, 
>others on this list may not. Please do not assume that everybody here has your 
>level of technical expertise and common sense. Shorting out a power supply of 
>any type with a screwdriver or other direct short should be avoided at all 
>costs, regardless of the voltages or capacitors involved. This is a very poor 
>work habit which can turn around and bite you in the @ss and broadcasting this 
>example of poor judgement to a public mailing list invites disaster and 
>possible lawsuits. In DIY, safety should ALWAYS come first, we sure don't want 
>to lose any listmembers to injury or worse. :-(
>
>Besides, doesn't fast discharging of electrolytic calacitors risk damage to the 
>capacitors themselves? Especially old tube amp capacitors?
>
>Take care,
>Doug
>______________________
>The Electronic Peasant
>
>www.electronicpeasant.com
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