[sdiy] RE: Tube amp discharge tool (important safety information)

[Harry Bissell Jr]

Sorry guys, you are all wet here.

Discharging a cap by "arc" is NOT anthing like
"hitting it with a square wave".  

First, a square wave does not go from 0 - 400V
in no time. Internal impedances in the transformer, as
well as the natural rise of current in the transformer
inductance, and the fact that you are pretty likely
to NOT always throw the switch at the peak of the AC
line... all conspire to slow the rate of charge into
the
caps.

If you could throw that 'square wave" you probably
just
blew the rectifier diodes.

Discharging the cap with an 'arc' or direct short
circuit is going to draw current according to ohms
law... with ESR of the cap as the only limit. The
power
is ALREADY stored in the cap. There is no limiting
inductance here, its pure resistance and precious
little of that.

The arguement that 'I have done this successfully'
does
not hold any merit... nor does saying that you 'won an
russian roulette'.  

The ARC itself can be dangerous, it can burn you,
leave
chunks of metal missing from your amp. Leave burn
marks on the chassis that would not inspire a
customer's confidence in your repair skills.

You ARE damaging the capacitor. Can you get away with
it once, maybe... twice... maybe...

Try this.  Take an electrolytic capacitor (maybe
50uF)and charge it to some reasonably low voltage
(maybe 24V).  Run a wire through a scope current
probe. Discharge the cap by shorting it with the wire.
Arc spark... what was the peak current ?   Maybe 50A

Keep repeating this process.  After ten times or so,
you'll notice that you never get that 50A peak, ever
again.

I know this becauce I needed a pulse current source
for testing a current sensing safety system. The first
few times it WORKED and tripped my circuit, then it
stopped working.  what I found was that a new
capacitor could
pass enough current to cause the trip, but that the
cap degraded every time after that.  Soon it was not a
good capacitor anymore.

I'm sharing this because most folks won't have the
current probe, or notice the degradation. Why put in
new caps and potentially ruin them ?

There was a post about shorting the supply through onr
of the plate connections of the power transformer...
at least here you have a bit of series inductance and
resistance (once saturation occurs). By then the cap
is probably discharged. Still, that would not be my
first choice.

Shorting a cap with a screwdriver is just plain bad
advice and should not be given.  Using a resistor to
discharge the cap at a controlled rate, and checking
it with a meter is GOOD advice that everyone should
follow.

H^) harry  (this is the ONLY smiley in this message
:^)