Re: Shock Info

RichL wrote:

<teemukyttala@xxxxxxxxx> wrote in message
news:1180480853.195722.10890@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

I don't even know why people ask questions like:


how long does it take that bleed-resistor to make the circuit
safe.....and can this be used on any tube amp?

You are asking for an information that can be found from every
respected book about electronics theory! I do hope that people take
time to do old-fashioned stuff like reading books or studying BASIC
electronics before they decide that they are competent enough to start
dealing with potentially LETHAL tube circuits. To answer the question:
The resistance is not the only variable in the related theory. You
need to calculate the time constant t and how long it is (in seconds)
from:

t = R C,

where C = the capacitance storing a specific charge and R = resistance
that shunts the capacitor to ground.

During the first time constant, voltage drops to 36,8% of the initial
value. Basically, the voltage drops according to following table:

1t = 36,8 % of initial value
2t = 13,5 % of initial value
3t = 5% of initial value
4t = 1,8 % of initial value
5t = 0,7 % of initial value

So you see, the voltage and capacitance are equally important as the
resistance of the "bleeder" resistor. This should give a rough
estimate of the time it takes for the capacitor to discharge to a
"safe level" - whatever that is. However, in practice you don't count
seconds but MEASURE the voltage with multimeter before you touch
anything with anything other than the measurement probes! The "bleeder
resistor" discharge method works with all electric appliances that
store charge to capacitors - and that definition includes tube amps.


And although the famous "screwdriver method" will provide a nice spark
show don't use it because:
1. Dielectric absorption may become a problem with high voltage
charges
2. You have a risk of ruining the capacitor since during the discharge
the only current limiting factor is ESR
3. You have even higher risk of destroying delicate solid-state
devices that the circuit might contain
4. You ruin the screwdriver


Just to add what is probably obvious to most and has been posted here many
times: the analysis you describe applies only approximately to filter caps
because of the memory effect. Once you think the right time has passed,
*measure*, and if the measurement indicates it's OK ground the damned things
and keep them grounded until you're done so they don't charge back up!


THAT is the most useful information.
.

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