Re: Rebias and Retube my Marshall?

Arlowe <bare.arsed@xxxxxxxxx> wrote:
After serious thinking Grinner wrote :
"White Spirit" <wspirit@xxxxxxxxxxxxxxxx> wrote in message
news:g777rq$t24$1@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Renli wrote:

On Aug 4, 7:12 pm, White Spirit <wspi...@xxxxxxxxxxxxxxxx> wrote:
Lord Elpus wrote:

You'll go far, Renli.

If you see some large cylindrical components, they'll be the
voltage dissipators. These are specially designed to make the
amplifier safe when
you do your own modifications. You simply need to place your
tongue squarely across both contacts, although you'll need to
wear an anti-static
wrist strap connected to a good earth. Remove the 1 Meg ohm
resistor from
the strap before commencing however as it is only required for
micro components.

That's not really funny. Those large cylindrical components are
called filter condensers and they store enough charge to stop
your heart.

There have been cases where people have gone to jail over saying
stuff like that.... I can see why someone might think it's funny,
but it's also extremely dangerous misinformation in the hands of
someone who might not be old enough to recognize the "joke".

It's not that I think anyone would be silly enough to try
tongue-ing a couple of electrodes - it's just that the filter
condensers are one of the most dangerous parts of the amplifier as
they can still kill when it is switched off and unplugged.

roughly how long though is safe after switched off and unplugged?
how long does it take for the power to discharge. i mean it takes a
while for the red power led to fade out on an ac30cc2, up to a
minute. as silly as i am, i'm not going to take that as guide and go
'choice! now i can open her up and have a look at her guts'.
That will depend on the size of the capacitors and the value of the
bleed resistors.
if you are bleeding off a single capacitor it very easy to determine
the time it takes to discharge

1 time constant= capacitance X resistance
In theory a capacitor can never be completly discharged, but for
practical reasons it is considered to be discharged in 5 time
constants.
So the time it takes to discharge a capacitor will be:
(5 X Capacitance in farads X Resistance in Ohms)seconds.
So if you have a 10 uf (0.00001 farad) and used a 10k ohm bleed
resistor to discharge (5 * .00001 * 10000 = 0.5)it would take a half
second to discharge the capacitor.

3 time constants should be OK...if the cap's charged to 400 V then three
time constants brings the voltage down to exp(-3)*400 V = 20 V. There's
not too much bite in that. At that point it should be safe to place a
short across the cap while you work, but check with a meter first! But
I'd use a longer time constant, it eases the power requirement on the
bleed resistor.

A typical filter cap value is 25 uf. At 400 V the stored energy is
(1/2)CV^2 = 2 J. If you discharged that with a resistor that gave a 0.1
second time constant you'd need a resistor that could handle 20 W!
(That would be a 4 K resistor in this example).

Using the 25 uf value, if you wanted a 1 second time constant you'd need
R = 40 K. Using the 400 V value, with 2 J energy stored, the peak power
would be 2 W. Much better!

Best to overkill on the power rating of the resistor. This site
recommends a 25 K resistor rated at 10 W, which should be plenty for a
wide variety of filter caps.

http://studentweb.eku.edu/justin_holton/caps_p.html

The problem with something like an amplifier there are several
circuits with multiple caps on each circuit Some may have bleed
resistors, some may not... Again, that is where experience come in to
play....
It will be the High voltage power circuit that you will want to
discharge, they usually have big electrolytic capacitors and they
bite.

Good idea to never assume there's a bleed resistor built in even if the
schematic says there is. And yeah, the filter caps are the killers!

(btw I know this stuff because I know electronics but I for one wouldn't
be caught dead or alive with my fingers in there....I take mine to a
pro!)


.

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