Re: Remote Power Supply Connection

Trevor Wilson wrote:
> 
> "Ian Thompson-Bell" <ruffrecords@[EMAIL PROTECTED]
> wrote in message
> news:g0rmo0$27co$1@[EMAIL PROTECTED]
> > Trevor Wilson wrote:
> >
> > -- Trevor Wilson www.rageaudio.com.au "Ian Thompson-Bell"
> > <ruffrecords@[EMAIL PROTECTED]
> wrote in message
> > news:g0ovq7$dq3$1@[EMAIL PROTECTED]
> > > > flipper wrote:
> > >> >> On Sun, 18 May 2008 07:36:56 +1000, "Trevor Wilson"
> > >> >> <trevor@[EMAIL PROTECTED]
> wrote:
> > >> >>
> > >>> >>> "Ian Thompson-Bell" <ruffrecords@[EMAIL PROTECTED]
> wrote in message
> > >>> >>> news:g0mu6g$2vgp$1@[EMAIL PROTECTED]
> > >>>> >>>> Trevor Wilson wrote:

 snip, 

> > > > This clearly shows that at low currents (e.g HT currents) and high
> > > > voltages (e.g.HT voltages) dc arcing is very unlikely to occur.
> >
> > **Remind me: How much energy is able to be stored by large (unstated
> > values)
> > capacitors?
> >
> > 0.5*C*V^2
> >
> > In a 100uF capacitor at 300V that's a mere 4.5 joules. Not worth
worrying
> > about.
> 
> **Points:
> 
> * 4.5 joules is a substantial amount of energy. My small rotary hammer
drill
> is rated at 2.5 joules per strike and can pulverise bricks. 4.5 joules
can
> seriously damage connectors.
> * I've worked on valve power amps which employ MUCH larger capacitors
than
> 100uF. More like 2,000uF. (Just a reminder: The amp under discussion was
a
> power amp and B+ figures under discussion have ranged up 700 Volts) That
> makes it more like 490 joules. And that is a figure well worth worrying
> about.

I design and build tube amps with huge amounts of capacitance and up to
650Vdc present.

You don't design or build anything, and have never been forced to make
the best
decision where separate chassis are necessary so that the amps can be
moved.

If you remove a plug on a cable going to a tube amp with charged up
large value electros,
and immediately grab the pins, the high resistance of the body will make
the shock a sustained one
by dc flow until the Vdc subsides.

If you place 1,000V rated diodes at the amp cable entry,
this high dc flow does not occur. 

Best design and construction is something you have never comtemplated
for
where there is potentially lethal Vdc and stored energies.

Saying to us all, like a sick featherless parrot,
"use ac only" is an entirely unacceptable answer to our concerns.

But where the PSU is separate from an amp chassis,
the vast majority of filter caps and inductors should all
be within the PSU case. The capacitance at the CT in the amp chassis
should be
a small value, perhaps 2uF, or just enough to shunt the 
inductance of the umbilical cable length, and if there is a 100k
discharge resistance,
the time to bring it from say 500V to 150V is 0.2 seconds.
Its down to about 50V in 0.4 seconds.

The tube current present would vastly exceed the 5mA with 100k 
across the 500v rail, so normally, the Vdc with 2uF would drop
much faster than 0.4 seconds.

An arc from a 2uf cap doesn't damage much.

The usual design of tube amps has the early stages
fed by B+ through R&C filters where C will be a lot more than 2uF.

So if you have say 2k2 to 470uF for the driver amp, the 100k discharge 
R won't pull down the voltage much

So if the plug is removed, a relay can be used to 
shunt the B+ to 0V BUT THROUGH A SUITABLE RESISTANCE TO LIMIT CURRENT.
I have often utilised one side of a DPDT mains switch to 
turn on a discharge resistance in the 'off' position.
But one must not rely on the amp being turned off for the discharge to
still occur
asap if the live plug is removed, and grabbed.
The R must be able to take the peak current discharge, and the current 
must not exceed a safe value for the relay.
And a 0.01uF should be across the relay contact points.
One doesn't want an arc, and doesn't what a large "Blatt!" inside the
relay.
Noticed how relays rated for say 6 amps and 250Vac are 
usually reated for much lower values of switching DC?
Its the arc, and the huge peak currents involved that corrode
contacts, and can ruin them.

The really good designer will have separate DC rails to each stage of
the amp
supplied by separate rails in the PSU, with all the large caps in the
PSU,
and with only low value caps for bypassing in the power amp chassis
which are very easily discharged in less than 0.1 second.
If you have a relay and discharge resistances and diodes to prevent back
currents
there isn't much possibility of a shock, no?

Of course the the tube currents that normally are present will very
quickly discharge the 
low value bypass caps on the power amp chassis.
The relay + R method really only does much if the PSU is turned on
without 
tubes present, and then the discharge is entirely reliant on relay & R.
  

So its not hard to do it Trevor.

You can spout your ****ing unacceptable bull**** long as you like, and
maintain
a position of NEVER offering a suitable solution with all its ***precise
details.***
Be a stupid **** if you want Trevor.

But we all know how idiotic your stubborness and stupidity appears to
everyone here.

You pontificate on a newsgroup about tube crafting yet you have never
designed or built any amplifier
since about 1970.

Patrick Turner.

 
> 
> --
> Trevor Wilson
> www.rageaudio.com.au