Luk,
At least for amplifiers I've seen here in North America, the isolation
between the chassis metal and the power mains neutral typically come
from the HV transformer (and any additional transformers that are used
for producing things like filament voltage, control voltages, etc). Do
you have any connections between the lines on the primary side of your
transformers and the grounding conductor? In other words, I am trying to
understand why you need an isolation transformer to prevent RCD/GFCI
trip when the transformers in the amplifier should provide that function?
73, Mike W4EF
On 11/28/2024 3:52 PM, Lukasz wrote:
Hi Jim,
It is similar here in the EU as far as I know.
A typical way devices, are constructed here is that the ground (including
the case if made of metal) is not used as a normal current path in the
device. One sees current flow on that ground only in a fault scenario.
This means RCD/GFCI protections can work as when it see any discrepancy in
the live/neutral current balance it shuts the circuit down.
These rules normally mean one always has a return current path within the
device that is separate from ground (neutral, DC minus etc), but this is
difficult when the amplifier is entirely made of metal and one would prefer
to have a current return plane (for example the floor if the case) rather
than route separate connections for DC minus and RF ground.
This is not how OLD devices were made. I have a couple of old lathes and
mills that do use the steel frame as neutral/ground(one connector called
PEN) . This is how, it seems, all tube amps are built I saw pictures so
far.
So my question is, if one wants to build a tube amp in a modern way.
Properly grounded, is building a case within a case and electrically
isolating the two the only way? Or has anyone managed to build a tube
amplifier in a single grounded steel case. That works normally within the
usual electrical systems (with gfci/rcd protected circuits). If so, please
describe how are all the current return paths done.
73,
Luk, SP4IT
On Wed, 27 Nov 2024, 23:05 Jim Brown, <jim@audiosystemsgroup.com> wrote:
Hi Lukasz,
In North America, the protective earth MUST be carried in the same cable
with the current-carrying conductors, and MUST be bonded to the chassis
at the point where that cable enters. I'd be surprised if it's different
in EU -- it must be done that way to be protective!
There's another requirement in NA -- all grounds (earths) MUST be bonded
together. Again, protective against both lightning and electrical shock.
So when power goes through ANY transformer that's not part of equipment,
protective earth MUST be carried from one side of the transformer to the
other.
I don't remember about EU, but I do remember that in the UK, no earth
connection to the power system is permitted -- that happens only within
the power distribution system.
When thinking about this and antennas, remember that the earth is a big
resistor, and parts of antennas like radial systems are NOT intended to
couple to the earth, but rather to shield the EM field from the earth,
and to provide a low resistance path for the antenna's return current.
The only antennas that benefit from an earth connection are SOME
receiving antennas, like Beverages and loops.
73, Jim K9YC
On 11/26/2024 8:10 AM, Lukasz wrote:
Hi,
Is there some way to build a tube amp in a metal case, using the case as
RF
ground, and not have current flow out of the amp through the grounding
conductor? (other than isolating that ground from the case entirely,
which
presents it's own problems - for example capacitive coupling to the
case).
I'm currently experimenting with an amp and I have to power it via an
isolation transformer (the high voltage is 3 phase on its own circuit
with
no GFCI temporarily) or my RCD will trigger.
The high voltage power supply is an old military device that used its
chassis as ultimate DC ground (hv ground connects to it through a
overcurrent protection coil).
Then I also used the chassis as ground for DC HV, screen and grid
supplies.
This I could swap, but it's the RF ground (especially the Pi tank) I'm
concerned about.
Is essentially isolating amp RF ground from the grounding conductor
(PEN/EGC or whatever it's called where you are) the only way?
73,
Łukasz, SP4IT
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