Re: Why is my amp blowing up?

On Apr 26, 5:55 pm, Peter Wieck <p...@xxxxxxx> wrote:
On Apr 26, 2:39 pm, Bret Ludwig <bretld...@xxxxxxxxx> wrote:



On Apr 26, 5:38 am, Peter Wieck <p...@xxxxxxx> wrote:

On Apr 25, 9:42 pm, Clueless Newbie <c...@xxxxxxxx> wrote:

Received a tube amp with no power cord...under the chassis there's a tab
with a black and a red wire. The fuse inside amp is attached to black
wire.

So I attached a 3 prong power cord with black, white, green wires. I
attached black to black, white to red, and green to the chassis.

I plug it in and my circuit breaker trips! But, If I unhook the green wire
from chassis, I can plug it in and it seems to work fine, though I haven't
touched the thing without gloves on.

Did I screw something up? I can leave green wire off but if it's unsafe I
don't want to mess with it.

Thanks for any help

Lemme see if I get this straight....

a) You found an amp with a cut cord.
b) Color-coding was not (from your description) either RMA or NEMA or
NEC
c) You did no circuit tracing of any nature.
d) You made some sort of connection and let 'er rip.
d) You are surprised when fuses blow.

Some Pacific-rim and Euro lighting equipment and fixtures use black
for neutral and red for hot... I have never observed this in
electronics, but it may be so. The amp may be a home-brew where the
builder did not follow "normal" color-coding.

So, before you do anything else, you need to draw out the circuit and
connect the power-leads where they make sense. As it seems (again from
your descriptive), you have reversed the polarity going in, so
naturally when the hot is connected to the ground, things go *POP*
when you apply power.

Seems a few posts back, West suggested an interesting use for a
Variac. More to the point in this case would be an effective ISOLATION
TRANSFORMER that would allow you to do some testing with at least a
modicum of additional safety. THEN use a metered variac for additional
diagnostics. You really do need to be careful with tubed equipment. B+
can run to 500V or more with very nasty results if you happen to
insert yourself into it. An isolation transformer will prevent *some*
(but not all) of that.

An isoformer will just give you an artificial sense of safety because
as soon as you hook anything up to anything else in the outside world
you are referenced to ground once again. The OP apparently does not
have a schematic, has no idea of pretty much what goes where, seems
not to possess so much as a DMM. At this point he needs to take the
box to someone who can and will get it squared away for him and
provide some tutoring. I suspect he doesn't really have basic AC
electrical practice down.

The oldtimers never used isolation transformers. They had a bench
with a ground stud and a hot terminal or rail. They learned early on
to find the hot side with a neon test light. There were no guffers so
usually they ran their own known good ground, which today you
couldn't. Rarely did they get zapped.

Untutored people (and some who should know better) do a lot of damage
with variacs.- Hide quoted text -

- Show quoted text -

Bret, an isolation transformer is a BENCH SAFETY DEVICE in any
application as may apply to items discussed in this venue. It is
useful for diagnostic purposes only. It prevents obvious types of
danger in a way that makes such diagnoses more convenient and somewhat
easier to perform. It can be a life-saver just as a vehicle air-bag.
It can also be dangerous. Just as a vehicle airbag. But it is not
meant to be taken as a cure for anything.


The problem is that once you connect any AC-powered test equipment to
the set, you have redefined chassis ground. And if you don't redefine
it you can put mains or B+ to the test equipment. I own some real test
equipment and have seen what happens when other people do stuff like
this. You haven't seen a repair bill until you have seen one from
Agilent, Aeroflex, or Tektronix.

You should have a bench set up with defined ground and defined AC
power off ground. In some cases you actually do want a big isoformer
running all your stuff, in which case you want to start looking for
surplus ones. But usually you will want to set one terminal of the
output as ground. Pro audio techs may want to rig up 110V balanced
power to see if studio gear that tests fine on normal AC supplies work
fine on balanced power too, because 60-0-60 balanced 120V is used in a
lot of studios today. You need to get a four-way 220-110-110-220
surplus transformer and set it up as 220-to-220 and feed it 110.
You'll need to derate it substantially, but it's still a lot cheaper
than a new transformer, especially since they are commonly pulled out
of "legacy" minicomputer installs-yes, many still come out each year,
there are still DG Eclipse and MicroVAX shops.

Hams, being cheap, raised the art of floating things off ground to
new levels. I've seen people wire a DMM in the plate circuit of a
linear amplifier running a 3-500Z, setting it on a piece of innertube.
If someone had touched it they could have struck an arc to ground
right through the plastic case and it could have killed them or as
sometimes happens permanently burnt the nerves in their hand and arm.


.

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