OK, exactly why don't trannies (on valve amps) like it being run without a
load?

It's not like they are going to burst open, is it??

Also does quick 'hot-swapping' speakers while an amp's hurt it any? (Oi dun
a lot o' thaat in tha parrst.... ;-)

Keith G wrote:
OK, exactly why don't trannies (on valve amps) like it being run without a
load?

It's not like they are going to burst open, is it??

Also does quick 'hot-swapping' speakers while an amp's hurt it any? (Oi dun
a lot o' thaat in tha parrst.... ;-)



Well it starts with load lines and reflected loads and goes on from
there. how far back do you want to go ?

--
Nick

"Nick Gorham" wrote in message
...
Keith G wrote:
OK, exactly why don't trannies (on valve amps) like it being run without
a load?

It's not like they are going to burst open, is it??

Also does quick 'hot-swapping' speakers while an amp's hurt it any? (Oi
dun a lot o' thaat in tha parrst.... ;-)



Well it starts with load lines and reflected loads and goes on from there.
how far back do you want to go ?


Wot ho Nick, that wuz quick!!

OK, not too *deep* but more along the lines will it damage the amp if a
speaker swap is quite quick (while it's *on* - omitted that in the original
post)...???

Perhaps I should say will it damage the transformer?

Keith G wrote:
OK, exactly why don't trannies (on valve amps) like it being run
without a load?

I think it has something to do with the xformer's secondary trying to drive
a very large impedance (ie, open circuit). I think this translates into
excessive current in the primary (but I could be wrong).


--
Wally
www.artbywally.com
www.wally.myby.co.uk

"Wally" wrote
Keith G wrote:
OK, exactly why don't trannies (on valve amps) like it being run
without a load?

I think it has something to do with the xformer's secondary trying to drive
a very large impedance (ie, open circuit). I think this translates into
excessive current in the primary (but I could be wrong).

I'd have thought quite the reverse; usually, I'd have thought the
primary current would decrease when the load was removed from the
secondary.

Mind you, if there's feedback and so on happening round it all, goodness
only knows.



Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

Fleetie wrote:
"Wally" wrote

Keith G wrote:

OK, exactly why don't trannies (on valve amps) like it being run
without a load?

I think it has something to do with the xformer's secondary trying to drive
a very large impedance (ie, open circuit). I think this translates into
excessive current in the primary (but I could be wrong).


I'd have thought quite the reverse; usually, I'd have thought the
primary current would decrease when the load was removed from the
secondary.

Yes, it will, without the reflected load the primary reverts to
effectivly choke loaded. But the secondary is still a inductor, and so
does what comes naturally to it.

Think about how ignition coils in cars work, its the rapid di/dt that
causes a high voltage.


Mind you, if there's feedback and so on happening round it all, goodness
only knows.


Its not a feedback issue, just inductors at play.

--
Nick

"Nick Gorham" wrote in message
...
Fleetie wrote:
"Wally" wrote

Keith G wrote:

OK, exactly why don't trannies (on valve amps) like it being run
without a load?

I think it has something to do with the xformer's secondary trying to
drive
a very large impedance (ie, open circuit). I think this translates into
excessive current in the primary (but I could be wrong).


I'd have thought quite the reverse; usually, I'd have thought the
primary current would decrease when the load was removed from the
secondary.

Yes, it will, without the reflected load the primary reverts to effectivly
choke loaded. But the secondary is still a inductor, and so does what
comes naturally to it.

Think about how ignition coils in cars work, its the rapid di/dt that
causes a high voltage.


Mind you, if there's feedback and so on happening round it all, goodness
only knows.


Its not a feedback issue, just inductors at play.



OK, tells me not a lot other than it's probably not too damaging if it's
quick and there's no signal present??

In article , Nick Gorham
wrote:
Fleetie wrote:



Mind you, if there's feedback and so on happening round it all,
goodness only knows.


Its not a feedback issue, just inductors at play.

Not quite. The o/p transformer is a pair of *coupled* inductors, and there
may be feedback from either/both sides. This may then control the voltage
gain and prevent problems. Depends upon the details of the design. Removing
the speaker load may drop the current levels, but this does not *guarantee*
that the plate swing will rise to a dangerous level. There is a risk of,
this, though. So you must assume it possible with an 'unknown' design.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

"Jim Lesurf" wrote in message

In article , Nick Gorham
wrote:
Fleetie wrote:



Mind you, if there's feedback and so on happening round it all,
goodness only knows.

Its not a feedback issue, just inductors at play.

Agreed. If there is a current flowing through an inductor, and the source of
that current is removed, then the voltage across the inductor will tend to
increase until the given level of current flow is maintained across whatever
impedance effectively parallels the inductor, at least briefly. If the
parallel impedance is very high, then the voltage kick will tend to be very
high.

This is the well-known inductive kick-back, which is commonly used as a
basis for automotive ignition systems, for example. Many people do not
realize that while the DC voltage applied to the primary of an automotive
ignition coil may only be 12 volts, when the points or their modern SS
equivalent opens up, there may an inductive kick of several hundred volts.
(Note: other technology may be used.)

Not quite. The o/p transformer is a pair of *coupled* inductors, and
there may be feedback from either/both sides.

Agreed. The amount of mutual coupling between the primary and secondary can
vary.

This may then control
the voltage gain and prevent problems. Depends upon the details of
the design. Removing the speaker load may drop the current levels,
but this does not *guarantee* that the plate swing will rise to a
dangerous level. There is a risk of, this, though. So you must assume
it possible with an 'unknown' design.

If there is an effective negative feedback loop, then its unlikely that
dangerous voltages will be developed in the output transformer, no matter
what the speaker load is, ranging from zero to infinity.

Trouble can begin when the amp lacks loop feedback, which is strangely
enough increasingly popular.

Or, the feedback loop can be present but ineffective, for example if the amp
is operating in a highly nonlinear way.

Jim Lesurf wrote:

In article , Nick Gorham
wrote:
Fleetie wrote:

Mind you, if there's feedback and so on happening round it all,
goodness only knows.


Its not a feedback issue, just inductors at play.

Not quite. The o/p transformer is a pair of *coupled* inductors, and there
may be feedback from either/both sides. This may then control the voltage
gain and prevent problems. Depends upon the details of the design. Removing
the speaker load may drop the current levels, but this does not *guarantee*
that the plate swing will rise to a dangerous level. There is a risk of,
this, though. So you must assume it possible with an 'unknown' design.

I've certainly seen the classic output stage burnout occur where feedback *was*
taken from the secondary side - so I wouldn't advise that as an indicator of it
being safe to hot swap.


Graham