One for the valvies 2/2....
 

Stewart Pinkerton wrote:



Flippin' 'eck, I thought you guys were supposed to be the experts on
valve amps! What happens is that you have a situation similar to that
in a car ignition system. A transient hits the primary, it gets
transformed massively in the secondary, and the energy has to go
somewhere when the magnetic field collapses, so the secondary arcs
over, damaging the insulation and destroying the tranny in quick time.

Didn't I just say that ?

its the rapid di/dt that causes a high voltage.

:-)

--
Nick

One for the valvies 2/2....
 

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

Depends upon the design. However the risks a

1) Excessive plate/transformer voltages that might cause damage.

2) Instability - which may cause damage.

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

Well, they might burst into flames. :-)

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

Pass. The difficulty is that this depends upon the design and the
circumstances. May be fine in some cases, and fatal for the amp in others.

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

One for the valvies 2/2....
 

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

One for the valvies 2/2....
 

"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??

One for the valvies 2/2....
 

"Nick Gorham" wrote in message
...
Stewart Pinkerton wrote:



Flippin' 'eck, I thought you guys were supposed to be the experts on
valve amps! What happens is that you have a situation similar to that
in a car ignition system. A transient hits the primary, it gets
transformed massively in the secondary, and the energy has to go
somewhere when the magnetic field collapses, so the secondary arcs
over, damaging the insulation and destroying the tranny in quick time.

Didn't I just say that ?


You getting a bit of reverb, Nick??

:-)

One for the valvies 2/2....
 

"Pooh Bear" wrote in message
...


Keith G wrote:

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

Because with no load on the secondary, the primary of the transformer just
behaves like a huge choke.


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

The current swing can cause huge 'back emfs' that can damage both output
valves
and the transformer itself.


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

If there's any significant signal, the damage can occur.


OK, thanks for that.

One for the valvies 2/2....
 

"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.

One for the valvies 2/2....
 

On Fri, 05 Nov 2004 07:53:16 +0000, Nick Gorham
wrote:

Stewart Pinkerton wrote:



Flippin' 'eck, I thought you guys were supposed to be the experts on
valve amps! What happens is that you have a situation similar to that
in a car ignition system. A transient hits the primary, it gets
transformed massively in the secondary, and the energy has to go
somewhere when the magnetic field collapses, so the secondary arcs
over, damaging the insulation and destroying the tranny in quick time.

Didn't I just say that ?

its the rapid di/dt that causes a high voltage.

You did, but you seem to be one of the few who's clued in.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

One for the valvies 2/2....
 

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

Depends upon the design. However the risks a

1) Excessive plate/transformer voltages that might cause damage.

2) Instability - which may cause damage.

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

Well, they might burst into flames. :-)


What, more than they usually might? :-)



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

Pass. The difficulty is that this depends upon the design and the
circumstances. May be fine in some cases, and fatal for the amp in others.


I must assume that I've got away with it then, on the amps that I've done
it.

One for the valvies 2/2....
 

Keith G wrote:

"Pooh Bear" wrote in message
...


Keith G wrote:

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

Because with no load on the secondary, the primary of the transformer just
behaves like a huge choke.


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

The current swing can cause huge 'back emfs' that can damage both output
valves
and the transformer itself.


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

If there's any significant signal, the damage can occur.

OK, thanks for that.

YW.

I keep a busted KT88 in my little " rogue's gallery " of broken bits btw, that
experienced being hot-swapped when the bass guitarist shouldn't have been
playing.

The anode is actually *molten* and the glass envelope cracked from the heat !
The output transformer survived in this case luckily. 4 x KT88s was a pricey
enough mistake to make even 25 or so yrs ago.

Graham