One for the valvies 2/2....
 

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.... ;-)

One for the valvies 2/2....
 

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

One for the valvies 2/2....
 

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

One for the valvies 2/2....
 

"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?

One for the valvies 2/2....
 

"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

One for the valvies 2/2....
 

"Keith G" wrote in message


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

There is a possibility that the output transformer(s) will arc over
internally and be permanently damaged.

One for the valvies 2/2....
 

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

One for the valvies 2/2....
 

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.


Graham

One for the valvies 2/2....
 

On Thu, 04 Nov 2004 19:25:34 +0000, 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.... ;-)

If you want to do some speaker swapping turn the volume right down
or, better, switch to standby (HT off) while you do it. That way the
output stage won't be driving into the open load and it will be ok.

Running a signal into an open load can do a few things:
1 - nothing (quite possible as some circuits put sufficient load on
anyway, or you may have insufficient signal to cause a problem).
2 - internal arcing in one or more valves - usually ok.
3 - insulation breakdown on one or more turns on the output transformer
primary. If this causes one or more shorted turns (quite possible) then
the tranny is usually dead. If it causes an open circuit then it is
definitely dead! It probably won't look damaged though.

--
Mick
(no M$ software on here... :-) )
Web: http://www.nascom.info

One for the valvies 2/2....
 

On Thu, 04 Nov 2004 20:50:43 GMT, "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.

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

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.
--

Stewart Pinkerton | Music is Art - Audio is Engineering

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

One for the valvies 2/2....
 

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

One for the valvies 2/2....
 

In article , Pooh Bear
wrote:
Jim Lesurf wrote:

In article , Nick Gorham
wrote:


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.

Agreed. Hence the "may" in my comments. ;-)

My personal approach would be to avoid having the output o/c when a valve
power amp was on. Even with designs that 'should' be OK, as such mistakes
can be expensive.

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....
 

"Pooh Bear" wrote


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.


Oops.


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.


Yes and they're not too cheap nowadays!

One for the valvies 2/2....
 

Keith G wrote:

"Pooh Bear" wrote

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.

Oops.


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.

Yes and they're not too cheap nowadays!

It *only* cost him about £60 back then - but was enough that he had to pay me
for the repair in installments !


Graham

One for the valvies 2/2....
 

On Sat, 06 Nov 2004 07:32:15 +0000, Pooh Bear
wrote:



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

A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!

d
Pearce Consulting
http://www.pearce.uk.com

One for the valvies 2/2....
 

Don Pearce wrote:



A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!


You may be joking, in which case sorry, but AFAIK, the only way you
could get such radiation is to accelerate the electrons quite a bit
more, and the only way you could do that was with more voltage. More
current would melt a anode, its quite easy to get them glowing, as being
in a vaccume it can only loose heat by radiation.

Someone correct me, but I think you need somewhere in the region of 25k
to start producing x-rays.

--
Nick

One for the valvies 2/2....
 

On Sat, 06 Nov 2004 16:33:25 +0000, Nick Gorham
wrote:

Don Pearce wrote:



A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!


You may be joking, in which case sorry, but AFAIK, the only way you
could get such radiation is to accelerate the electrons quite a bit
more, and the only way you could do that was with more voltage. More
current would melt a anode, its quite easy to get them glowing, as being
in a vaccume it can only loose heat by radiation.

Someone correct me, but I think you need somewhere in the region of 25k
to start producing x-rays.

No, you're right. I was thinking back to my early days when I designed
colour TVs. We would do an overvoltage test on line output stages that
would have the anodes glowing red with about 30kV on board. We used
lead screening around that box, and an x-ray dosimeter nearby.

d
Pearce Consulting
http://www.pearce.uk.com

One for the valvies 2/2....
 

"Don Pearce" wrote in message
...
On Sat, 06 Nov 2004 16:33:25 +0000, Nick Gorham
wrote:

Don Pearce wrote:



A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!


You may be joking, in which case sorry, but AFAIK, the only way you
could get such radiation is to accelerate the electrons quite a bit
more, and the only way you could do that was with more voltage. More
current would melt a anode, its quite easy to get them glowing, as being
in a vaccume it can only loose heat by radiation.

Someone correct me, but I think you need somewhere in the region of 25k
to start producing x-rays.

No, you're right. I was thinking back to my early days when I designed
colour TVs. We would do an overvoltage test on line output stages that
would have the anodes glowing red with about 30kV on board. We used
lead screening around that box, and an x-ray dosimeter nearby.

d
Pearce Consulting
http://www.pearce.uk.com

Was there an appreciable increase in measurement of x-ray emission with 30kV
on LOP stage compared to normal HT/EHT supply/s?

Mike

One for the valvies 2/2....
 

On Sat, 6 Nov 2004 17:15:33 -0000, "Mike Gilmour"
wrote:


"Don Pearce" wrote in message
...
On Sat, 06 Nov 2004 16:33:25 +0000, Nick Gorham
wrote:

Don Pearce wrote:



A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!


You may be joking, in which case sorry, but AFAIK, the only way you
could get such radiation is to accelerate the electrons quite a bit
more, and the only way you could do that was with more voltage. More
current would melt a anode, its quite easy to get them glowing, as being
in a vaccume it can only loose heat by radiation.

Someone correct me, but I think you need somewhere in the region of 25k
to start producing x-rays.

No, you're right. I was thinking back to my early days when I designed
colour TVs. We would do an overvoltage test on line output stages that
would have the anodes glowing red with about 30kV on board. We used
lead screening around that box, and an x-ray dosimeter nearby.

d
Pearce Consulting
http://www.pearce.uk.com

Was there an appreciable increase in measurement of x-ray emission with 30kV
on LOP stage compared to normal HT/EHT supply/s?

Mike


With the normal drive voltage of 25kV there were no detectable x-rays.
at 30kV the anode glowed cherry red, and they poured out. 30kV was
just part of a stress test - not a normal operating condition.

d
Pearce Consulting
http://www.pearce.uk.com

One for the valvies 2/2....
 

"Don Pearce" wrote in message
...
On Sat, 6 Nov 2004 17:15:33 -0000, "Mike Gilmour"
wrote:


"Don Pearce" wrote in message
...
On Sat, 06 Nov 2004 16:33:25 +0000, Nick Gorham
wrote:

Don Pearce wrote:



A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!


You may be joking, in which case sorry, but AFAIK, the only way you
could get such radiation is to accelerate the electrons quite a bit
more, and the only way you could do that was with more voltage. More
current would melt a anode, its quite easy to get them glowing, as being
in a vaccume it can only loose heat by radiation.

Someone correct me, but I think you need somewhere in the region of 25k
to start producing x-rays.

No, you're right. I was thinking back to my early days when I designed
colour TVs. We would do an overvoltage test on line output stages that
would have the anodes glowing red with about 30kV on board. We used
lead screening around that box, and an x-ray dosimeter nearby.

d
Pearce Consulting
http://www.pearce.uk.com

Was there an appreciable increase in measurement of x-ray emission with
30kV
on LOP stage compared to normal HT/EHT supply/s?

Mike


With the normal drive voltage of 25kV there were no detectable x-rays.
at 30kV the anode glowed cherry red, and they poured out. 30kV was
just part of a stress test - not a normal operating condition.

d
Pearce Consulting
http://www.pearce.uk.com

I'm fascinated :-) Of what duration were the tests? Did you get much
corona discharge/ozone smell? Any flashovers during tests?

Thanks,
Mike

One for the valvies 2/2....
 

On Sat, 6 Nov 2004 17:44:25 -0000, "Mike Gilmour"
wrote:

With the normal drive voltage of 25kV there were no detectable x-rays.
at 30kV the anode glowed cherry red, and they poured out. 30kV was
just part of a stress test - not a normal operating condition.

d
Pearce Consulting
http://www.pearce.uk.com

I'm fascinated :-) Of what duration were the tests? Did you get much
corona discharge/ozone smell? Any flashovers during tests?

Thanks,
Mike


The test was run for about ten seconds. If you left it on for more
than twenty, there would be a sucking sound and the valve would gently
collapse in on itself. As for ozone - yup, plenty of that. We didn't
really know how poisonous it was then. Flashovers happened, but they
were generally more to do with bad transformers than valves.

We had a specific flashover tester that would induce flashover at any
selected point during the scan. A couple of big metal spheres were
connected to ground and EHT. The ground one had a small extra
electrode like a spark plug set into it, and when we wanted a
flashover, a quick burst of high voltage on this electrode would
provide enough ionisation to seed a big flash from EHT.

d
Pearce Consulting
http://www.pearce.uk.com

One for the valvies 2/2....
 

"Don Pearce" wrote in message
...
On Sat, 6 Nov 2004 17:44:25 -0000, "Mike Gilmour"
wrote:

With the normal drive voltage of 25kV there were no detectable x-rays.
at 30kV the anode glowed cherry red, and they poured out. 30kV was
just part of a stress test - not a normal operating condition.

d
Pearce Consulting
http://www.pearce.uk.com

I'm fascinated :-) Of what duration were the tests? Did you get much
corona discharge/ozone smell? Any flashovers during tests?

Thanks,
Mike


The test was run for about ten seconds. If you left it on for more
than twenty, there would be a sucking sound and the valve would gently
collapse in on itself. As for ozone - yup, plenty of that. We didn't
really know how poisonous it was then. Flashovers happened, but they
were generally more to do with bad transformers than valves.

We had a specific flashover tester that would induce flashover at any
selected point during the scan. A couple of big metal spheres were
connected to ground and EHT. The ground one had a small extra
electrode like a spark plug set into it, and when we wanted a
flashover, a quick burst of high voltage on this electrode would
provide enough ionisation to seed a big flash from EHT.

d
Pearce Consulting
http://www.pearce.uk.com

Adds some excitement to routine testing - except for the poisonous ozone
that is ;-)
I'm not surprised the transformers went, LOP's were always a seat of
problems under normal conditions (called IHVT on schematics now I believe -
why change what everyone knows?) Some sets run at around 30kV now I
believe. It's been a while since I've worked on TV's ( Did repair TV's on
ships from time to time & I worked for Redifusion whilst on leave repairing
TV sets because I got bored with too much time off living it up!).
Many shipborne radars had particularly vicious EHT's as I once discovered
on changing a CRT after the normal discharge procedure - I usually avoid the
EHT point on the CRT due to it 'recharging' - but it got me once when
lifting out a particularly long tube, luckily the neck was clear :-)
I built a Wimshurst machine once just for fun.... using an LP :-) worked
very well :-))
Thanks for all the info, very interesting.

Mike

One for the valvies 2/2....
 

Don Pearce wrote:

On Sat, 06 Nov 2004 07:32:15 +0000, Pooh Bear
wrote:



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

A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!

More a case of lead knickers for his *roadie* actually. He was the culprit !


Graham

One for the valvies 2/2....
 

Mike Gilmour wrote:


Adds some excitement to routine testing - except for the poisonous ozone
that is ;-)

I thought sea air was good for you !


Graham ;-)

One for the valvies 2/2....
 

Don Pearce wrote:

With the normal drive voltage of 25kV there were no detectable x-rays.
at 30kV the anode glowed cherry red, and they poured out. 30kV was
just part of a stress test - not a normal operating condition.

Didn't production sets of that era have an overvoltage cut-off ?


Graham

One for the valvies 2/2....
 

On Sun, 07 Nov 2004 09:02:05 +0000, Pooh Bear
wrote:

Don Pearce wrote:

With the normal drive voltage of 25kV there were no detectable x-rays.
at 30kV the anode glowed cherry red, and they poured out. 30kV was
just part of a stress test - not a normal operating condition.

Didn't production sets of that era have an overvoltage cut-off ?


Graham

Yeah - the LOP valve blew up.

d
Pearce Consulting
http://www.pearce.uk.com

One for the valvies 2/2....
 

On Sun, 07 Nov 2004 08:53:43 +0000, Pooh Bear
wrote:

Don Pearce wrote:

A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!

More a case of lead knickers for his *roadie* actually. He was the culprit !

Since you need in excess of 25kV to produce X-rays, lead knickers will
be the *least* of your worries if you're working on an amp with those
sort of rail voltages! :-)
--

Stewart Pinkerton | Music is Art - Audio is Engineering

One for the valvies 2/2....
 

"Pooh Bear" wrote in message
...
Don Pearce wrote:

On Sat, 06 Nov 2004 07:32:15 +0000, Pooh Bear
wrote:



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

A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!

More a case of lead knickers for his *roadie* actually. He was the culprit
!


Graham


Join us in a tale of daring. Experience 30kV!!! Smell the Ozone!!! Handle
toxic Lead!!! and see live anodes melt before your very eyes!! 'Valvies
On The Edge' coming to a cinema near you :-)

One for the valvies 2/2....
 

Mike Gilmour wrote:
"Pooh Bear" wrote in message
...

Don Pearce wrote:


On Sat, 06 Nov 2004 07:32:15 +0000, Pooh Bear
wrote:



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

A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!

More a case of lead knickers for his *roadie* actually. He was the culprit
!


Graham



Join us in a tale of daring. Experience 30kV!!! Smell the Ozone!!! Handle
toxic Lead!!! and see live anodes melt before your very eyes!! 'Valvies
On The Edge' coming to a cinema near you :-)


Thats a altogether different bunch of nutters :

http://homepage.ntlworld.com/electri...uktjune04.html

--
Nick

One for the valvies 2/2....
 

"Nick Gorham" wrote in message
...
Mike Gilmour wrote:
"Pooh Bear" wrote in message
...

Don Pearce wrote:


On Sat, 06 Nov 2004 07:32:15 +0000, Pooh Bear
wrote:



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

A molten anode? Chances are there was a pretty hefty burst of x-rays
as that one went phut. Lead knickers for you!

More a case of lead knickers for his *roadie* actually. He was the
culprit !


Graham



Join us in a tale of daring. Experience 30kV!!! Smell the Ozone!!!
Handle toxic Lead!!! and see live anodes melt before your very eyes!!
'Valvies On The Edge' coming to a cinema near you :-)
Thats a altogether different bunch of nutters :

http://homepage.ntlworld.com/electri...uktjune04.html

--
Nick

Excellent!! Hair standing on end and melted mosfets...next stop the
National Grid :-)