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

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

"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!

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

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

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

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

"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

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

"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