In article , Don Pearce
wrote:


Just one thing to remember. When working on live valve circuits, keep
one hand in the pocket at all times.

You beat me to that. :-)

I'd add: make sure the mains switch on the wall is within easy reach. There
will be times when you wish to switch off the unit in front of you with
minimal delay. :-)

Also, "do not poke components with a pencil".

Crepe-soled shoes are a help, too. (I know, that's two things). I found
all that out the hard way in my early days of design.

One of my ex-colleagues used to work on HV discharge lasers 'live' whilst
wearing rubber boots. Mind you, he was a bit crazy. Doubt the
health-and-safety people would let you do it now.

I have always been much more slow-and-cowardly. If kit has mains, or above
about 50Vdc I am inclined to keep shutting everything down when I want to
move probes about, etc. Slower. But you get time to think about what comes
next, and may live longer to do more thinking. :-) Also checks out that
the kit can be turned on and off a lot without problems.

I also tend to prefer bench supplies when developing or testing an amp, and
only connect to the working PSU when doing more protracted tests. Bench
supplies give more safety options, and can also be useful for checking
other effects, etc.

Hence my other advice tends to be "pause and think again, and try to work
out what devious ways the kit in front of your has just devised to try and
kill you." This is also what tea breaks are for.

Just one more thing - don't hit the 500VDC supply with a spanner while
hanging onto the metal rack with the other hand. You travel a long way,
and you don't find the spanner again.

My personal estimate is that about 2kV equals about one rotation of the
human body and you bowl over backwards (if lucky). This is roughly what I
did one day courtesy of someone else rewiring a klystron without telling
me.

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 article , Don Pearce
wrote:



Just one thing to remember. When working on live valve circuits, keep
one hand in the pocket at all times.


You beat me to that. :-)

I'd add: make sure the mains switch on the wall is within easy reach. There
will be times when you wish to switch off the unit in front of you with
minimal delay. :-)

Also, "do not poke components with a pencil".


Crepe-soled shoes are a help, too. (I know, that's two things). I found
all that out the hard way in my early days of design.


One of my ex-colleagues used to work on HV discharge lasers 'live' whilst
wearing rubber boots. Mind you, he was a bit crazy. Doubt the
health-and-safety people would let you do it now.

I have always been much more slow-and-cowardly. If kit has mains, or above
about 50Vdc I am inclined to keep shutting everything down when I want to
move probes about, etc. Slower. But you get time to think about what comes
next, and may live longer to do more thinking. :-) Also checks out that
the kit can be turned on and off a lot without problems.

I also tend to prefer bench supplies when developing or testing an amp, and
only connect to the working PSU when doing more protracted tests. Bench
supplies give more safety options, and can also be useful for checking
other effects, etc.

Hence my other advice tends to be "pause and think again, and try to work
out what devious ways the kit in front of your has just devised to try and
kill you." This is also what tea breaks are for.


Just one more thing - don't hit the 500VDC supply with a spanner while
hanging onto the metal rack with the other hand. You travel a long way,
and you don't find the spanner again.


My personal estimate is that about 2kV equals about one rotation of the
human body and you bowl over backwards (if lucky). This is roughly what I
did one day courtesy of someone else rewiring a klystron without telling
me.

Slainte,

Jim


Many thanks all - I appreciate your words of caution. I have no
intention of fiddling with the Beard amp I've got - that's going to a
professional, what with its 2kV voltages and all!

Given my known ****wittery I'm going to go verrrry slow.

:-)

Rob

Beard amp I've got - that's going to a
professional, what with its 2kV voltages and all!


2Kv on a Beard amp????????? Where?

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Andy Evans wrote:
Beard amp I've got - that's going to a
professional, what with its 2kV voltages and all!


2Kv on a Beard amp????????? Where?

=== Andy Evans ===
Visit our Website:- http://www.artsandmedia.com
Audio, music and health pages and interesting links.

Er, you would ask! It says on some technical notes that i have:

"The voltages found at the anodes of the output valves approach 1.1kV on
load and 2kV off load"

Then it witters on about blocking capacitors in test gear probes.

So - the anodes i suppose - keep well clear!

Rob

On Sat, 11 Dec 2004 16:49:19 +0000 (GMT), Jim Lesurf
wrote:

In article , Don Pearce
wrote:

Just one more thing - don't hit the 500VDC supply with a spanner while
hanging onto the metal rack with the other hand. You travel a long way,
and you don't find the spanner again.

My personal estimate is that about 2kV equals about one rotation of the
human body and you bowl over backwards (if lucky). This is roughly what I
did one day courtesy of someone else rewiring a klystron without telling
me.

After we'd checked that he wasn't seriously hurt, one the funniest
things I ever saw was in my days at Marconi Space and Defence. One of
the guys was kneeling into the back of a rack cabinet, working on a
200 volt DC PSU which had about 30,000uF of reservoir capacitance on
it. He accidentally touched one of the cap terminals, which threw him
backwards. His head hit the top of the cabinet doorway, which threw
him forward so that he hit the cap array, which threw him back into
the doorframe, which threw him......

He must have completed about five oscillations before he eventually
missed the doorframe and flew across the lab floor! After the intial
rush to check that he was basically OK, we absolutely howled.... :-)
--

Stewart Pinkerton | Music is Art - Audio is Engineering

On Sat, 11 Dec 2004 16:49:19 +0000 (GMT), Jim Lesurf
wrote:

I also tend to prefer bench supplies when developing or testing an amp, and
only connect to the working PSU when doing more protracted tests. Bench
supplies give more safety options, and can also be useful for checking
other effects, etc.

Apropos of the actual power supply. WIth valve voltages, during
development, I always had biggish resistors wired across the main
electrolytics to ensure that they discharged fairly quickly after
switchoff - about a couple of seconds was my target. These came out
for the final design, of course.

d

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

"Don Pearce" wrote in message
...


Apropos of the actual power supply. WIth valve voltages, during
development, I always had biggish resistors wired across the main
electrolytics to ensure that they discharged fairly quickly after
switchoff - about a couple of seconds was my target. These came out
for the final design, of course.

Many old studio amps had these bleeder resistors permanently installed.
220k seemed typical. They dissipated only a couple of watts, which was
negligible in the overall scheme of things, and discharged the 'lytics
well within the time it took the service man to remove the amp from
its rack, and take of the bottom plate.


Iain

"Jim Lesurf" wrote in message
...

I have always been much more slow-and-cowardly.

That's the only approach:-)
Whatever your level of experience, Be afraid, very afraid.
It is conducive to longevity:-)

I have a 220V neon across my variac which fires at 70V
- the danger threshold.


If kit has mains, or above
about 50Vdc I am inclined to keep shutting everything down when I want to
move probes about, etc. Slower. But you get time to think about what comes
next, and may live longer to do more thinking. :-)

SS amps do not take kindly to the open end of a croc clip lead making
electrical contact in the wrong place. They tend to fail the smoke
test immediately. Valve amps, despite their much higher
operating voltages, seem to be a little more forgiving.

One of the first pieces of bech equipment I built for myself was a
discharge box on which is mounted a 50W 10k resistor, with a
voltmeter in parallel and a switch to put them in and out of circuit.
It has screw tags (no croc clips!) and is across the reservoir capacitor
all the time the amplifier is open on the bench. After switching off
the amp and switching on the box, the B+ across the electrolytic
drops to a very low potential in a matter of seconds.


I also tend to prefer bench supplies when developing or testing an amp,
and
only connect to the working PSU when doing more protracted tests. Bench
supplies give more safety options, and can also be useful for checking
other effects, etc.

Yes indeed. I bought a 350V 1A Farnell supply just recently for GBP50.
A good investment I thought. It can supply 2A at 6.3V for heaters, and
this supply remains uninterrupted regardless of the state of the HT supply.
The HT has a variable control plus a selector to increase the potential
in steps of 50V, and with HT off, one can safely poke about, assuming
there are no large caps downstream.


y other advice tends to be "pause and think again, and try to work
out what devious ways the kit in front of your has just devised to try and
kill you." This is also what tea breaks are for.

"Valve amps look so innocent and appealing" (quote, my wife:-)

Hmm:-)

Kunniottaen
Iain

In article , Iain M Churches
wrote:

"Jim Lesurf" wrote in message
...

If kit has mains, or above about 50Vdc I am inclined to keep shutting
everything down when I want to move probes about, etc. Slower. But you
get time to think about what comes next, and may live longer to do
more thinking. :-)

SS amps do not take kindly to the open end of a croc clip lead making
electrical contact in the wrong place.

Nor do some valve circuits. :-)

Fortunately, in both cases this should not be a problem in normal
use.

They tend to fail the smoke test immediately.

Depends on circumstances. The problem with BJT's is the mix of secondary
breakdown, their willingness to die in your service, and a tiny actual
active device.[1] In some cases, though, using a bench supply with limiters
may help avoid immediate trouble. In the end, resistors will release the
magic smoke, though, at the command of either SS or valve... ;-

Valve amps, despite their much higher operating voltages, seem to be a
little more forgiving.

Depends on the circumstances. I've certainly seen smoke, flames, flashes,
and accompanying noises emerge from valve circuits pretty swiftly. Probably
not on the same timescale as secondary breakdown, but still faster than
anyone can leap to the power switch! :-) Hence on a 'human response time'
basis the difference may not always be that significant. And SS devices can
often be cheaper than large valves to replace.

If comparing the safety and reliability of SS devices and circuits with
valve ones, you would need to consider more factors. For example, the
higher sensitivity of valves to mechanical damage, ageing effects, etc.

However, if the design is a decent one, and the units are appropriately
used these things are more of a worry for the development engineer
than the end-user.


Slainte,

Jim


[1] Plus the innocent assumption of the designer that no-one has just
dropped a metal bar into the circuitry. :-) This makes me think of the
old BEAB/BS tests that used to include things like lowering a metal
jewellery chain into the set to see what it might find...

--
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