Safety Practices, General and Ultra-High Voltage

[nigelwright7557]

Originally Posted by tankcircuitnoise

So, regarding Audio Power Amplifiers: whether tube or solid state,
the power stored in those 100uf 450Volt caps
or in those 100,000uf 90volt caps
is dangerous.

I have a test unit set up for testing amplifier modules.
I bought in a lower power transformer than the amplifiers need in case of shorts, the output transistors should be able to take an ampere permanently.
However the energy stored in the power supply capacitors is enough to kill the mosfets.

[Wavebourn]

It is not about tubes, though... And translation is not needed!

Elektriker Horst (5 Sicherheitsregeln) Part2/Teil2 - YouTube

[stajo]

It has become popular to feed tubestages through FETs. Different types of CSS etc is almost in any DIY tube project nowadays. So a few lines about that might be appropiate.

Given that all takes part on the secondary side, i.e with a transformer with safe isolation between the application and your holes in the wall, this formula is considered as a standard, at least where I live (with the polarbears in Sweden).

d = 10^(0.78log(U/300))

d is the least safe distance in millimeters between the two differentials in voltage. That might be the FET and the grounded sink or chassie, wherever it is mounted. U is of course the difference in volts.

That said, if wanted to stay on the safe and recommended side, any bolts thru the FET is not recommended, since the hole of the FETs radius often is close or less then this distance, regarded B+ feeding for tube stages.

If pushing for example 360 V thru a FET gives a minimum safe distance of 1,15 mm, rounded up to 1,2. Try fit a bolt thru a FETs hole there.

There we get the next dilemma. Dissipation. We must use a pad that takes our heat well over to the sink over a pad that is minimum 1,2 mm thick. Thermal conductivity becomes important.

Btw, nylon bolts etc still takes a hole in the isolation, and makes the distance between the FET and the sink closer. Think CLAMP instead of bolt.

Important notice 1: Those recommendations is for applications EITHER on the primary or on the secondary side. Distance between differentials between primary and secondary must be greater, 3 mm minimum by some standards Class 1 (grounded chassie) 6 mm Class II (non grounded). Some standards says 4/8.

Impotrant notice 2: Safely ground the sink in any Class I application.

Best
Staffan

[Megaudio]

regarding safety....there is a small capacitor on switching the amp on & off.This are MPX 2X capacitors for EMI surpression and this small thing is saving other capacitors and semicondactors in the amp and all equipement conected with the amp from dangerous overvoltage spikes caused by sparks when switching amp on and off...the question is..does anybody knows can I use a biger capacitance ( the original has value of 0.01mf) or can I use even 0.1 mf instead ?

[cogsncogs]

These are for EMI suppression and to protect the switch contacts from arcing. Most caps made for this purpose have a 220-100ohm resistor in series with the cap. There will be no net gain going from .01uf to .1uf. Doing so will just increase the current through the cap when the switch is in the off position.

[Megaudio]

Originally Posted by cogsncogs

These are for EMI suppression and to protect the switch contacts from arcing. Most caps made for this purpose have a 220-100ohm resistor in series with the cap. There will be no net gain going from .01uf to .1uf. Doing so will just increase the current through the cap when the switch is in the off position.

Thanks.I was told the same and yes caps do protect switch from arcing but even more important ,the rest of device from high spikes coused by arcing - so called switch transients - in some cases even 6 kv was measured moving trough the lines - EMI are big killers for capacitors

[krokkenoster]

Originally Posted by Layberinthius

Yeah "Too" all components including wire insulation have a maximum allowable voltage and indeed resitors.

If you don't know how to figure out a resistor's voltage rating, ask for more help! you are no where near ready...

If you were using 250volt 10amp rated wiring in an amplifier that was going to have a 1000v B+ you could not use the wire and not expect electricity to shoot out of the side through the insulation and onto something nearby, like a finger or more commonly a chassis.

Also, don't ever service a TV set, don't even open one up as the chassis in two-conductor televisions are live and will kill you as they are just as if you stuck a fork into the power point, only there is much more contact area. Of course, it may be something simple and you want to see your fav programme, pay a qualified TECH to do it!! Don't take it to your friend's grandfather or your father!

Do you think a retired welfare guy has enough cash to afford all the equipment needed to fully and properly service your TV/VCR?

Believe me, you'll be doing them a favour! especially with all the new TV's out, plastic here plastic there shotty cheap components everywhere... digital this digital that. mongrels...

And never service old radio/amplifier equipment using the same wire which was in there for 50 years, the insulation /will/ fall apart
someday or even worse catch fire from excessive heat.

Always replace electrolyctic capacitors in equipment 10-15+ years old, just because an old set is 'working' doesn't mean it's going to be working for another month or 6. THESE EXPLODE PEOPLE!

Same goes with the old resistors, they are a very high fire risk especially if old-man sprays WD40 over the pots and fires it up straigt afterwoods :P Generally this is okay if you leave it for a few seconds, as it cleans the dirt and corrosion off the pot contact.

However some variants may have a low-evaporative rate and a high flammability, WD40 dries out eventually..

Generally on the output transformers the secondary voltage is too low to become dangerous, however with high impedance taps like 50-150-300 ohm taps for PA work the voltage is stepped up to god-only-knows hundreds of volts, this will give you a nasty tingle or start a fire with poor quality speaker cable!

Aswell, If a tube has a WHITE 'powder' inside it means that the tube has lost it's vacuum!! completely!
if the powder is silver or black it is FINE black actually means it's
got a better vacuum than a silver one so I recommend you don't
return it just because the powder looks 'stressed' or 'burnt'.

You're more than welcome to return it due to any concern, I'm not stopping you. Like a lop-sided tube, wriggly pin, looks weird inside, when you shake it you can hear a ting or ping, etc.

Just my $5, if anyone would like to comment, just notify a moderator or send me an email and I will modify this posting for the next 30mins, I don't get angry if I've been fed bad information at the person who notified me of that fact.

Anyone at any time can place a notification to the moderator about a specific post just by clicking "Report" on the bottom righthand side of the post-box..

Cheers.

When testing on live sets keep your fancy digital or transistorised instrument in your cupboard and use instruments of the era like 20000ohms per volt instruments and KEEP ONE HAND IN YOUR POCKET when doing tests especially those old AC/DC sets I talk out of experience and the supply is 240 volts here and it BITES

[M Gregg]

From the above post..
----------------------------------------
However some variants may have a low-evaporative rate and a high flammability, WD40 dries out eventually..
------------------------------------------------

How to stuff a pot...
Dirt magnet...

As long as the plastic used can take it Servisol takes some beating..

Regards
M. Gregg

[Megaudio]

When switching a tube amp off,there is a short soft noise coming from right loudspeaker - is it normal? ...in the very begining there was no noise but now there is...just to mention...my amp was hit by strong EMI and I changed all capacitors...but this noise is still there

[krokkenoster]

Originally Posted by cogsncogs

These are for EMI suppression and to protect the switch contacts from arcing. Most caps made for this purpose have a 220-100ohm resistor in series with the cap. There will be no net gain going from .01uf to .1uf. Doing so will just increase the current through the cap when the switch is in the off position.

DEAR FELLOW EXPERIMENTERS
I TRIED THESE CAPACITORS WITH THE RESISTOPRS BUT RELAY CONTACTS DID KEEP ON ARCING SO I FITTED TRANSORBS OR TRANSIENT SUPPRESSORS AND THEN THE TRANSIENT PROBLEM WAS SOLVED ESPECIALLY WHEN THE LOAD WAS VERY INDUCTIVE LIKE MOTORS AND XFRMRS GET ONE THAT IS AT THE SAME WORKING VOLTAGE BUT DO NOT SKIMP ON THE POWER ABSOBTION ABILITY I USED THEM ON MY OLD 1980 FORD V6 WITH BREAKER IGNITION AND THE POINTS LASTED MORE THAN 100,000 KM