• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Safety Practices, General and Ultra-High Voltage

Bench Isolating Transformers

Bench Isolating Transformers are 1:1 ratio transformers, with good insulation between primary and secondary, with a usually comparatively big power rating. The idea is this: Your incoming mains supply has one end of supply company's transformer secondary connected to earth as close as possible to the transformer, and usually via a spike stuck into the ground. This is then the NEUTRAL wire - because of the connection to earth, it is usually pretty close to earth potential. The other end of the supply company's transformer secondary, the one which isn't connected to earth is the LIVE - it's voltage is 110 volts wrt (with respect to) ground (US) or 240 volts (UK). If you touch it, current will flow through you, through the earth, and back to the transformer. You have completed the circuit, and possibly died. The Bench Isolating Transformer gives you a supply that is "floating" wrt what you're standing on - theoretically, you can touch either end of its secondary without being shocked. Assuming the transformer is correctly wired and not defective. However, you are probably better protected by using a Residual Current Device, that looks for imbalance between the current flowing in the Live and Neutral wires, and disconnects the power if it sees a difference. BUT, and it's a big but, this all applies only to the incoming AC mains supply. Neither RCDs or Bench Isolating Transformers will protect you against being shocked by the sort of high voltages typically found in valve/tube equipment - and they're usually higher voltage than the incoming mains - especially if you're on a 110v supply. There just isn't an easy answer to this one.....so be careful!
 
I would like to put in a plug for isolating transformers.
I use a high-quality 5kVA isolating transformer for my bench supply.

The isolating transformer has a big advantage over the RCD, in that with the transformer the equipment that you're working and your test gear are all isolated from the mains earth or ground.

This means that if you accidentally touch the high-voltage DC there is no complete circuit through the ground that you're standing on. This is because the negative of your DC supply is not physically connected to the incoming mains earth.


The RCD, on the other hand, does not have this advantage. The gear that you're working on plus all of the test gear has a direct connection to the incoming mains earth.

The RCD does not sense earth current, it senses a difference between the incoming mains phase and neutral wires. It will not, therefore, sense the DC flowing down the earth wire from your DC power supply through your body etc.


rgds, M.
 
The isolating transformer has a big advantage over the RCD, in that with the transformer the equipment that you're working and your test gear are all isolated from the mains earth or ground.


rgds, M.

You also have the advantage that you can put your scope ground on any part of the circuit. For an irs2092 it has various voltages referenced to different places so a floating unit helps.
 
When I worked on a high-voltage amplifier with a 2660 V supply delivering 37 mA and having a much greater short-circuit current, see Elektrostatic Loudspeakers , I always wore rubber gloves of the type that are used to keep your hands dry when you wash dishes. I don't know if they would actually have helped against 2660 V, but I figured they wouldn't harm. In fact I tried to keep as much as possible of my skin covered with something that insulates.

More importantly, I was very scared of the thing and therefore very careful.
 
Ouch!

When I worked on a high-voltage amplifier with a 2660 V supply delivering 37 mA and having a much greater short-circuit current, see Elektrostatic Loudspeakers , I always wore rubber gloves of the type that are used to keep your hands dry when you wash dishes. I don't know if they would actually have helped against 2660 V, but I figured they wouldn't harm. In fact I tried to keep as much as possible of my skin covered with something that insulates.

More importantly, I was very scared of the thing and therefore very careful.

That sort of voltage would most likely poke a hole straight through those gloves.

I've worked on a lot of electrostatic power supplies. Many are very low current (Quad) but some can supply a lot of juice (Acoustat), so one needs to be very careful with them.

The 50KV EHT on old CRT video projectors was most hazardous!

By the way, I never wear gloves, I find them too restrictive and basically useless. Unless you get the real ones that the power company guys use and they are THICK!

rgds, M.
 
"I was very scared of the thing and therefore very careful" Wise Words. When working on powered-up equipment I try never to forget that one wrong move might be my last. Sobering, but sane and safe(r) Without the chassis being earthed, a lot of valve equipment turns into a sensitive, broadband AM radio receiver, which is a big problem. The chassis is usually connected to the HT DC 0v line, and in any case fatal shocks need not involve your feet at all - one hand to the other, through the heart works fine. Please take this on board! Don't see an isolating transformer as a panacea - it isn't! I'm not saying that they're never useful, sometimes they are. The best piece of safety equipment you will ever possess is found between your ears (despite the attempts of certain legislators to outlaw its use!).......
 
One of the most dangerous things may be a -presumed- dead power supply. Watch them bleeder resistors! One time on a 10KW AM transmitter one of the bleeders on a HV cap opened up. Hot sticks are my only true religion. Saved my butt when I went poking around with one on the cap with the open bleeder on it. WABANG! Piece of 1/8" aluminum hook 1/2" wide vaporized about 9/16" off the end. Got an extra dose of hot stick religion.

Even on TV sets, had a cracked circuit board issolate rectified line cap from bleeder. Got 161V on cap discharged arm to arm. Not fun. Now I always poke a high watt low ohm resistor across any caps BEFORE I start working on things.
Doc
 
Had an IC explode at 18volts. A CMOS "power driver" capable
of 12 amps for 50 nanoseconds or so. Long Vdd/GND wires (2 inches)
provided enough inductive ringing (above 30 volts for some nanoseconds)
to trigger a "bipolar snapback latchup".

The lab team was using an unusually hunky HewlettPackard bench supply,
and all the energy in the bench supply capacitors
was dumped
into the rather small
(2mm * 3mm) silicon IC.

The top of the plastic DIP-8 package flew to the ceiling, along with 2 of
the 8 leads.

From then on, the team placed a sheet of paper atop the setup.

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.
 
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.
 
HV FETs on sinks

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
 
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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 ?
 
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.
 
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

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
 
Disabled Account
Joined 2010
From the above post..
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However some variants may have a low-evaporative rate and a high flammability, WD40 dries out eventually..
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How to stuff a pot...:D
Dirt magnet...

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

Regards
M. Gregg
 
CONTACTS ARCING AND CAPS BLOWING

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:smash: