Pots at B+ Voltage

[defaced]

Thanks Ted. I'll grab a couple of those in my next order and see what shakes out.

There's not rocket science at power scaling, we are talking very simple HV regulators here.

After playing with this in LTSpice and seeing what happens, that's kinda what I was starting to suspect.

This whole project is forcing me to learn alot more about MOSFETs that I didn't know before. Good stuff.

[guitarmike2107]

it's been said before, the standby switch sits on the front panel of many amps with full B+ sitting on it, and most DIY builds I have seen have done nothing to prevent wire creepage should a connection break/come loose. plus many of those toggle switches have very nice conductive levers.

Would be interesting to do the safety tests to find out if it would pass CE/UL standards, maybe there are loads of DIY builders building fire bombs… I don’t think so though.

[Steve Conner]

There's not rocket science at power scaling, we are talking very simple HV regulators here.

Some of them are regulators with an output voltage referred to a constant reference, others are just a potential divider with a source follower, putting out a fraction of the input voltage.

It's an important distinction: with the regulator type you have to do the analysis to ensure it won't drop out under the worst-case combinations of line voltage and loading. (Well, except in practice you don't really notice if it drops out )

But the potential divider type can't drop out, because if the input voltage decreases the output will decrease in proportion.

[R.G.]

it's been said before, the standby switch sits on the front panel of many amps with full B+ sitting on it, and most DIY builds I have seen have done nothing to prevent wire creepage should a connection break/come loose. plus many of those toggle switches have very nice conductive levers.

Would be interesting to do the safety tests to find out if it would pass CE/UL standards, maybe there are loads of DIY builders building fire bombs… I don’t think so though.

I have some experience with exactly this issue.
1) In safety testing, the switch toggle will be tested for continuity to chassis ground. If it is almost a short circuit to chassis ground, and can support 25A (!) of current at less than some small fraction of a volt, it likely passes; this is usually solved by choosing a switch which is itself UL/CE/TUV/Yada/Yada tested and approved. I believe that the majority of DIY amp builders probably get approved parts largely by accident. Many of the switches that look right are actually approved. Some are copies with the approval stamps on them but put there as decoration and never tested.

The nice, conductive levers are actually a safety feature, so long as they are in direct metal contact with the solid metal bushing that is solidly screwed into the metal chassis. However, if there is for instance a paint layer on the chassis, then the same switch will likely fail the ground bonding test, and represent a clear and present shock hazard. For real safety, there needs to be a toothed washer on the inside of the chassis on the switch bushing, and the chassis needs to be solid metal and continuous to the switch mounting point.

Notice that for this to provide any safety, the chassis must have a solid metallic bond to AC third-wire safety ground. There is still a group of builders that resist this and make two-wire amps, or resist converting two-wire to three-wire amps, even when it could save their lives. If the chassis is not solidly bonded to third-wire ground, a solidly bonded switch lever only serves to make the whole chassis hot if there is an internal flaw in the switch. I guess in many ways this is a Darwinian process, but it's slow.

2) The wiring to the switch must be set up such that any one wire breaking will not cause a voltage hazard or a fire hazard. This is generally solved by tying the whole bundle of wires to the switch together so the non-broken wires hold the broken one in place. In the Boutique-ey amps I've looked in, this is randomly the case; most often by twisting, sometimes with ties.

3) The switch must be secured against turning if the nuts come loose. This may be the case, but again, since most builders haven't a clue that it's needed, I think it's purely accidental. I have not been able to tell by looking into various amps because you actually have to loosen the nuts on the bushing and twist.

It would indeed be interesting to have the results of safety testing on DIY and boutique amps to see if they pass. I'd be willing to bet a modest amount of money that less than 10% would. That 10% would be composed of a few second-career veterans who've been through formal safety testing before and the remainder of the purely lucky; that's my guess anyway.

From a safety standpoint, it is never a good idea to run high voltage to the control panel, period. Historically, builders of electrical equipment were forced to do this by the unavailability of anything to do remote switching at a price they could afford. Power switches live there, which forces switch makers to live up to a certain standard or lose their customers to electrocution and their money to lawsuits. But with the remote-switching that's available today, it is a good idea from the safety point of view to never take high voltage (that is, over 42VDC or AC peak, by the safety standards) to a user panel at all. Boutique and DIY builders are so focussed on making it "vintage but better" that they mostly never give the slightest thought to this.

[jpfamps]

It would indeed be interesting to have the results of safety testing on DIY and boutique amps to see if they pass. I'd be willing to bet a modest amount of money that less than 10% would. That 10% would be composed of a few second-career veterans who've been through formal safety testing before and the remainder of the purely lucky; that's my guess anyway.

Well I've seen several amps from major manufacturers that, in my interpretation of the regs, would not pass EU safety regulation despite carrying a CE mark, eg the safety earth is soldered to a tag, which is held in place with a self tapping screw.

Of the boutique amps I see, most of the US ones wouldn't pass the regs (nor ROHS) and many aren't CE marked. I've also seen some real horrors in CE marked UK-based builders; eg soldering the safety earth to a piece of tag strip to which the main filter caps were also soldered.

I get the impression that US builders are slightly more cavalier than in the UK as mains is only around 120VAC.

Regardless, whilst we shouldn't be complacent, the fact that guitarists aren't dropping like flies would suggest that guitar amps are a fairly safe consumer item.

[R.G.]

Well I've seen several amps from major manufacturers that, in my interpretation of the regs, would not pass EU safety regulation despite carrying a CE mark, eg the safety earth is soldered to a tag, which is held in place with a self tapping screw.

Yep. The major amp builders probably self-certify, as allowed under EU regulations for the CE mark.

Of the boutique amps I see, most of the US ones wouldn't pass the regs (nor ROHS) and many aren't CE marked.

Yep. Not required in the USA. If they were referred to a safety lab for testing, they'd probably fail EU testing. Notice that no safety testing at all is required by law in the USA.

I've also seen some real horrors in CE marked UK-based builders; eg soldering the safety earth to a piece of tag strip to which the main filter caps were also soldered.

Ugh. Not good.

I get the impression that US builders are slightly more cavalier than in the UK as mains is only around 120VAC.

I think it's worse than that. I think they're clueless in many cases.

Regardless, whilst we shouldn't be complacent, the fact that guitarists aren't dropping like flies would suggest that guitar amps are a fairly safe consumer item.

That's largely because the DIYers and Boutiquers are copying designs that didn't massively kill people, and because transformer and wire insulation is much more consistent today. After all, Fender amps with two-wire and power on the panel didn't kill a whole lot of people back in the 50s. But their sale would be forbidden on safety issues in the EU and most places with mandatory safety certification today.

"Fairly safe" is no comfort to the guy who gets shocked.

[jmaf]

I've seen some home made amplifiers with the death cap using regular 600V polyester capacitors, some have the third ground in on the IEC connector unsoldered....if it's crazy in Europe / US you can imagine elsewhere.

[jpfamps]

Yep. Not required in the USA

Yes, but try importing something into the US.......

[loudthud]

UL Recognition requires a High Pot test on certain types of equipment. Is it required on Guitar Amps? Has anyone seen a High Pot rubber stamping on any UL logoed amp? Can anyone find a link to UL regulations online?

[R.G.]

UL Recognition requires a High Pot test on certain types of equipment. Is it required on Guitar Amps?

Yes.

Has anyone seen a High Pot rubber stamping on any UL logoed amp?

I have not. In general, UL tests samples and type approves based on samples. They don't hand test each one. The use of the UL logo on the amp is an indication of passing all the UL tests.

Notice that UL is NOT a government agency, and is NOT mandatory by federal law in the USA. However, some local jurisdictions make it mandatory for general public sales of consumer stuff.

Can anyone find a link to UL regulations online?

No, they can't. UL and all the other safety places have pulled the fast one of forcing manufacturers to comply by various means, and then SELLING their specifications/regulations. You will not find either the UL nor the IEC regs on line - I looked for a long time. You can only buy them. They're copyrighted, and change often enough that the issuing bodies also sell subscriptions.

[guitarmike2107]

It’s easy to spot non CE compliant amps even just from the outside, most of the US boutique amps imported here don’t even get the markings right in the first place, even if they get the CE mark on the back the others are lacking.

I helped an amp builder in London-shire who is wants to sell amps that are fully CE marked, I got a copy of the regs from him to work through. Even started drawing up check sheets to make the design records easy to complete. But I got a bit stuck when it got to the more advanced tests requiring special equipment. Problem is there isn’t really anyone you can talk to without costing an arm and a leg and the forums generally get you all sorts of opinion based answers or non at all.
It would be ideal if there was a forum section based on these issues, but unlikely for obvious reasons
Not easy if you just want to sell a few amps a year

The EU regs are not online, but the Low Voltage Directive is, this is the legal part that the Harmonised standards(Regs) ensure you comply to. Basically it says don’t kill or harm anybody?

[Steve Conner]

I've been through the CE marking hassles with a few products I was involved in at work. Basically if you're a mom and pop shop, you just don't do anything stupid, make a safe, conservative design with good quality components, ponder EMC while sacrificing a chicken at midnight, stick a CE sticker on the back and keep your fingers crossed.

And don't forget to use lead-free components, lead-free solder, and add the obligatory Weee "crossed out wheelie bin" sticker. (My American Standard Strat came with one of these on the neck plate, which I thought was neat.)

For larger things you really should get them tested according to the EMC and LVD stuff. It costs a couple of grand, and they will do the FCC testing at the same time as the European standards. I have designed things that passed it: the hardest part for my stuff was the electrostatic discharge test, which a tube amp should breeze.

We always used wall warts on our products, even though I hate them passionately, because the mains electronics inside the wart have already been tested, so you don't have to do the LVD.

If someone wants to start a forum section on CE, UL and the like, I am happy to contribute.

There is also a CE Marking Association aimed at small companies in the UK. I got sent to one of their seminars and got a bunch of useful stuff.
CE Marking Association – CE Marking & Product Compliance Specialists

[jrfrond]

A few things here:

1) Unless you are doing large-scale production with widespread distribution, RoHS specs will fly under your radar. This includes just about all boutique builders.

2) I can't see ANY reason why someone would build a new amp with a death cap design, even if using a UL-approved AC cap, unless it is based on ignorance..... or stupidity.

3) If you've ever taken apart a typical toggle switch, you'd see that the contacts are FAR more removed from the outside world than your typical pot. Under certain conditions, I could picture arc-over in a pot under high-voltage conditions, particular in pots where worn carbon tracks have created a film of conductive dust.

4) Generally, though not always (unfortunately!), people who have service enough amps know the difference between good and bad and/or unsafe designs.

5) The protection from afforded by good grounding practices IN the amp is only as good as the electrical ground in the wall, not to mention that some players will use a 3/2 ground adaptor improperly, or pull the third prong altogether.

[R.G.]

1) Unless you are doing large-scale production with widespread distribution, RoHS specs will fly under your radar. This includes just about all boutique builders.

... except builders with competitors, ex-spouses, or others who complain about the "unsafe" items to an independent testing lab. The testing labs are allowed to test the thing and if it does not pass, bill the maker, as I (dimly) understand EU law.

2) I can't see ANY reason why someone would build a new amp with a death cap design, even if using a UL-approved AC cap, unless it is based on ignorance..... or stupidity.

Me either, but then I don't see the logic in posting to technical forums threads which begin "What's the best/worst/most/least/yada/yada [whatever]."

3) If you've ever taken apart a typical toggle switch, you'd see that the contacts are FAR more removed from the outside world than your typical pot. Under certain conditions, I could picture arc-over in a pot under high-voltage conditions, particular in pots where worn carbon tracks have created a film of conductive dust.

Yep. Pots are going to be worse, not least because they are generally not expected to handle AC power line connections. Switches are.

4) Generally, though not always (unfortunately!), people who have service enough amps know the difference between good and bad and/or unsafe designs.

Like cops, they tend to deal mostly with the failures. 8-O

5) The protection from afforded by good grounding practices IN the amp is only as good as the electrical ground in the wall, not to mention that some players will use a 3/2 ground adaptor improperly, or pull the third prong altogether.

Yep. I keep harping on players to carry a $5 wall-plug tester to save their lives. Some of them even do.

[jpfamps]

... except builders with competitors, ex-spouses, or others who complain about the "unsafe" items to an independent testing lab. The testing labs are allowed to test the thing and if it does not pass, bill the maker, as I (dimly) understand EU law.

Actually it's the importer's problem in the EU (assuming that the amp is made outside the EU, in which case it would be the manufacturers), which for many US boutique amps would also be the retailer.

In the UK, for an item to be investigated for being non-RoHS or non-CE compliant, it would require a complaint to be made to the local trading standards office who would then investigate it. This would initially simply require the importer to provided the requisite documentation, assuming of course that this exists......

Most retailers I've dealt with don't a clue about CE/RoHS, even though it's their responsibility (hardly surprising as many builders don't have a clue; at a recent trade show I had an extensive chat with a very well-regarded UK-based amp builder who had absolutely NO idea about either CE or RoHS).

As RG alludes, you also have the potential for "malicious" investigations. I heard on the grapevine of one UK amp builder how had to stop building amps for a while due to a trading standards investigation.