Necessary safety features

[Cliff Schecht]

What do you guys consider necessary safety features in an amplifier?

The big ones I know of are a 3 prong cord with earth ground right next to the plug and having the power switch on primary side before the transformer, fuse after transformer (on "return"). Standby switch goes before the reservoir caps on the B+ line. For anything over 350V I use only 1/2W+ resistors or at least make sure what I'm using can handle the voltage swing. Signal and B+ caps are all rated above the highest voltage they will ever see (even if no tubes are plugged in).

What is the purpose of putting the fuse on the return side of the transformer? How about things like tube spacing (crosstalk and excess heat problems) and transformer placement (clearances)? I have my standards that I build to but not really a set one, more just common sense. I'm wondering what the ideology behind others safety features are (like KF's use of MOV's and flyback protection diodes, which I understand but don't think are necessary)..

[David Root]

Everything you said, except if you're building a Music Man iron Dumble with Vp=700 flyback protection is probably a good idea. I think HAD did that on his 150W and 300W amps.

I think DPST switch on both sides of the PT primary is a good idea above 100W output and/or 500Vp. As is a HT fuse a la Marshall.

I have also done Fender style with the standby switch after the plate PS caps, but not in a Music Man Dumble. This is not really kosher these days I know.

[Cliff Schecht]

Interesting about the DPST at over 500V and the other higher power things. I'm building a friend a SE KT88 amp using some iron I got from Allied for cheap ($50 shipped). After rectifier I'm expecting about 530V DC @ 150mA. I'm using one of those combined Off/Standby/On switches and I wasn't concerned about the voltage rating until you brought this up. Should I be worried about this switch not lasting (it's a Carling)?

[KT66]

Instead of switching the B+ you could make the standby feature switch the PT CT ala Ampeg, Matchless etc.

[paulster]

I think DPST switch on both sides of the PT primary is a good idea.

Depending on what region you're building for it's also a legal necessity.

It hot and cold are swapped at the outlet it's useful to be able to guarantee that you've properly isolated the amp.

Also, fuse in the hot wire before the power switch. Never in the cold wire as it completely defeats the purpose of the fuse blowing if the chassis becomes live.

Also the mains in should go to the end of the fuseholder and the mains out to the power switch should go to the collar of the fuseholder. This stops children or guitarists with small digits sticking them in and coming across an energised fuseholder.

[Cliff Schecht]

Great stuff guys! Keep it coming, this will be a nice reference for future (customer) builds. I'm not so worried about stuff I build for myself..

I thought it made sense to put the fuse on the hot side. Somebody mentioned putting it on the cold side once and it didn't make sense to me.

[Aurora]

I think DPST switches are compulsory, as pr CE regulations, but I'm not 100% sure - at least it is good common sense.

[FYL]

I think DPST switches are compulsory, as pr CE regulations

Yup, dual pole handling live and neutral, placed *after* the fuse(s). Plus a thermal fuse (self resettable is OK) in the PT primary winding as well as a dedicated earth connection - you can't use PT bolts or any other tie point.

[rp]

Also the mains in should go to the end of the fuseholder and the mains out to the power switch should go to the collar of the fuseholder. This stops children or guitarists with small digits sticking them in and coming across an energised fuseholder.

Not so much the rarer poking, but simply inserting the fuse without putting it in the cap first, or pulling out a fuse that didn't stay in the cap. If you're holding one end of the fuse it's easy to hit the collar only half way in.

A clever thing I read either here or somewhere else, is to make the ground of the AC cord a few inches longer than the live and neutral. If the power cord ever gets yanked out from a fall etc the ground will be the last rip out.

[Structo]

I wonder if there isn't a place where the electrical code or safety practices aren't listed for AC powered equipment?

Like Underwriters or CE? Conformité Européene or European Conformity.

We pretty much go by word of mouth or forum to mouth as it were.

Most of it is common or practical sense the way the AC should be connected but there is some confusion among builders about where the fuse should be located as well as how the ground or earthing wire is connected to the chassis.

What I found with the CE marking for low voltage devices is:
The "Electrical Equipment" means any equipment designed for use with a voltage rating of between 50 and 1000 V for alternating current (A.C.) and between 75 and 1500 V for direct current (D.C.). Therefore, it is called often "Low Voltage Electrical Equipment" which includes the vast majority of electrical equipment in everyday use.

[Dr-Joned]

Labeling:

No user servicable parts inside. Service by a qualified technician only.

[Gaz]

No user servicable parts inside. Service by a qualified technician only.

This seems like a sarcastic label for a DIY amp

[Gaz]

Ground the input jack. This way if high voltage ever gets on the grid of the first preamp tube, it doesn't get to you. This is convienient for me because I use the input jack as the main 'star' point for all my signal grounds. I use a toothed lock washer to secure the jack, and run a short extra ground wire to a lug screwed to the chassis just in case the jack ever comes loose.

I also use 2 stacked nyloc nuts to secure my main Earth ground...however, I also use that point as a very convenient ground for the artificial center-tap for the heaters. Am I going to Hell?


1+ to everything else so far.

[guitarmike2107]

I wonder if there isn't a place where the electrical code or safety practices aren't listed for AC powered equipment?

Like Underwriters or CE? Conformité Européene or European Conformity.

We pretty much go by word of mouth or forum to mouth as it were.

Most of it is common or practical sense the way the AC should be connected but there is some confusion among builders about where the fuse should be located as well as how the ground or earthing wire is connected to the chassis.

What I found with the CE marking for low voltage devices is:
The "Electrical Equipment" means any equipment designed for use with a voltage rating of between 50 and 1000 V for alternating current (A.C.) and between 75 and 1500 V for direct current (D.C.). Therefore, it is called often "Low Voltage Electrical Equipment" which includes the vast majority of electrical equipment in everyday use.

I have a copy of the BS EN 60065 standard, which is the EU low voltage Audio specific standard ( and I bet the same as the American standard). its not your average builders bedtime reading, and it doesn’t specifically say build with X Y and Z. its more like build with x or y or z as long as A and B are both satisfied.

I have always thought it would be a good idea for the various forums to purchase these and make them available to forum members. It would promote safe practice for the forums/Clubs and give those members who wish to sell an amp or two the information they need to build to the current standard, without having to spend the hundred for the standard by themselves JMHO

[Aurora]

CE is jungle of different regulations - not only eceltrical.
This however, gives some insight to the outskirtsof teh jungle...
http://www.siracertification.com/UserDo ... es/lvd.pdf