Unsafe Wiring Instructions

[Reader]

I just read the article posted at https://www.mojotone.com/support/kno...=Last+2018+FIF .
I believe I noticed several instances of bad/unsafe advice plus missed opportunities to educate home builders in the proper wiring of the power line input circuit.
Am I way out of line in expecting that a commercial company should have higher standards in the information they publish?

One specific example of what I am talking about is that the article clearly shows the hot lead of the 120VAC feed passing through the fuse holder and on to the the PT primary and the ON/OFF switch being fitted in the neutral lead to the PT primary winding. The photo below is from the article. If I am correct I guess this happened because Mojo is a cabinet manufacturer and parts supplier but doesn't really have employees fluent in electrical things.

[Helmholtz]

One specific example of what I am talking about is that the article clearly shows the hot lead of the 120VAC feed passing through the fuse holder and on to the the PT primary and the ON/OFF switch being fitted in the neutral lead to the PT primary winding.

There is no distinction between hot ("phase") and neutral inside an amp and both wires have to treated as hot. This is typical for all "moveable" electrical equipment, as the power plug can be inserted into the wall outlet in two ways. So each wire could be hot.

[Justin Thomas]

At this point I would hope that no new amplifier, kit, clone, or homebrew would come with a 2-wire cable that can be inserted 2 different ways. And even if it's a 2-wire cord then 99% of the time they are polarized (prongs 2 different sizes).

Personally I'd rather be sure to wire my amp in the currently accepted fashion of hot => fuse tip => fuse end => ower switch => tranny w. the other (neutral) straight to tranny, using a 3-wire cable of course. I know there are different standards in Europe; I'd follow them too if I were exporting.

As it is, I believe this amp in this video is a clone; my attitude towards clones is that although the audio circuit should remain intact, there are certainly some things that can & should be updated - the mains wiring being one. Just because Fender did it one way in 1966 doesn't mean we should keep doing it. From Reader's description the erroneous wiring is being duplicated.

Better for me to wire my amp correctly - if there IS a problem, I don't want to be liable. I guess if it's a DPST switch, it's a different matter, but in this case it's not.

Justin

[Helmholtz]

I don't think equipment manufacturers can rely on keyed 3-wire plugs. The house installation could be wired wrong as well as any extensions.
Our safety 3-wire plugs are symmetrical/not keyed as it does not really increase safety.

[Reader]

I think that the equipment (The amp) should be wired in conformance with the electrical code. In this case the US code. If it is later plugged into a wall socket that is wired incorrectly that would be unfortunate but at least the amp builder would not be at fault for any consequences having done the build job correctly.

[Helmholtz]

I think that the equipment (The amp) should be wired in conformance with the electrical code. In this case the US code. If it is later plugged into a wall socket that is wired incorrectly that would be unfortunate but at least the amp builder would not be at fault for any consequences having done the build job correctly.

The wire colors inside the equipment do not influence safety. As said, both mains leads and their connections need to be treated equally as either could be hot. That's what international (IEC) safety standards require.

[Dave H]

Our safety 3-wire plugs are symmetrical/not keyed

UK mains plugs are non reversible with a fuse in the live.

[Reader]

I think we have different perspectives regarding the USA wiring code and that which you use. In the USA the black line lead is "Hot" and the white line lead is treated as "neutral". If all is wired correctly the neutral in a 120V line circuit in connected to the earth ground at the breaker box.

[minim]

Well, we don't become lazy and not maintain good safe practice just because "the house installation could be wired wrong as well as any extensions'.

Where I come from (Australasia), the power switch in a grounded appliance (i.e. a non- double insulated appliance) MUST be placed in the 'hot' line. This is to avoid electric shock if someone were to begin working on the appliance without pulling the plug from the wall (we have switches on wall outlets over there). If the switch was in the neutral, the entire line into the appliance including fuse, transformer, and whatever else would all be live up to the switch in the neutral line.

In working in refrigeration and air con, I have indeed encountered some appliances, particularly European, where it is acceptable to place the power switch in the neutral line, as Justin said. That is their business, and good luck to them. I for one will always have the power switch in the active.

"There is no distinction between hot ("phase") and neutral inside an amp and both wires have to treated as hot. This is typical for all "moveable" electrical equipment, as the power plug can be inserted into the wall outlet in two ways. So each wire could be hot." - Helmholtz

I'm not really sure why you said this. Most guitar amps are quite clearly not 2-pronged appliances (small modelling amps run off a wall-wart being one exception), they are not double-insulated and therefore must be grounded. Grounded plugs can only be oriented one way. A two-prong, reversible plug would quite clearly not be appropriate if fitted to non-double-insulated amps.

Having said all that Reader, I see you're studying electronics and are probably up to speed on US codes. Perhaps you could contact Mojo directly if you think they would benefit from modifying their practices?

[Dave H]

In the USA the black line lead is "Hot" and the white line lead is treated as "neutral". If all is wired correctly the neutral in a 120V line circuit in connected to the earth ground at the breaker box.

It's the same in the UK but the colours are different. When I first went to the USA I thought black was neutral! because my house is wired to the old UK standard of black=neutral, red=live.

[Reader]

It's the same in the UK but the colours are different. When I first went to the USA I thought black was neutral! because my house is wired to the old UK standard of black=neutral, red=live.

The USA color code does cause confusion here too when "electronic" types try to be "electricians" and work on their house wiring. It takes some re-learning after growing up working on electronics where the "ground" wires are traditionally black.

[frus]

Grounded plugs can only be oriented one way.

Not in Europe. Grounded (in fact any) plug can be reversed, therefore DPST power switch should be used in equipment (which is much safer than trusting some guy who wires the wall sockets)

[Reader]

Not in Europe. Grounded (in fact any) plug can be reversed, therefore DPST power switch should be used in equipment (which is much safer than trusting some guy who wires the wall sockets)

I agree that best practice is to use DPST power switches. My point in starting this thread though was to discuss the best wiring practice for amps in the USA that use the modern IEC Type B plug, run on the 120VAC single phase source and already have SPST power switches installed.

[minim]

Not in Europe. Grounded (in fact any) plug can be reversed, therefore DPST power switch should be used in equipment (which is much safer than trusting some guy who wires the wall sockets)

Roger that.

[Pedro Vecino]

In Europe there are symmetrical and also asymmetrical plugs in relation to ground pin. But: the 230V can be obtained from two 230V phases or with a 400V phase and a neutral. At least in Spain (I imagine that in other countries the same thing happens).
It makes no sense to try to identify phases and neutral in the assembly of amplifiers or other 230V devices.