Tube amp - AC power safety

[Music1969]

On another forum I saw this comment:

Most importantly you should inquire what power transformer he is using, and how its employed in the transformation. If the transformer is a dual primary, and he is using one of those primaries as the secondary... Thats a HUGE red flag, a UL violation... but most importantly its a fire and safety hazard. I don't mean to be an alarmist, but there have been some questionable circuits out there... parading to be high end, yet failing this most basic fundamental.

Can anyone explain how this is a fire and safety hazard?

And what is the correct way to wire co-primaries and secondary?

Anyone got a picture of correct wiring ? Simple case of dual primary transformer

@restorer-john @solderdude

 

[solderdude]

Most likely because the 115V windings may not be separated very well or that the primary and 'secundary' winding are awfully close together ?

How to wire it depends on 115V or 230V mains input voltage. Mind the 'dot' when parallelling (115V) or connecting the windings in series (230V)

 

[fpitas]

Certainly you'll only get half the power rating doing that. But why would someone do that? Just for isolated 115V?

 

[egellings]

On another forum I saw this comment:



Can anyone explain how this is a fire and safety hazard?

And what is the correct way to wire co-primaries and secondary?

Anyone got a picture of correct wiring ? Simple case of dual primary transformer

@restorer-john @solderdude

Most likely because the 115V windings may not be separated very well or that the primary and 'secundary' winding are awfully close together ?

How to wire it depends on 115V or 230V mains input voltage. Mind the 'dot' when parallelling (115V) or connecting the windings in series (230V)

Insulation breakdown between the two coils is a possibility, causing a shock hazard. Use the primaries only on the AC input side.

 

[Music1969]

Certainly you'll only get half the power rating doing that. But why would someone do that? Just for isolated 115V?

Half the power by doing what?

 

[fpitas]

Half the power by doing what?

Using only one winding as a primary input. You're supposed to parallel them for 115V, or series for 230V.

 

[Music1969]

Insulation breakdown between the two coils is a possibility, causing a shock hazard. Use the primaries only on the AC input side.

In terms of shock hazards - the only important thing in terms of wiring is AC safety earth connected to chassis?

I guess that is most important thing but anything else?

 

[egellings]

In terms of shock hazards - the only important thing in terms of wiring is AC safety earth connected to chassis?

I guess that is most important thing but anything else?

A properly grounded chassis would definitely improve safety. Problem could be that if the power transformer, which would likely be painted, might not be making a good connection to the grounded chassis. Touch the transformer itself and a shock could occur.

 

[Music1969]

A properly grounded chassis would definitely improve safety. Problem could be that if the power transformer, which would likely be painted, might not be making a good connection to the grounded chassis. Touch the transformer itself and a shock could occur.

Easy to check with simple continuity test ? With multimeter.

Check continuity from AC power cable's earth pin to all touchable parts of transformer and chassis?

If it all beeps, then all safe?

 

[egellings]

Easy to check with simple continuity test ? With multimeter.

Check continuity from AC power cable's earth pin to all touchable parts of transformer and chassis?

If it all beeps, then all safe?

A multimeter test would not be a good safety check because the meter applies a small test voltage that likely would not break over dodgy insulation. Insulation needs to be tested at or above the actual voltage it will see in its end use.

 

[solderdude]

In terms of shock hazards - the only important thing in terms of wiring is AC safety earth connected to chassis?

I guess that is most important thing but anything else?

The problem is when the 'secondary' winding (the other primary) would make a connection to mains then the audio ground could be in direct contact with mains.
When you grounded the enclosure then that is safe (assuming safety ground socket) but when audio ground is not connected to the metal enclosure there can still be a voltage on it.

In most transformers both primary windings are isolated from each other but that doesn't mean all are equally well made.