I just decided to test the voltages in my old tube guitar amp against the schematic. I live in Australia and it's a US amp, so I use a separate 240-110v step down transformer to run the amp. Upon testing the amp, I realised there was around 240v between the chassis of the amp (grounded) and the neutral line. I went back to the step down transformer and got the following readings

~110v between hot and neutral
~130v between hot and earth
~244v between neutral and earth

My wall plug is fine, with ~244v between hot and neutral, ~244v between hot and ground, and 0v between neutral and ground.

Does anyone know what's going on here? I've left the amp in bits and I'm not playing it until I've worked this out, as it seems I might end up playing with 240v on the guitar strings! Thanks in advance

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    \$\begingroup\$ Where the measures have been taken? Schematics, please. You are measuring same voltages with two different results? How is this possible. \$\endgroup\$ Mar 20, 2017 at 13:30
  • \$\begingroup\$ Schematics of what? The amp isn't really relevant in terms of my question, and I dont have one for the transformer \$\endgroup\$ Mar 20, 2017 at 13:41
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    \$\begingroup\$ Sounds like you may have the live and neutral swapped over in the wall plug. Are all the plugs 3 pin with earths connected? \$\endgroup\$
    – Finbarr
    Mar 20, 2017 at 13:41
  • \$\begingroup\$ Seems like thats the case. The wiring in this house is very old. I had the active and neutral positions muddled in my head, likely from thinking about US plugs. Thanks very much \$\endgroup\$ Mar 20, 2017 at 13:48
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    \$\begingroup\$ I meant the plug, i.e. what's on the end of the cable rather than the socket in the wall, but it would have the same result. You really need to check that they're all correct. \$\endgroup\$
    – Finbarr
    Mar 20, 2017 at 13:52

2 Answers 2


You don't specify whether the transformer is isolating or "auto-transformer". The auto-transformer isn't isolating and one of the output terminals is connected through to one of the input terminals.


simulate this circuit – Schematic created using CircuitLab

Important: the link between N and earth / ground is shown for reference only. This is done by the utility company at their transformer and possibly where the supply comes into the building. Do not link N and E anywhere in your circuit.

If you swap the L and N on your wall plug you may find the problem is solved.


simulate this circuit

Figure 2. Figure 1a configuration with the amplifier added.

The amplifier will, most likely, have an isolating transformer on the mains input. This means the output is isolated from both mains and neutral wiring.

Note the connections of the earth wire and the three symbols used.

  • The leftmost indicates a connection to ground or earth. This is usually done at the supply transformer or building incoming connection depending on local regulations. The neutral is so called because the link between it and ground "neutralises" the voltage on that line.
  • The central symbol is a chassis connection - usually a crimped ring terminal bolted onto the chassis somewhere. This is done to keep the chassis at ground potential.
  • The third symbol is the signal - audio in this case - ground. Typically there will be one connection between signal ground and the chassis. More than one runs the risk of ground or earth loops which can cause hum in audio circuits.
  • \$\begingroup\$ You nailed it. That is the problem, it's an autotransformer he has plugged in upside down (easy to do on some socket types). But that said, please don't draw a neutral-ground connection except through a meter. Neutral is not ground, except where it is bonded in the main service. Many people conflate them, and doing so creates a recipe for a dangerous situation. \$\endgroup\$ Jun 10, 2017 at 22:04
  • \$\begingroup\$ My drawing is intended to show that there is a neutral connection to ground. The first bullet point under Figure 2 explains that point and states that the neutralising link location depends on local regulations (or 'code' in USA). In Europe the ground does not pass through a meter but is, in domestic installations, usually linked to neutral at the incoming supply which is usually the meter point. \$\endgroup\$
    – Transistor
    Jun 10, 2017 at 22:05
  • \$\begingroup\$ @Harper: I added some shouting into my answer to make it clear. Thanks. \$\endgroup\$
    – Transistor
    Jun 10, 2017 at 22:14
  • \$\begingroup\$ Appreciated. I see what you mean, it's tough to illustrate because you need to show the N-G relationship in your illustrations. \$\endgroup\$ Jun 10, 2017 at 22:17

The output of your transformer is floating i.e. it is not galvanically connected to ground. However, it will be capacitively connected to the primary winding and, (it sounds like) the primary and secondary voltages are conspiring to produce a high impedance voltage of 240 V between secondary neutral and earth. If you put a 1 Mohm resistor between secondary neutral and earth, I expect this rogue AC voltage to drop to below 10 V RMS.

But, be careful; there could be a fault that is causing this and not just stray inter-winding capacitance. Use a 1 watt resistor that is capable of sustaining 240 V RMS in the test.


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