Shocking Amp but Why?

[misterc57]

I serviced an Airline Model 62-9012A Amp made by Danelectro (Same as Silvertone 1471) all tube combo, properly installed a 3 prong cord as I have done 100 times prior.

I tested the amp for AC on the chassis and there was none.

I shipped the amp to someone and they say they are getting a bad shock from the amp.

I am wondering what else could cause this to happen.

I would have installed the HOT to the fuse, switch, and PT. The neutral direct to the other side of the PT. The ground to chassis. If there was a death cap, I would have removed it.

Any ideas?

The amp has a power transformer. Tube line up 6X4, 6V6 12AX7. Very much like a Silvertone 1471.

Could not find an exact schematic but something like the attached image.

Thank you

[xtian]

Recipient's outlet is improperly wired/not earthed.

[The Dude]

^^^^^^ Yes, that.
Or, the amp is fine and another device he is touching (microphone wired to mixer, etc.) has chassis leakage and he is grounding it through his body and amp. If you installed a grounded AC cord and properly wired it to the chassis, it could not be your fault.

[Mick Bailey]

An outlet that's not earthed won't cause a voltage on the chassis where an isolation transformer is used and no death cap is present. I've had plenty of situations though where touching correctly grounded metalwork has given a shock or tingle where either there's a fault on adjacent or connected equipment or equipment using an SMPS is used and permitted leakage causes a shock in some individuals.

A grounded AC cord properly wired to the chassis can mask transformer leakage from the winding to the core, which will only show up if an earth leakage test is carried out or the equipment is connected to a supply with a faulty or disconnected earth. Usually any earth fault trip will operate if the fault is serious enough, but leakage can be low enough to give a mild shock but not trip the device.

I have a plug-in tester that breaks the mains earth connection and simply places a parallel resistor and cap in series with the earth. Leakage current can then be determined from the voltage drop across the resistor. What method did you use to test for AC on the chassis?

[misterc57]

I have a plug-in tester that breaks the mains earth connection and simply places a parallel resistor and cap in series with the earth. Leakage current can then be determined from the voltage drop across the resistor. What method did you use to test for AC on the chassis?

My procedure is to set my meter for AC, place one probe into the ground of an outlet on my bench, the other tip goes to any chassis metal (chassis, input plug nut, and even guitar strings if the guitar is plugged in). This test has shown me AC voltage on leaking amps.

[Chuck H]

I'll second the possibility that it's a combination of gear or improper AC wiring and not the amp you reworked that is responsible. Consider that what you have done should eliminate chassis shocks. If the amp didn't do this before, but does now then it's possible that NOT being wired for safety ground was preventing the shock before somehow.

[Mick Bailey]

Measuring from the chassis to an outlet ground measures the voltage across a low resistance - maybe just a few ohms. If the leakage source impedance is high compared to the resistance of the current path to ground, the voltage divider action won't give true indication of leakage. The shock possibility is only then realized if a leaky amp is disconnected from mains earth (due to a faulty outlet or wiring), where the voltage then appears across the resistance of the operator, especially if they touch another grounded body.

[drewl]

I had an amp with an open cathode connection of the first preamp tube that caused the input grid to rise to the plate voltage
that voltage was on the input jack tip and on the guitar cord

[Helmholtz]

I had an amp with an open cathode connection of the first preamp tube that caused the input grid to rise to the plate voltage
that voltage was on the input jack tip and on the guitar cord

Now that's strange.
With a standard input stage it seems clear that the cathode voltage will get high when not connected.
But I don't see how the grid voltage could get positive.
I guess the tube had an internal short.

[g1]

I had an amp with an open cathode connection of the first preamp tube that caused the input grid to rise to the plate voltage
that voltage was on the input jack tip and on the guitar cord

This would still not affect string ground, would it? And if so, still depend on amp chassis being un-grounded, or input jack isolated from chassis?

[Helmholtz]

I always use a mains/phase tester (screwdriver with integrated neon lamp) to identify components carrying a touch voltage >60V to ground.
The little tool can save lives.

[g1]

I always use a mains/phase tester (screwdriver with integrated neon lamp) to identify components carrying a touch voltage >60V to ground.
The little tool can save lives.

Something like this? These kind of devices are very inexpensive.
https://www.gardnerbender.com/en/p/G...-Circuit-Alert

[Helmholtz]

Mine is just the simple passive screwdriver type every electrician uses to find the "phase", e.g.
https://www.meteorelectrical.com/bra...instester.html

[Mick Bailey]

Establish what's plugged into the amp - is it just the guitar? Also, what's being touched when the shock occurs? If the input socket is directly grounded to the chassis there's no potential difference between the guitar and the amp and touching the grounded guitar would have the same effect as touching the grounded parts of the amp.

Some SMPS are capacitor-coupled from the secondary back to the primary live side and this can place 60v to 90v on the DC supply. This can cause problems in some circumstances, especially when there's a wiring fault with reversed live and neutral. The 'shock' is only felt by some people - I can't feel it with the ancient dried-out leather that passes for skin on my hands.

[Chuck H]

This^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

It could be that before the chassis of the amp was earthed there was no path for conduction. Now that it is, the stray voltage from ANOTHER source has a path to ground through the player. I agree with all above suggesting an AC outlet checker. They're cheap enough and saved my bacon on more than one occasion playing smarmy venues when I use to gig.