I was messing around with some multimeters here and noticed something while setting the bias on a tube amp I have in the basement with exposed tubes and is grounded thru the chassis and runs on a two prong plug.




First I had a non-contact voltage detector close to the amp.... the thing lights up within a foot of the amplifier. I suppose I understand that as there are exposed tubes.




I also noticed that there is .4 VAC (point 4 volts miniscule) between the chassis of the amp and the grounded conduit of the electrical system.




This was seen on a cheapo harbor freight multimeter. Thinking that this was phantom voltage I tried the two real multimeter I have. Using a fluke with True RMS which usually does not show phantom voltage when I work on house wiring, I also get the .4 VAC.

Now it gets strange.


The other multimeter has a amperage clamp on it and is not true rms. Without the leads even touching either the amp or the grounded conduit the voltage started to creep up to 3VAC with the multimeter held over the amplifier. When I touched the probes to the chassis and the ground the multimeter, still be held over the amp, it creeped to over 275VAC!!!


I assume phantom voltage. I also touched the chassis and the ground pipe barefoot on cement floor and I am still alive so I am wondering.... is this just phantom voltage caused by the RF interference or something?


The fluke however showed .4 VAC chassis to house ground none the less... is that miniscule amount something to be concerned about that the amp has a flakey ground or something?

 

I am surprised that an ungrounded tube amp has ONLY .4V ac to earth. Appears there are no Y caps from primary to chassis. Most types of ungrounded, power transformer type products should read closer to 60V to earth with a high Z input voltmeter, rms or avg responding.
point 2; if there is any RF in the chassis, there is a problem with the amp, certainly that would not be normal.
The current clamp multimeter: i have no idea. Who made this? The fact that you are getting wild readings makes the measurement, maybe even the test meter suspect.

PS; you should be aware that there are fault modes within the amp that can put hazardous voltages on the metal chassis. Most folks I know working with vintage electronics put on a 3 wire cordset. There is actually a fault condition that can put far higher than line voltage on the chassis.

 

Just to add to TG's reply.

A 6L6GC tube can have up to 500vdc on its plate connection.
You want to be real careful when measuring voltages inside a tubed amp.

 

Some older tube radios and tube amps have one side of the line (the intended neutral prong of the power cord) bonded to the chassis. If the plug is not polarized and not inserted correctly in the receptacle or if the house circuitry is incorreclty wired, then the chassis will be hot relative to house ground.

Modern and better electronics have a power transformer and the chassis is not bonded to the power cord hot or neutral. Use your multimeter continuity or ohms function to verify that there is no continuity between the amplifier chassis and either plug prong (amp not plugged in). A fault here could be unnoticed while the amp performs properly, until someone gets shocked touching exposed metal on the amp.

Some clamp on ammeters may be accurate only at approx. 60 Hz. Their behavior could be uinpredictable in the presence of audio let alone RF frequencies.

When the chassis is not bonded to a line conductor then measuring voltage between chassis and house ground with the equipment powered on is meaningless except to suggest the existence of a fault.

 

Most tubed guitar amps utilize a two wire power cord. That means the chassis is not grounded and will induce hum within the amp's audio stream. To reduce that hum.. a small capacitor (.05ufd) is used to connect from the chassis to neutral of the power cord.

That cap can cause a slight voltage on the chassis like you are seeing.

As being involved with pro sound..... I can recommend you look into having a three wire power cord installed on that amp. I've seen many guitar players get a wallop when touching their guitar and a grounded mic.

 

Phantom voltage in house wiring can be caused by the capacitance between conductors that run juxtaposed over long distances. A capacitor in tube amps and similar equipment connected between the chassis and what ends up being the hot power cord conductor has the same effect. I figure out that the 0.05 mfd (ufd) cap would allow a leakage of about 2 milliamps at 120 volts at 60 Hz, not enough to electrocute someone.