tube amp- grounding, stray voltages?

stereory

Member
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 using 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?
 
That happened by accident prior to bringing out the multimeters.

So as a retired electrician, what do you think? Phantom voltage or a problem?
 
Lots of stray voltage/current around amps,,, don't know what amp, but if its a series wired amp, there's 50/50 chance of the chassis being hot...
 
You are playing with hazards here !!
The amp will in stock form, get close to a voltage between mains live ( 120V ?)
and mains return (0V ?) due to leaking. This leakage is primarily in the mains transformer.

Using this in a damp space with concrete floor is dangerous. Convert them to 3-prong
cabling and make sure a proper earth is installed.
 
High impedance meters like a digital will pick up stray fields and read nonsense. Connect a lead from chassis to a grounded outlet and see what it tells you. A lot of tube amps had a line to chassis cap, and its pretty typical to get some reading. If the cord is flipped the wrong way, you'll get somewhere around 120 volts AC, the other way is generally under a volt. Stuff that is not quite so ancient has polarized or grounded cords. Properly wired, both should read near zero volts from chassis to ground.
 
Here's an old tip i learned from a Ham Radio operator. Connect a 10k resistor from chassis to chassis and read the voltage across it. It should drop to near zero volts, indicating stray, harmless AC leakage current. If the voltage doesn't change, you have a real leakage problem.

Just to reiterate, don't EVER touch any amplifier or gear that's plugged in while you are barefoot on the basement floor or any floor to be safe. That's how the famous shorted "death capacitor" in guitar amps. killed a guy!
 
Back
Top Bottom