Widowmaker or not?

[xtian]

I have a made-in-Japan Victoria lunchbox amplifier on the bench. I cannot find a schematic. It has no power transformer. It has a tube rectifier. Wall 120vAC service is not directly bonded to chassis, but, if plugged in "incorrectly" (plug is not polarized), I read 120vAC from chassis to earth.

Please watch my video.



I can't measure more than 2.75mA current from chassis to earth. So now, is this a widowmaker or not?

If I put in a three-conductor power cable and bond earth to the chassis, it will be safe where the power outlets are wired correctly, but could still be dangerous if outlet was wired wrong.

[pompeiisneaks]

Yeah that thing needs a 1:1 PT for sure, that is like some of the amps I've seen that use the 'voltage drop across the heaters' method of providing the power to the heaters, and then just rectifies the AC directly to the rest of the amp, very dangerous without an isolation transformer, otherwise they're too dangerous.

Also I noted your measurement of current. Most any multimeter that I've used that measures current requires you put the red lead over on the other side to measure current, does yours not work that way? I can't quite make it out, is it an aneng 8008?

I think that since there are tubes in etc, it's possible that no current flows because it is 'operational' it's when things go wrong that they can become extremely dangerous because curren't cant' flow through the amp correctly...

BUT I'm definitely not a super savvy 'old school transformerless amp' knowledge person

~Phil

[R.G.]

I think it's that "I don''t have a schematic" thing that is confusing the issue.

It is possible that the chassis is "isolated" from the opposite side of the AC power line by a capacitor. A small enough capacitor will limit current by its capacitive impedance. This is the fundamental idea behind transformerless power supplies for line-connected logic. Xc = 1/2*pi*F*C

That current limitation doesn't make it much less dangerous, probably. I'd still call it a widowmaker. Or a widower-maker.

[JMFahey]

Will expand later but as starter:
1) do NOT repeat NOT measure resistance with the amp plugged in, PERIOD

2) chassis IS connected directly to mains plug (directly here meaning no galvanic isolation) .

That they do that through a 47k current limiting resistor does not make it exactly "safe".

2.7mA current through chest means it´s below *average* "deadly value" so instead of electrocuting everybody and his dog it only remains dangerous to old guys with a pacemaker or needing heart medication. (hint hint )

Very much doubt Japanese were "original designers", so they must have been very close to what 60´s Americans were doing, such as:



Notice they added a small cheap 1:1 transformer to feed input 12AU6, probably to reduce hum injection in cathode but that was not an universal practice.

EDIT: answering the original question: yes, it IS a widowmaker.

[xtian]

Owner asked me to install a 1:1 iso transformer. Do I still add a 3-conductor power cable and earth the chassis, or just leave it floating?

[thetragichero]

DEFINITELY install 3 prong plug with ac safety ground bonded to the chassis. i personally would refuse to work on any amp with a 2 prong plug where the owner wished to keep that original (some "vintage amp" guys are more trouble than they're worth, wishing to keep unsafe operation/dried out dead electrolytics because they want it "original")

[TUBEDUDE]

Will expand later but as starter:
1) do NOT repeat NOT measure resistance with the amp plugged in, PERIOD

2) chassis IS connected directly to mains plug (directly here meaning no galvanic isolation) .

That they do that through a 47k current limiting resistor does not make it exactly "safe".

2.7mA current through chest means it´s below *average* "deadly value" so instead of electrocuting everybody and his dog it only remains dangerous to old guys with a pacemaker or needing heart medication. (hint hint )

This is not entirely accurate. The timing is more critical than the current.
Much smaller currents, sub mV, if applied when the ventricles are repolarizing can cause VFib. That is death without a defibrillator.
That amp is a murderer looking for an opportunity.

[Phil_S]

I personally would refuse to work on any amp with a 2 prong plug where the owner wished to keep that original...

Me too, though I don't do this sort of thing as a job.
Install the iso and 3 prong. I, for one, would be sorry to see TAG lose such a good member. Be safe, Aaron.