AC voltage in the chassis ??

[Studiogearlover]

Hey guys!

I am about to troubleshoot this amp on the pic and to my surprise:

1. amp switched on, measuring 6.3VAC to mains ground in the chassis

2.  approx 30 seconds  later measuring 60 VAC to mains ground  in the chassis

Tubes are brand NOS and checked. Filter cap brand new, checked.

Its a 6v6 push pull to VC.  Its two prong old stuff correctly plugged in. I have two Y cap replaced. Not properly grounding these could be an issue?

Where is this 60 VAC coming from? it is from the transformer secondary to ground?  Any thoughts would be greatly appreciated.


60VAC with 2 mA.

 

[Khron]

You're measuring that AC voltage between mains earth (a possibly clearer name than "ground") and... what point, in the amp?

 

[Studiogearlover]

You're measuring that AC voltage between mains earth (a possibly clearer name than "ground") and... what point, in the amp?

Thanks for your help!

Between the mains earth and the same with also the negative (cold) side of the mains socket....and to any point on the chassis ground. ...the transformer center tap is also grounded with a 300 Ohm maybe 5w or 10W ceramic resistor and this is at the pin 1 on one of the 6v6GT.

 

[Khron]

At least in theory and by standard, mains neutral should be connected to mains earth, at the breaker box; the difference between the two is that the earth wire is not allowed to carry any current outside of fault conditions (unlike the neutral).

I don't suppose the power supply ground of the circuit (on the secondary) side of the power transformer) is connected to mains earth, is it? And possibly neither is the chassis?

If that's the case, the "ground reference" of the secondary will float up to half of the incoming mains - 60V AC leads me to believe the mains is 120V over there.

You'll find the same with stuff like phone chargers, which have no earth pin, and where the only connection between the output and the mains is through a Y2-capacitor (if you're lucky; if you're not, it's a bog-standard ceramic).

 

[JohnRoberts]

May be stray voltage... maybe put a 10k ohm shunt across the meter leads during measurement.

JR

 

[Studiogearlover]

thanks guys...

it is not me who built this amp and no schematic..however following a similar 6v6 Pp design, i am trying to figure this out what am I dealing with...

1. So the transformer has two C.T.  on for the heater voltage and one for the B+ ( i hope I am right with that)

2. at the 6SJ7 tube, the pin 2 and 1 should normally be shorted together to chassis..( i hope I am right?) but here is not. Instead it is also connected to the heater voltage across all the two 6v6 and the 6SN7 through the pilot light to the secondary heater.  Is this correct?

3.  I can not locate any cathode capacitor at the 6V6s pin8. There is a 300 Ohms large ceramic resistor to chassis ground and this is the point where the CT tap is also connected from the main transformer...., but should not be here a cathode bypass capacitor?  of 50 uF/ 50 V ?

 

[Studiogearlover]

At least in theory and by standard, mains neutral should be connected to mains earth, at the breaker box; the difference between the two is that the earth wire is not allowed to carry any current outside of fault conditions (unlike the neutral).

I don't suppose the power supply ground of the circuit (on the secondary) side of the power transformer) is connected to mains earth, is it? And possibly neither is the chassis?

If that's the case, the "ground reference" of the secondary will float up to half of the incoming mains - 60V AC leads me to believe the mains is 120V over there.

You'll find the same with stuff like phone chargers, which have no earth pin, and where the only connection between the output and the mains is through a Y2-capacitor (if you're lucky; if you're not, it's a bog-standard ceramic).

Hi thanks for that.

No the secondary is not connected to mains earth.. it is floating....and yes,  you are correct that it is 120 VAC ( its a Us made small mono 6v6 PP ) 

 

[Studiogearlover]

Hi thanks for that.

No the secondary is not connected to mains earth.. it is floating....and yes,  you are correct that it is 120 VAC ( its a Us made small mono 6v6 PP ) 

..but with this amp, it is important how I do connect it. I need to check the live side of the socket as if I flip it around, without turning the unit on having 120 VAC right away in the chassis, regardless if the Y2 " safety" cap...

 

[scott2000]

1. So the transformer has two C.T.  on for the heater voltage and one for the B+ ( i hope I am right with that)

sounds normal..

2. at the 6SJ7 tube, the pin 2 and 1 should normally be shorted together to chassis..( i hope I am right?)

Not really.. Sometimes it can be like this....

Instead it is also connected to the heater voltage across all the two 6v6 and the 6SN7 through the pilot light to the secondary heater.  Is this correct?

sounds normal

3.  I can not locate any cathode capacitor at the 6V6s pin8. There is a 300 Ohms large ceramic resistor to chassis ground and this is the point where the CT tap is also connected from the main transformer...., but should not be here a cathode bypass capacitor?  of 50 uF/ 50 V ?

There doesn't "have" to be a cathode bypass cap....

Does this amp work?

 

[Studiogearlover]

sounds normal..

Not really.. Sometimes it can be like this....

sounds normal

There doesn't "have" to be a cathode bypass cap....

Does this amp work?

Woooah... thanks Scott. It seems that I am overthinking this. It does work with some hum tho... there is a base hum with volume pot down and there is also much stronger one with the volume pot controlled

I did put a brand new old stock (never used) 5Y3GT in. It is making some internal sizzling sound...is this also normal? The guy who sold me that said I need to burn the tube in and this could be normal..  ? but is it really?

Amp works but will do another quick test in a bit... so should I not worry about the floating 60VAC in the chassis?

Sorry if I am overthinking this but better be safe then sorry  ;D

 

[radardoug]

Gosh, did no one else pick up that this amp has a TWO WIRE POWER CORD!!!
You need to ground that chassis. Take off the two wire cord and put a 3 wire cord on, taking the ground wire to the chassis.
Electricity can be dangerous stuff! Dont become a dead statistic!