Power transformer shield

[sbirkenstock]

Hi everybody,
just putting a new amp together, with a Classic Tone 18096 power transformer. (Vibrolux Style).
The fuse burns, when I switch it on.
I norrowed it down to the orange wire.
If I do not ground it, everything seems fine.
I did use this transformer before and I did ground the orange wire.
The orange wire is called SS in the specs sheet. (I attached the .pdf)
Elsewhere I found it as "shield" and that it should be grounded.
When I apply power to the transformer, I can measure 120V AC on the orange wire.
My line has 235 V DC, so it´s about half of that.
But the orange wire has no connection to any other wire. It´s obviously completely isolated.
And interestingly, when I ground the orange wire and put a light bulb limiter before the amp,
nothing happens....

Any idea or info about the ss / shield?
Stephan

[Stevem]

If that static shield has voltage on it you then must find that there a resistance between that orange wire and another wire .
There’s no other way for any level of voltage showing up there like you have .

[2tone]

If it's an internal shield, and recommended to be grounded, there may be an internal short in the transformer??

[LOUDthud]

First, is your 235VAC balanced as it is in the USA (Each side being roughly 120V to Earth) or a single ended arrangement with one "Neutral" side being very close to Earth and the other side being 235VAC to Earth ?

Second, which Primary wiring did you use, 230VAC or 240VAC ? Did you make doubly sure of the non-connection to the unused wires ?

What exactly were the conditions when the fuse blew, when the light bulb limiter was used and when you measured 120VAC on the Orange wire ? Were the Secondary wires connected your circuits in your amp ? Were the Grn/Yel and Red/Yel wires grounded ?

It's not unusual to measure substantial Voltage on an un-grounded shield wire due to capacitive coupling within the transformer. However, it shouldn't blow a fuse when connected to ground.

[TUBEDUDE]

Open up the tranny and look for stray wire across the terminals.

[sbirkenstock]

Hi,
I checked all wires again, no resistance between the orange and the others (tested with a Fluke 179).

235 VAC is not balanced, 300mV on one side, 235VAC on the other.

Now surprise:
I figured out that fuse did not burn after switching on the amp and then touching the orange wire to the ground bus.
This happend repeatedly.... (I could not believe it so I did it again, kind of an expensive experience )
And the amp is not populated yet, only bias is there, LED pilot light, a few parts but no filter caps, no tubes.

The burning fuses were 500mA. My mistake, should have been 1A for a 6L6. (235VAC = half A compared to 117VAC).
With an 1A fuse everything is stable.....

And again, no filter caps, no tubes.....

I have no explanation.

Does anybody know what is shield is shielding?

Thank you very much,
Stephan

[R.G.]

The burning fuses were 500mA. My mistake, should have been 1A for a 6L6. (235VAC = half A compared to 117VAC).
With an 1A fuse everything is stable.....
And again, no filter caps, no tubes.....
I have no explanation.

Actually, you have probably just explained the problem. The first connection of a power transformer to the AC mains can cause a very large current for a few cycles of the AC mains. I once measured the inrush first-cycle current peak on an especially large variable transformer at over 200A. Its fuse was 15A, and the fuse did not open; but the AC mains breaker did.
The rating and type of mains fuse is important. A fuse with half the rating, and possibly more important, not rated as "time delay" or "slow blow" could be the entire problem. If the fuse was on the edge of blowing, connecting the shield could have provided just enough additional current to make it open.

This is speculative. Proving it would require doing some careful measurements of the inrush currents.

Does anybody know what is shield is shielding?

It is common for transformers to include an electrostatic shield, a copper foil between the primary and secondary. This intercepts and grounds much of the AC power line leakage and noise in the primary.