Grounding output transformer common lead??

[sabredude]

Does anybody know why you are supposed to ground the secondary of the output transformer? What is the harm in leaving it ungrounded? I've accidentally left the common lead of the output transformer isolated from ground using cliffs jacks and the amp seemed to work fine. Then I would add a ground and notice no difference. I've got to believe there's a reason but I can't quite figure what that would be.

[ampgeek]

What type of rectifier topology were you using with the CT ungrounded experience?

I was under the impression that the HT winding needs to find ground directly somewhere to get current to flow. Putting the standby switch between the CT and ground is a fairly common arrangement that demonstrates the point.

When using a full wave bridge type rectifier, the CT connection (if there is one) can be left hanging because one side of the bridge is connected to ground.

Cheers,
Dave O.

[brewdude]

I think he's asking about the OT not the PT.

I think you need that wire to complete the circuit for the speaker. Perhaps it works fine floating and not tied to ground. I really don't know if there is any harm or not.

[ampgeek]

Oh gosh! So sorry. Spacing out a tad this afternoon.
Cheers,
Dave O.

[sabredude]

Not sure I follow you.
I was referring to the secondary common connection of the output transformer, not the primary CT. I don't recall what power supply I had and I don't see the relevance because the current would flow from the CT to the Anodes of the Power Tubes and be coupled onto the secondary taps regardless as to how I generated the DC potential. If I missed something, please enlighten me.

[Phil_S]

Does anybody know why you are supposed to ground the secondary of the output transformer? What is the harm in leaving it ungrounded? I've accidentally left the common lead of the output transformer isolated from ground using cliffs jacks and the amp seemed to work fine. Then I would add a ground and notice no difference. I've got to believe there's a reason but I can't quite figure what that would be.

Are you bringing the common lead the the sleeve of the jack? If so, that is a floating ground, not no ground. I am ill equipped to explain further. Sorry.

[sabredude]

Yes, I am attaching the common lead to the sleeve but on a Cliffs, that doesn't mean grounded as I'm sure you're getting at...

Some will say it is floating, others will get all technical and say there's no such thing as a floating ground.

I'll just say its not referenced to ground in anyway shape or form!

The speaker is seeing the difference in potential between the ends of the coil. This to me actually seems safer than grounding the common lead and referencing the speaker to ground. If a voltage were to get somehow applied to the speaker cable shield, it would not be felt by the amp, the output would simply be riding the voltage.

[sluckey]

Does anybody know why you are supposed to ground the secondary of the output transformer?

There is no need to reference the OT secondary to ground if all it will be doing is driving a speaker. But... If there will be a NFB loop connected to the secondary, you must reference the secondary winding to ground to make the NFB circuit work properly.

[Jana]

There is a reason to ground the OT common even if there is no negative feedback being used. A floating winding can build up a charge. A ground prevents that from happening.

[printer2]

There is a reason to ground the OT common even if there is no negative feedback being used. A floating winding can build up a charge. A ground prevents that from happening.

And I almost thought I was going to be the one that said it first. What can happen is the voltage gets high enough that it flashes over at some point. I never seen it do it but I did measure the output winding being a couple hundred volts above ground.

[katopan]

Flash over is very unlikely, because it's static potential build up. But still enough to give you a surprise when you touch the speaker terminals. Think of the capacitance between the windings and the OT frame, and keep in mind the primary has full HT voltage on it. The capacitances to ground just act like voltage dividers and the secondary winding will float up to whatever voltage in between given a particular transformer design.

Also think of the single mode fault of break down of insulation between the primary and secondary. Then you really want one end of the secondary grounded to blow a fuse before you find out the bad way it's sitting at B+ voltage by direct connection.

[gingertube]

The secondary should always be connected to 0V.
If using NEG Feedback then the ideal place to do this is at the 0V point adjacent where the feedback is applied, usually across the bottom resistor in the diffamp tail or in an amp with a concertina splitter across the bottom resistor on the preceding common cathode stage.

Connect the secondary common to this 0V point regardless of if you then add negative feedback or not.

R=That way the amp is all set up REAEDY to add NFB if you so decide AND the secondary is no longer floating

Cheers,
Ian

[VacuumVoodoo]

If isolation between primary and secondary winding fails you will see full B+ or more on secondary. If it is grounded it will short the high voltage to ground and blow the HV fuse if there's one. Or it will take a couple seconds until it blows the main fuse. In any case, HV winding should always be fused.

[sabredude]

Thanks guys! Very helpful!