Output Transformer Ground Question

[odourboy]

In my latest build, I've used jacks with isolated contacts so I find I have the option of isolating the common lead on my OT from the ground of the rest of the amp. Good idea, bad idea or no-op?

[paulster]

I always ground the OT secondary regardless as, if you had a transformer failure, there's always the possibility of getting B+ on the speaker jack without tripping the fuse otherwise. If you happened to be plugged into a metal-cased attenuator that someone touched, for example, it could put a major downer on their day.

Now this is a fairly unlikely scenario, but it does illustrate the sort of thing that could happen. And it's always in an 'unlikely' scenario that somebody gets hurt.

[LOUDthud]

It's a good idea to ground one side. Depending on the transformer interleveing, there could be a large common mode voltage that will cause strange problems.

[odourboy]

Thanks guys. Actually, I think I'm about to answer my own question. If I don't tie the common to the amp ground, how can the global NFB voltage be properly referenced at the phase inverter? I'd better try it.

[drz400]

Thanks guys. Actually, I think I'm about to answer my own question. If I don't tie the common to the amp ground, how can the global NFB voltage be properly referenced at the phase inverter? I'd better try it.

With negative feedback you have to ground it, without it is still a good idea.

However, do use isolated jacks, this way you take OT ground to the jack ground, then run the ground to the same ground point as your phase inverter.

[paulster]

Thanks guys. Actually, I think I'm about to answer my own question. If I don't tie the common to the amp ground, how can the global NFB voltage be properly referenced at the phase inverter?

You have answered your own question! I'd assumed that you were talking about something similar to a Vox/Matchless without global NFB and there was actually a decision to be made on whether or not to ground the OT secondary. With global NFB there isn't a choice.

Follow drz400's advice on the isolated jacks and the PI ground point though, as he makes a very good point regarding the NFB loop.