Transformers covers

[emk2]

I built a 2 stages differential pp amp (uf86->el86, became later 6s3p-ev > 6p43p-e) with toroid output transformer.

If I touch with hand the (steel) cover of OPT, it start to hum in the speaker.
If I remove the cover and touch the OPT, it hums even higher.

First question: why?

Second question: what is the better, connect the cover at signal ground (0V) or connect it to safety ground (earth)?

Last one: Am I supposed to ground the screw fixation of toroid OPT? the plate is not made of steel, but wood.

For your information there is a Sowter 8920 at input used as phase splitter, so NO connection between signal ground and safety ground.

Thank you.

 

[Lingwendil]

I would do safety earth, since the chassis is attached as well (usually!)

 

[Alan4411]

... and one side of the secondary (output) is grounded?

 

[DF96]

High frequency parasitic oscillation?

 

[emk2]

Thank you.

I would do safety earth, since the chassis is attached as well (usually!)

OK, will do.

... and one side of the secondary (output) is grounded?

Yes, negative speaker binding post is at ground.

High frequency parasitic oscillation?

Do I need a LF generator or just shorted input and try to see something in HF range with the oscilloscope?

 

[DF96]

HF oscillation can show itself in various ways:
unexpected hum
unexpected distortion
unusual DC conditions - does the valve bias change when you touch the cover?

Sometimes you can see the HF with a 'scope, sometimes you can't.

 

[JMFahey]

Quote:
High frequency parasitic oscillation?

Do I need a LF generator or just shorted input and try to see something in HF range with the oscilloscope?

Neither LF generator nor shorted input, and your hum is not continuous, so scoping the output while not humming will show you nothing.

So first attach scope probes to output terminals (because you hear hum, so it´s going to speakers) and then touch transformer both ways , with cover and straight to windings, exactly what you did before.

*When* you hear hum, you will see "something" on the screen.

Describe what you see and even better have somebody else (not you) take a scope screen picture, post it here.

 

[emk2]

Thanks.

I didn't measure anything yet.

But it's weird, I changed the drivers (UF86 > 6S3P) just before opening this thread.

Now this behaviour has disappeared.. I don't get it.

Oh well..