DC on secondary of output transformer

[tardishead]

Can someone explain whats going on here
I build a valve preamp like many I have built before and I am getting some DC on the secondary of a Trimax output transformer.
About 15v
When I ground the transformers screen/shield it goes down but it is still 2-3vdc
I have never seen this phenomenon before. is it a problem?

 

[emrr]

Sounds like a DC offset relative to your test equipment, without knowing more.

 

[joaquins]

Just to be clear, is it across the secondary or between the secondary to ground?

I guess the last one, in which case you could put it to ground and see no problem, you don't need to, the receiver equipment should do the job accommodating the floating winding wherever it want's it to be.

If the first one you have found a transformer that goes down to DC!!!  8) Or you just have it miss wired somehow so check that.

JS

 

[tardishead]

actually I just got a small shock off touching the output connector

 

[gyraf]

Disconnect and Ohm primary-to-secondary.

 

[tardishead]

Ok so let me just fill you in on my experiences
I built 2 RCA BA2c preamps about 6 years ago with NOS Trimax output transformers 20k:600 rated at 6.5ma
I biased them like the original to 4-5ma so well under current rating
They have been great sounding and I used them almost every day.
This year both of them started crackling one a couple of months before the other one.
I trouble shooted for days checking the obvious suspects. caps resistors etc. And then I started to look at the output transformers.
They both exhibited that strange DC on the outputs but when I grounded the transformer shields this lessened the DC to about 1v but did not stop the crackling.
I finally found some transformers exactly the same and swapped them - fixed the problems immediately.
What the hell happened here?
I read one of PRR's posts somewhere where he mentioned that the paper can break down.
Is it possible that the enamel wire coating could have broken down also?
Its pretty strange that both of them started having problems after exactly the same amount of use.
Can anything speed up the arrival of these problems? Temperature ? Humidity?
Most of the time they have been sitting in a rack of tube equipment. They were howeevr stored in a garage for about a year which got pretty cold and damp sometimes.
Help me understand this strange phenomenon

 

[emrr]

The only instances of crackling I've seen/heard have been current related.  In one case I moved B+ off of the unobtainium output transformer and changed it to parallel feed with a choke, cracking went away.  I don't recall seeing DC leak to the secondary, which suggests insulation breakdown somewhere.  The garage could be the culprit, temperature and humidity swings can cause failures from expansion and contraction, humidity moving in and out along with.  I've noticed you can measure inductance at different temperatures and get different readings, has to be coil size change.