Output transformer ambiguity

"Steve Sousa"

** No.

The ratio between rated primary Z and winding resistance is never less than

5:1 and is typically 10:1 or more.

.... Phil

On 11/25/2010 12:01 PM Phil Allison spake thus:

You may have a point there.

OK, let's climb down into N. Cook's pit of bodgery and post mortem analysis and look at this.

It all depends on the expected failure mode of a transformer, correct? Two main possibilities here: open windings, or shorted windings (or I suppose a combination). Obviously not an open winding, so probably shorted ones.

Shorted windings could be within the primary or secondary, or between. Let's look at the simpler case, shorted within the primary, which should be 4K ohms.

It seems most likely that any short would be between adjacent layers of windings. Not likely that there'd be a short from close to one end to close to the other end. Which should limit the effect of the short. So if the 4K ohm primary is now 900 ohms, there's something else going on.

But what? Could there be a short from the winding to the iron core?

Has the O.P. checked for a short between primary and secondary? How about shorts to the core/frame of the xfmr?

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

"David Nebenzahl = Idiot "

** Shame about the third and most likely scenario - one that makes the primary R go way high.

Think " vertigris " .....

..... Phil

chassis!

What is the result of hot-air heating and knocking the transformers with copper/hide hammer or simulating with hammer and wood block while monitoring the ohms? Certainly don't power up with new valve without using a variac and probably filament bulb telltales(acting as fuse if serious problem) in place , if you cannot establish which Tx is wonky.

I would go for shorted turns on the "900" ohm one especially if heating and knocking fails to show any variation. Shorted turns tend to be "welded" together . I only have test data for the EL80 but 25mA per pentode and looking at some Premier Tx data for range of use 7 watt to 60W, DC of HT side ranging from 1500 to 16000 ohm so 4000 ohm seems more likely. What is the physical dimensions of your ones?

On 11/25/2010 5:22 PM Phil Allison spake thus:

You mean *verdigris*.

So that could happen if the insulation got nicked and moisture gets in, corroding the wire. How does that work? Narrow the wire and increase the resistance (eventually causing an open)?

--
The fashion in killing has an insouciant, flirty style this spring,
with the flaunting of well-defined muscle, wrapped in flags.

- Comment from an article on Antiwar.com (http://antiwar.com)

"David Nebenzahl" Phil Allison

** Can happen any number of ways including at the termination points of the enamelled wire where it is soldered to a flying lead. Solder flux is then the trigger and moisture from the air can get to it.

The high resistance is because for some small distance, conduction is via the verdigris.

.... Phil