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Old 9th May 2012, 12:26 PM   #11
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Indeed - by measuring the the wire resistance of the primary against the secondary - whilst cold - and accounting for the step up ratio which should be matched by the resistance ratio - you should have a fairly good idea of whether the reverse connection will work without passing any signal through the transformer.

My gut feeling would be that they will probably use exactly the same wire - to make their product as flexible as possible.

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Old 9th May 2012, 03:46 PM   #12
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Thank you guys for the inputs

The interstage in question is the old Tango NC-18.

I attached the datasheet.
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File Type: jpg Tango NC-18.jpg (412.1 KB, 107 views)

Last edited by HP8903B; 9th May 2012 at 03:59 PM.
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Old 9th May 2012, 03:47 PM   #13
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Notice that the primary DCR is 600 ohm and secondary DCR is 1.5k ohm, both in series.
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Old 9th May 2012, 04:07 PM   #14
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They should be very nearly the same - so it seems it might not be such a good idea to reverse them.

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Old 9th May 2012, 04:20 PM   #15
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Quote:
Originally Posted by Shoog View Post
They should be very nearly the same - so it seems it might not be such a good idea to reverse them.

Shoog
Difficult to say.
Primary DCR of 600 ohms is a good specification when tubes with Rp's in the 7k range (6SN7) can be used; it is very well possible that the choice of wire gauge was to get this DCR to keep copper losses small, and that actually the current rating has more to do with core saturation than wire gauge.
One experiment that can be done is to measure secondary DCR cold and after let's say 20 mA of DC current; when "hot" DCR is not much higher than cold DCR I don't expect problems by reversing primary and secondary.
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Old 9th May 2012, 11:05 PM   #16
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I want to use the Tango NC-18 as a preamp OPT for the Western Electric 437A.

The WE 437A will draws ~12mA

The turn ratio will be 1:3; wired in reversed then 3:1.

5400 : 600 ohms

Can these transformers be used in this application ?
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Old 10th May 2012, 12:05 AM   #17
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Yes.
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Old 10th May 2012, 01:23 AM   #18
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Thank you
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Old 10th May 2012, 03:22 AM   #19
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Quote:
Originally Posted by nazaroo View Post
I'm still having trouble with this:

Wire is rated strictly on current carrying capacity.
If only the primary is meant to carry the extra DC current (on top of the AC),
then one of the transformer maker's concerns will be current carrying capacity of the wire,
and he will use heavier wire in the primary, possibly even if the ratio were 1:1.

When you reverse an interstage xformer,
you are transferring the DC standing current to the other winding,
the one which could easily have smaller wire, especially if its a step-up.

Although the transformer doesn't care which side is given the source
of the AC signal, and which side passes it on, (reverse the general signal flow
and core saturation stays the same: think time reversal).

The transformer wire may care which side is carrying
the additional D.C. current, which is the whole reason
the transformer is needed (isolation).

It seems to me this must be checked on a transformer by transformer basis.
Yes, you're right. I hadn't thought about the power dissipation aspect of the DC.

I absolutely agree that transformers must be considered case by case. Too many parameters, and not enough common understanding for most people to make sensible predictions. OTOH, building valve amps seems to be about having fun these days, and the wrong transformer is more fun than the maths.

The original question is unanswerable without a lot more information. Had the information been available, the question wouldn't have needed asking.
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Old 10th May 2012, 04:36 AM   #20
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I see.

The specific question now is about the Tango NC-18; whether it can be reversed and used as a preamp OPT with 12mA current draw ?
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