yet another transformer question (terminating)

[rafafredd]

Well,

Iīm using some vintage transformers like WE, UTC, Triad, etc.. that are from companys not in business anymore. So, I canīt get termination info from them! And I donīt even know where they were used before... This are used units bought alone.

Is there any way of determining what would be good in terms of terminating the windings? Caps and Resistors values to put across the windings for proper performance...

Iīve heard that one could find aprox values by measuring the DC resistance of the windings, but I would like to have more info about this method, if itīs really functional. At least I would be able to figure out values to start trying with... I hope I donīt have to just try all the values possible and hear the results for comparation, as this would take lots of work.

Solder, listen, desolder, solder, listen, desoulder, solder.... [img]graemlins/.gif[/img]

Thanks for all again guys.

[Kev]

I think some of us may have some UTC data sheets. I may have posted this before. Have you checked Group DIY's trafo pages ?

[vintagedesign]

There's nice readings about terminating at Geoff Tanners page in the "Neve Secrets" You have to sign-up to see the secret folder!

[rafafredd]

Yes, but the UTCs I have are very uncommon and most I have many other that are not UTCs...

Is this Geoff pages info aplicable to any transformer or just the NEVE transformers?

[rafafredd]

Hey, can you link to the page?

I canīt find it on auroraaudio.net...?!

Thnaks

[Flatpicker]

I’d just put a high frequency square wave through it and experiment with different loading values to see which gives the least ringing, overshoot, and undershoot on the leading edge.

[NewYorkDave]

And if you plan to use it as a tube preamp input, be sure to take the input capacitance of the tube into account. A transformer that overshoots when terminated in a simple resistive load might be just fine when working into that same resistance shunted by the Miller capacitance of a triode grid.

[rafafredd]

Yes, but how can I determine such a resistance to terminate a secondary winding on a tube pre?

Isnīt it related in any matter with winding inductance, dc resistance and load impedance?

[NewYorkDave]

The resistance should normally be ten times (or more) the reflected impedance on the secondary.

For example, suppose you have a 1:10 turns ratio mic input transformer, for use with a 200 ohm (nominal) mic. The impedance reflected to the secondary (with the primary terminated by the mic) is the square of the turns ratio: 200 * 100 = 20K. So you'd want to use a grid resistor of at least 200K.

The reason for this is that you want to reflect a "bridging" impedance to the primary to maximize voltage transfer from the microphone. With a 200K resistance across the secondary, the impedance looking into the primary is 2K, which is ten times the nominal source impedance of the mic.

The mic is actually terminating the transformer; the grid resistor is bridging under normal use. If you truly "terminated" the transformer on the secondary as well, you'd have a double termination and a loss of signal.

The miller input capacitance of the tube is in parallel with the grid resistor. Nominal grid-to-plate and grid-to-cathode capacitances are listed in the tube manuals. Take the grid-to-plate capacitance (plus add several pF to account for "strays") and multiply it by the gain of the tube in the circuit you plan to use. Then add this to the grid-to-cathode capacitance. The total is the approximate capacitance that will appear across your grid resistor.

[rafafredd]

Man, Thanks a lot!

Now I have something to begin experimenting.

So, with solid state input stages, I would have to add a capacitor paralel with the resistor on the sondary of the mic input transformer.

Am I right? Any tips to find the apropriate pF value for this?