Push Pull Output Transformer - Impedance determination

[coylum]

I have an output transformer suitable for distributed loading with EL84's in push pull mode.

The secondary appears to have two separate windings with the potential for 3 different output impedance combinations. Snag is I have no idea what the impedances are - I guess 4, 8 and 16 ohm but which one is which?

I know that I can measure turns ratio by injecting low AC voltage, getting a ratio and then calculating impedance. I understand how to do this for a single ended output where the primary is just two wires with no centre tap or screen grid taps but how do I do it with a push pull transformer? Do I inject the AC voltage across the two anode connections or between anode and centre tap?

 

[Tom Bavis]

End-to-end, full primary. Probably around 8-10K primary. 8K to 16 is 500:1, square root of that is about 22, which will be the voltage ratio. So 120V on the primary will give about 5V on the 16 Ohm tap, 2.5V on the 4V tap, and 8 Ohms will be in between, about 3.5V.

 

[coylum]

End-to-end, full primary. Probably around 8-10K primary. 8K to 16 is 500:1, square root of that is about 22, which will be the voltage ratio. So 120V on the primary will give about 5V on the 16 Ohm tap, 2.5V on the 4V tap, and 8 Ohms will be in between, about 3.5V.

Thanks for that - much appreciated. I will give it a go tomorrow...

 

[coylum]

Thanks for that - much appreciated. I will give it a go tomorrow...

So I gave it a go and got some really odd results which seem to indicate the transformer is faulty - but I would appreciate confirmation/explanation!

The transformer theoretically has a primary impedance of 8K with screen grid taps at 40% (?) and secondary impedances of 16 and 4 ohms ( two separate windings either in series or parallel)

To start off with I applied 5vac across the 16 ohm secondary and this resulted in 62vac anode-anode on the primary - a voltage ratio of 12.4 only about half of the expected figure.

I then checked the voltages across all the primary tappings (with the same 5vac input on the secondary) with the following weird results:

Centre tap to anode 1 25vac
Centre tap to anode 2 38vac
Centre tap to screen grid 1 12vac
Centre tap to screen grid 2 22vac
Anode to anode 61vac

Does this indicate shorted or open primary windings?

I then checked the DC resistance of the primary and could not believe the results!!

Centre tap to anode 1 Open circuit
Centre tap to anode 2 Open circuit
Centre tap to screen grid 1 2.7K Ohms
Centre tap to screen grid 2 70 Ohms
Anode to anode Open circuit

So perhaps this primary is only functioning (??) by virtue of a screen grid coil that may still be intact..

It seems I should have done my testing in reverse order....

I assume I need a new output transformer.....!!!

 

[GordonW]

Are you sure you're on the right scale on your meter? I wonder if "open circuit" is just out of bounds for the ohm range you happened to be on. Is the meter in "auto range"?

That said, the drastic difference between screen grid 1 and screen grid 2, to the center tap (70 ohms vs. 2.7K) raises a red flag for me... those should be very close to the same (within 20% or so).

Regards,
Gordon.

 

[coylum]

Are you sure you're on the right scale on your meter? I wonder if "open circuit" is just out of bounds for the ohm range you happened to be on. Is the meter in "auto range"?

That said, the drastic difference between screen grid 1 and screen grid 2, to the center tap (70 ohms vs. 2.7K) raises a red flag for me... those should be very close to the same (within 20% or so).

Regards,
Gordon.

The meter is on the Auto Range setting and I have tried a second meter just to be sure.... I would have expected the overall primary DC resistance to be in the 100's not 1000's or more ohms

 

[GordonW]

Yes- 70 ohms for a screen-to-CT sounds reasonable. The other numbers are out of the norm, IME.

Regards,
Gordon.

 

[NOSValves]

You should be able to determine the impedance from low to high by simply measuring the resistance from the zero ohm black to each tap you will need to utilize a very good meter. The lowest tap will measure the least resistance and mmost likely be 4 ohms. The defference in each tap will be very small and the overall resistance will usually be about 1 ohm. So were talking 1/10 of an ohm difference between the taps. To be absolutely sure of it being 4, 8, and 16 you will need to go the turns ratio method though....