WEM Control ER15 Output transformer turns ratio question

[marlinster]

Hi experts,

I need to ask about the turns ratio of a push-pull output transformer for my WEM Control ER15 head. This is normally spcified for 15 Ohm speaker, but there is another lug available on the output transformer and if this is for 8 Ohms I would like to use it. But when measuring turns ratio it doesn't make sense.

The transformer has a center tapped primary winding, and three secondary lugs. The way it was set up originally for 15 Ohm is using the most outer lugs, as seen in the picture.

When applying 78V I got 1.5V on the used lug for 15 Ohm, and 1V for (the presumable 8 Ohm) lug. But this means a turns ratio of 1:52 for the 15 Ohm tap(?). What am I doing wrong?

Thanks in advance,

[Chuck H]

Are you testing your voltages going in and out with the unit under test? Often the input voltage will be loaded down by the transformer so you need to hook up the test and THEN test the voltage going in to see if that 78V is being loaded down. Conversely, try the test in reverse by putting a low voltage into the secondary and measuring at the primary to see if the results change. Again, test the input voltage with the unit under testing conditions. Source impedance for the test signal will almost always allow the test signal to be loaded down some.

[Helmholtz]

Voltages transform according to Nprim/Nsec, but impedances transform as the square of the turns/voltages. In other words the voltage ratio between 15 Ohm and 8 Ohm is sqrt(15/8)=1.4. So your measured voltage ratio between the secondaries is in the ballpark.

But this means a turns ratio of 1:52 for the 15 Ohm tap.

Yes, this would result in a primary impedance of 52²*15 Ohm=40k, which seems way too high. EL84s in PP typically use 8k. This would mean a turns ratio of 1:23 for the 15 Ohm tap.

Please check you meter (batteries) and the measuring setup. It is probably better to feed the secondary with a couple of volts and measure the primary voltage as Chuck suggests.

[marlinster]

Are you testing your voltages going in and out with the unit under test? Often the input voltage will be loaded down by the transformer so you need to hook up the test and THEN test the voltage going in to see if that 78V is being loaded down. Conversely, try the test in reverse by putting a low voltage into the secondary and measuring at the primary to see if the results change. Again, test the input voltage with the unit under testing conditions. Source impedance for the test signal will almost always allow the test signal to be loaded down some.

Hi, I tested it unhooked. I just tested it again by putting voltage on the secondary side and get the exact same result.

[marlinster]

Voltages transform according to Nprim/Nsec, but impedances transform as the square of the turns/voltages. In other words the voltage ratio between 15 Ohm and 8 Ohm is sqrt(15/8)=1.4. So your measured values are about right and confirm the 8 Ohm tap.

Ok many thanks for the info, but what am I missing in the math here? With turns ratio 1:78 for the 8Ohm tap the impedance load will be very high(?) for both taps. It's an EL84 push pull amp, 8k load.

[Chuck H]

Hmm.?. The test I know is to determine the turns ratio, square it and then multiply by the secondary load to figure impedance. So, with a turns ratio of 52:1 you have 52 X 52 = 2704 then 2704 X 16 = 43,264. Which seems high, right?

Why would the two secondaries be amalgamated for figuring impedance reflected on the output tubes operating from a single secondary tap?

[marlinster]

Hmm.?. The test I know is to determine the turns ratio, square it and then multiply by the secondary load to figure impedance. So, with a turns ratio of 52:1 you have 52 X 52 = 2704 then 2704 X 16 = 43,264. Which seems high, right?

Yes, shouldn't it be around 8000?

And for 8 ohm, 78x78=6084, 6084x8= 48672

[Chuck H]

Hi, I tested it unhooked. I just tested it again by putting voltage on the secondary side and get the exact same result.

You need to test your input voltage with the transformer already hooked up to that voltage because the transformer may be loading it down.

[marlinster]

You need to test your input voltage with the transformer already hooked up to that voltage because the transformer may be loading it down.

Maybe it's a misunderstanding, English is not my native language Just to clarify, all leads to the amp was unsoldered and the transformer is totally unhooked. Then I applied the AC voltage from a separate transformer connected to a variac and hooked up two multimeters to check input voltage and output voltage on secondary and primary.

[Chuck H]

Maybe it's a misunderstanding, English is not my native language Just to clarify, all leads to the amp was unsoldered and the transformer is totally unhooked. Then I applied the AC voltage from a separate transformer connected to a variac and hooked up two multimeters to check input voltage and output voltage on secondary and primary.

Then you have done the test correctly. It would seem to be that the transformer has a secondary short somewhere before the taps. Which seems odd since secondary shorts are rare anyway and your taps are correctly related. So I think we're overlooking a different possibility.

[Chuck H]

Is it at all possible that the OT secondary was connected incorrectly and you may be testing from the wrong end of the secondary to the taps?

[Helmholtz]

Hi, I tested it unhooked. I just tested it again by putting voltage on the secondary side and get the exact same result.

Do both meters give the same results when interchanged?

(I edited my first post above.)

Have you tried to measure output power?

[marlinster]

Do both meters give the same results when interchanged?

(I edited my first post above.)

Have you tried to measure output power?

I have changed batteries in both DMM's, no change.

I measured the resistance for the transformer. Primary: 96 Ohm and 83 Ohm from centertap to end. Ends are 179 Ohm. Secondary: 0.8 Ohm to unknown(8?) lug and 0.4 Ohm to 15 Ohm. Nothing to chassis.

No I haven't measured power....

Is it at all possible that the OT secondary was connected incorrectly and you may be testing from the wrong end of the secondary to the taps?

From what I have seen when looking at gut pics from other ER15's they all seem to be wired in this way. Never seen one using the middle lug though. But I have "heard" that this is the 8Ohm tap, but I want to make sure.

[Helmholtz]

Now if the measurements are correct, the only explanation I can think of would be shorted turns at the secondary as Chuck mentioned.
This will show in low output power and/or overheating with a primary test voltage of say 100V.
You may want to do the neon bulb transformer test.
https://www.premierguitar.com/articl...ormer-tester-1

[Reader]

I have changed batteries in both DMM's, no change.

I measured the resistance for the transformer. Primary: 96 Ohm and 83 Ohm from centertap to end. Ends are 179 Ohm. Secondary: 0.8 Ohm to unknown(8?) lug and 0.4 Ohm to 15 Ohm. Nothing to chassis.

No I haven't measured power....



From what I have seen when looking at gut pics from other ER15's they all seem to be wired in this way. Never seen one using the middle lug though. But I have "heard" that this is the 8Ohm tap, but I want to make sure.

That "Secondary: 0.8 Ohm to unknown(8?) lug and 0.4 Ohm to 15 Ohm" is suspicious. It suggests that the unknown lug has more secondary turns than the 15 Ohm lug and that therefore it is not an 8 Ohm lug. (Assuming that the 15 ohm lug is really 15 ohms)