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Thread: WEM Control ER15 Output transformer turns ratio question

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    WEM Control ER15 Output transformer turns ratio question

    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,

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    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.

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    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 52*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.

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    Last edited by Helmholtz; 11-01-2018 at 03:21 PM.
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    Quote Originally Posted by Chuck H View Post
    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.

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    Quote Originally Posted by Helmholtz View Post
    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.

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    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?

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    Quote Originally Posted by Chuck H View Post
    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

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    Quote Originally Posted by marlinster View Post
    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.

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    Quote Originally Posted by Chuck H View Post
    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.

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    Quote Originally Posted by marlinster View Post
    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.

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    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?

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    Quote Originally Posted by marlinster View Post
    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?

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    Last edited by Helmholtz; 11-01-2018 at 03:43 PM.
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    Quote Originally Posted by Helmholtz View Post
    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.

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    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

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    Quote Originally Posted by marlinster View Post
    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)

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    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)
    Yes, I measured it again and the readings are correct.

    Quote Originally Posted by Helmholtz View Post
    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
    Ok so I tested that super secret method, thanks for the link. The neon bulb flashes if I connect the bulb to the primary side and battery to primary. But not the other way around...

    I tried this method on a output transformer I know is working and the same result, working when bulb connected to the secondary but not the other way around...? Is this correct? I tested with a 9V battery.

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    Yes, I measured it again and the readings are correct.
    What is the DCR between the "8" Ohm lug and the 15 Ohm lug?

    The strange thing is that the voltage at the 15 Ohm lug was higher but the DCR to the ground lug is lower than at the "8" Ohm lug.

    Here is a modified neon tester that sometimes gives more positive results (neons have some variation regarding breakdown voltages):
    https://www.vintage-radio.net/forum/...ad.php?t=14317

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    Last edited by Helmholtz; 11-01-2018 at 05:36 PM.
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    The resistance test of the secondary should be close to this.
    4 ohm tap,.5 ohm.

    8 ohm tap, .71 ohm

    16 ohm tap, 1 ohm.

    Please note that to get the correct readings for these low ohm test you can not hold both metal tips of your meter other wise the meter will read thru you.

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    Quote Originally Posted by Helmholtz View Post
    What is the DCR between the "8" Ohm lug and the 15 Ohm lug?

    The strange thing is that the voltage at the 15 Ohm lug was higher but the DCR to the ground lug is lower than at the "8" Ohm lug.
    Between the 16 Ohm lug and the "8 Ohm" lug it measures 0.6 Ohm.

    When I search for pictures of other WEM's I see that sometimes the ground lead seem to go to the in my case 16 Ohm lug, and sometimes all three lugs are used. This is very strange.

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    Last edited by marlinster; 11-01-2018 at 05:56 PM.

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    Please note that to get the correct readings for these low ohm test you can not hold both metal tips of your meter other wise the meter will read thru you.
    No problem at low Ohms. The body resistance is in the 100k range and thus won't change the reading.

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    Quote Originally Posted by Helmholtz View Post
    What is the DCR between the "8" Ohm lug and the 15 Ohm lug?

    The strange thing is that the voltage at the 15 Ohm lug was higher but the DCR to the ground lug is lower than at the "8" Ohm lug.

    Here is a modified neon tester that sometimes gives more positive results (neons have some variation regarding breakdown voltages):
    https://www.vintage-radio.net/forum/...ad.php?t=14317
    Ok, I tested with that Improved method, 47k resistor and a 9V battery.

    When applying this to the primary winding the neon flashes.

    When applying to the secondary the neon does not flash.

    I tested this on another otput transformer and it flashes for both primary and secondary.

    So maybe there is a short...?

    Update - It flashes for both windings! Only not so bright for the secondary winding. I had to shut down some lights

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    Last edited by marlinster; 11-01-2018 at 07:34 PM. Reason: Update

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    Quote Originally Posted by marlinster View Post
    Ok, I tested with that Improved method, 47k resistor and a 9V battery.

    When applying this to the primary winding the neon flashes.

    When applying to the secondary the neon does not flash.

    I tested this on another otput transformer and it flashes for both primary and secondary.

    So maybe there is a short...?
    Update - It flashes for both windings! Only not so bright for the secondary winding. I had to shut down some lights

    So then perhaps it is not short? But what can it then be??

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    Last edited by marlinster; 11-01-2018 at 07:34 PM. Reason: Update

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    There can be different "degrees" of short (number of turns shorted, contact resistance) in an OT. All of them mean that it should be replaced (or in rare cases repaired).

    But if it delivers rated output power in the amp (and your voltage and DCR measurements were wrong) and shows no signs of overheating, it still may be O.K.

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    Quote Originally Posted by Helmholtz View Post
    There can be different "degrees" of short (number of turns shorted, contact resistance) in an OT. All of them mean that it should be replaced (or in rare cases repaired).

    But if it delivers rated output power in the amp (and your voltage and DCR measurements were wrong) and shows no signs of overheating, it still may be O.K.
    True..

    Does anyone know a transformer rewinding service in Germany or England?

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    Last edited by marlinster; 11-01-2018 at 11:34 PM.

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    If the amp is working ok there's no need to do anything; any of the faults mentioned would surely completely hobble the amp's output?
    No reason to fix something that isn't broke.
    Re-winding will inevitably be more expensive than replacing it.

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    Quote Originally Posted by pdf64 View Post
    If the amp is working ok there's no need to do anything; any of the faults mentioned would surely completely hobble the amp's output?
    No reason to fix something that isn't broke.
    Re-winding will inevitably be more expensive than replacing it.
    I did some more testing today and actually the output is not really what I would expect. Also the resistors by the OT had been replaced and I suspect it's getting hot. I will rewind it, I wanna keep it in original state as much as possible. Thanks

    Many thanks to everyone with all the tips, very helpful.

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    The output power of tube amps is usually less than expected, eg 10 watts from a '15' watt amp,
    If the OT was bad, eg shorted turn, it would be crippled, a tiny fraction of the intended power, as the primary inductance would collapse.
    The stock snubber resistors were 1/2 watt and they overheat when the amp gets overdriven, so by now they will look burnt or be replacements.

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    THIS^^^^^^^^^^^^^^^^^^^^^^^

    I have a 2xel84 design that uses a snubber across the primary. Though not two snubbers each to the CT like this design. My design is snubbing at a much higher frequency too. Anyway... I took great pains selecting the components for the snubber circuit to avoid the problem Pete is talking about. No such care has been taken in this amp. So I would expect the snubber parts to fail occasionally.

    Also in keeping with Pete's post... I have two of these amps that are schematically identical. Both are working perfectly using the same PT, the same circuit and the same tubes. One puts out 13W and the other puts out 17W. The only difference between the two is the layout and the output transformers. The output transformers are both 8k primary. One amp has an off the shelf Hammond (13W) and the other is using a proprietary Heyboer model (17W). I have a hard time reconciling that the output transformer construction could be the cause for the wattage difference, but I haven't swapped the OT's to eliminate that possibility. More importantly, both amps sound pretty much the same and I don't recognize a significant difference in volume.

    Maybe there'll be an "AHA!" moment some day on the difference in wattage. It's just not a problem such as to prioritize it right now. So... How many watts is YOUR amp making? Your opinion of "output is not really what I would expect" isn't something we can sink our teeth into.

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    You've used each meter on each test point (swapped meters) and get the same voltage?

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