Page 1 of 2 12 LastLast
Results 1 to 35 of 41

Thread: Testing Power Transformers

  1. #1
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12

    Testing Power Transformers

    Hello. I wonder how can I perform a test for a power transformer performance ,please ? Asking because I found one which is not power rated but just nominal secondary currents. The transformer is sell as a Marshall replacement and it have same physical size but claimed to have 600mA instead original 300mA for nominal voltage.(356V C.T./600mA). I really wanna know if I can use it at rated current for an kt88 project. It is a short test over 600 ohm load just enough, please ? Thanks

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-03-2019 at 10:38 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  2. #2
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,572
    Thumbs Up/Down
    Received: 1,732/18
    Given: 1,333/29
    Rep Power
    26
    IF it has same physical size, same voltages, but claims twice the current it simply is not true.

    SS electronics advances day by day, tubes are basically same as 50/60 years ago, average silicon steel same as 120 years ago, no new revolutionary advances allowing the doubling of performance for same weight of steel.

    Even less performance change for same weight of copper.

    Design equations, graphs and tables from 1947 Radiotronīs Designer are perfectly valid today.

    If anything, itīs hard to get steel as good as old Armscor and similar, not because "we know less", far from that, but because we donīt want to pay for it, go figure.

    Consider that same size same weight iron and copper standard Marshall transformer as a same specs clone, not a miraculously doubled performance improvement.

    Now if it were an SMPS, yes, modern Ferrites can reach even a MHZ, some even more, garden variety types are comfortable with a couple hundred kHz, while 1947 flyback cores "just" could be used at 17kHz so there a modern core can easilyprovide 10X the old performance, but .... on laminated silicon steel? ... forget it.

    1 Not allowed! Not allowed!
    Juan Manuel Fahey

  3. #3
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Thanks JMF. The transformer is on the way. I will full load the secondaries with resistors loads at nominal current they claimed leaving for couple of hours under load and post of full review. I have no problem to return a toasted transformer if will be the case.

    0 Not allowed! Not allowed!
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  4. #4
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,762
    Thumbs Up/Down
    Received: 1,329/1
    Given: 780/2
    Rep Power
    4
    Quote Originally Posted by catalin gramada View Post
    Thanks JMF. The transformer is on the way. I will full load the secondaries with resistors loads at nominal current they claimed leaving for couple of hours under load and post of full review. I have no problem to return a toasted transformer if will be the case.
    Your load resistors will have to absorb/dissipate around 200W in total at nominal current.

    1 Not allowed! Not allowed!
    - Own Opinions Only -

  5. #5
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Quote Originally Posted by Helmholtz View Post
    Your load resistors will have to absorb/dissipate around 200W in total at nominal current.
    This is true. Six pieces of 100 ohm/50w will do the job.

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-03-2019 at 02:47 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  6. #6
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,762
    Thumbs Up/Down
    Received: 1,329/1
    Given: 780/2
    Rep Power
    4
    Quote Originally Posted by catalin gramada View Post
    This is true. Six pieces of 100 ohm/50w will do the job.
    If used within their power derating temperature limits. This might mean mounting on a large heatsink for some resistor types (e.g. the golden aluminum encased ones).

    1 Not allowed! Not allowed!
    - Own Opinions Only -

  7. #7
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    This is true also.For any power resistor the sheets shows nominal power with and without heat sink relative to different ambient conditions.The aluminium encased resistors are made to use mounted on a certain surface heat sink, exactly like power transistors need to be mounted to a minimum surface heat sink for certain heat dissipation.

    0 Not allowed! Not allowed!
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  8. #8
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,762
    Thumbs Up/Down
    Received: 1,329/1
    Given: 780/2
    Rep Power
    4
    Quote Originally Posted by catalin gramada View Post
    This is true also.For any power resistor the sheets shows nominal power with and without heat sink relative to different ambient conditions.The aluminium encased resistors are made to use mounted on a certain surface heat sink, exactly like power transistors need to be mounted to a minimum surface heat sink for certain heat dissipation.
    I see you're well prepared.

    0 Not allowed! Not allowed!
    - Own Opinions Only -

  9. #9
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Does it exist a graph sheet/calculator regards maximum power transfer for a certain core section relative to "standard" lamination in respect of VA capabilities ? Thinking if can use thicker wires you can have an idea by core section if the iron is able to...

    0 Not allowed! Not allowed!
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  10. #10
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,762
    Thumbs Up/Down
    Received: 1,329/1
    Given: 780/2
    Rep Power
    4
    Thinking if can use thicker wires you can have an idea by core section if the iron is able to..
    A well designed transformer uses the complete coilformer space for the windings. As turns numbers are dictated by the application, increasing wire cross section is typically no option.

    2 Not allowed! Not allowed!
    - Own Opinions Only -

  11. #11
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Quote Originally Posted by Helmholtz View Post
    A well designed transformer uses the complete coilformer space for the windings. As turns numbers are dictated by the application, increasing wire cross section is typically no option.
    This exclude the possibility to get more power even you add lamination to the core and will be necessary to do a step up by lamination size ?

    0 Not allowed! Not allowed!
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  12. #12
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    I did a sort of empirical research and found for this standard lamination - EI 114 there are produced standard plastic bobbins. From my point of interest found 50mm deep for stacked EI 114 for power transformers between 200-260 VA and 60mm deep for PT between 250-360 VA. This should be a general indicator if the iron have enough power transfer capabilities in respect with secondary currents states...I think.

    1 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-03-2019 at 07:35 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  13. #13
    Lifetime Member
    Join Date
    May 2006
    Posts
    4,425
    Thumbs Up/Down
    Received: 401/1
    Given: 3/0
    Rep Power
    21
    The limit on a transformer's power throughput is difficult to actually pin down. It's determined by the combination of the surface area exposed to the outside air and the temperature withstanding of the interior insulating layers and wire insulation. The copper is good up to its melting temp, and the iron functions fine up to the iron Curie temperature at about... um, 770C? ... both higher than you'd ever get. What fails is the insulation between windings, and that's why hte idea of insulation classes was born.

    Unrated materials are assumed to die below 105C, the first step in the insulation temperature classes. No respectable maker (that is a significant limitation, by the way) would use anything less than Class 105. It goes up from there; see what-is-meant-by-class-in-insulation. The higher temperature you can let your insulation run at, the more power you can get out of the transformer before the insulation melts and makes 10,000F arcs inside to melt the copper.

    So a transformer's power rating is murky. There is no one good number without careful inspection. The best way to find out whether the transformer can power X watts is to (1) measure the resistance of the copper wires in the windings accurately, (2) load it with X watts and re-measure the copper resistance every so often and plot the temperature rise curve for the copper, (3) stop when you can project from the temperature rise curves that you've either found the top temperature or that you're going to exceed the temperature rating of the wires and internal insulation.

    Copper wire makes a great temperature sensor, as its resistance changes by +0.393 percent per degree C. So for example, if the 1.300 ohm wire in a hypothetical winding rises by 10% to 1.430 ohms, the 10% rise is 25.44 times 0.393, or a rise of 25.44C if I did the math right. If you started at 25C outside the transformer with the transformer absolutely at 25C through and through, the copper wire has heated up to 50.44C. So even Class A/105 is fine.

    There are some gotchas. You MUST accurately measure the copper wire resistance to be able to tell the rise, and that often means using a low-ohms meter, with four-lead measurement. A cheap DMM may not be up to the task for heater winding. You must also do a good job of watching the temp rise long enough to accurately predict the ending of the (typically) declining exponential temp rise curve AND the thermal time constant of the transformer. A several hundred watt power transformer may have a thermal time constant of an hour or more, and the temperature won't reach its final value for several hours. That's why you want to use the rising temp curve to guess how long you have to watch to get good numbers.

    Multiple secondary windings are another gotcha that isn't all that easy to handle. Obviously, if temperature defines the current limit, and you load only one winding, then the heat from other windings not being used isn't heating the inside of the transformer, and it will both take longer for the temperature to settle and be able to withstand much more current because it's the only heat source in there, not one of many. So you need to make a best guess about loading for your testing, and use those loads to get the overall temperature rise. You can test them one at a time, but the resulting data will be substantially worthless. They all have to be loaded to some semblance of the load they will actually be running so you get moderately worthwhile results.

    Murky.

    8 Not allowed! Not allowed!
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

  14. #14
    Supporting Member loudthud's Avatar
    Join Date
    May 2006
    Location
    Near Dallas Texas
    Posts
    3,513
    Thumbs Up/Down
    Received: 186/0
    Given: 104/1
    Rep Power
    18
    There is a way one vendor could rate a transformer at 300mA and another could rate the same transformer at 600mA. It has various names, Marketing, Specmanship, lying, fraud, you name it. The specs quoted are most commonly incomplete. There are too many unknowns. If they are talking about DC current after a rectifier and cap input filter, the current in the secondary could change by a factor of two to one simply by changing the capacitance. Do they give you this information ? Not a chance. Is a Full Wave Center Tap or Full Wave Bridge (like the one being talked about here) configuration being used ? They usually don't say. That could be another two to one difference in the spec.

    Transformers marketed to people building guitar amps are the biggest offenders. You never really know what you are getting. Your best guess (for tube amps) is to get something with a VA rating two to three times the power output in Watts.

    4 Not allowed! Not allowed!
    WARNING! Musical Instrument amplifiers contain lethal voltages and can retain them even when unplugged. Refer service to qualified personnel.
    REMEMBER: Everybody knows that smokin' ain't allowed in school !

  15. #15
    Senior Member trobbins's Avatar
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    704
    Thumbs Up/Down
    Received: 71/0
    Given: 0/0
    Rep Power
    10
    The manufacturer also has the choice of wire enamel thickness to use - typically identified by winding wire being grade 1 or 2. Thicker enamel coating means less turns per layer, so possibly a higher winding resistance.

    You also need to factor in your intended amp ambient temperature situation, and if it may typically operate at a significantly higher temperature due to restricted free-air ventilation, or heating from nearby hotter valves, or being in a location where ambient temp is noticeably higher than 20-30C (such as in a rack, or in a hot climate).

    If you go down the temperature test path, then I'd recommend testing for as long as possible (hopefully to when winding resistance stabilises) - its amazing how long the upwards creep in temp can continue, and is helped by the positive temp coef of the wire.

    Note that resistive loading of the transformer is more benign as far as winding loss than if you loaded the main secondary with rectified and filtered circuitry and then in to a loading resistor.

    1 Not allowed! Not allowed!

  16. #16
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Thanks for You answers gents. It is not the case to blame the product as I didn't tested yet. But simply this transformer is suspect by its size in respect with similars. I ordered it by curiosity (due to its claimed specs) to serve in a DIY project. But I want to perform a test in respect with most tech agree. I can put it with all secondaries on resistive batteries bench (have a full shelf of power resistors laying around), I can do precise voltage measuring and temperature aso. It is a regular Marshall 100 size, the specs are:
    Pri: 120V, 230V, 240V @ 50Hz
    Sec1: 356 VCT @ 600mA
    Sec2: 6,3 VCT @ 7,5A
    Sec3: 98 V @ 150mA
    Sec4: 12,6 V @ 1A
    Should be able almost 300va by its nominal rating...will see
    Any ideas to perform a objective proper test fully appreciated. Should be very interesting if I can collect enough data to have a picture of its characteristics. It will be very instructive aso (for me at least). Thank You.


    ...and No, it is not a MM btw....

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-04-2019 at 08:30 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  17. #17
    Lifetime Member
    Join Date
    May 2006
    Posts
    4,425
    Thumbs Up/Down
    Received: 401/1
    Given: 3/0
    Rep Power
    21
    If you're interested in pursuing this further in pursuit of the knowledge here's how I would set up the test.

    Arrange a place where it is safe for the test to run to completion. This is an issue because a full test will run many hours, even a day or two. Trobbins is correct - it is amazing how long a proper test to stable temperature will take. The thermal time constant for that much mass of iron and copper is likely hours, and a simple exponential rise is assumed to be close enough at five time constants. This is not a simple exponential rise, given that the real underlying heat transfer is not a simple exponential rise, but better described by a complex differential equation involving the fluid thermal dynamics of air moving past the outside of the tranformer to carry heat away and whatever amount of radiation can happen. It's going to run for a long time. That's why I mentioned approximating the end temp by continuously mapping the temperature rise curve so you can make a decent estimate of the final temperature and call off the test when you get a "good enough" answer.

    Since it will run a long time, and have exposed high voltages, you need to exclude other people and pets from the test area so they don't get electrocuted. Also, unattended high power tests always involve some danger of fire, so worry about that a little. I've worked with AC mains power long enough to be paranoid about leaving anything plugged in and running when I'm not there to deal with a smoke-and-flames situation.

    Second, think about how you will measure the resistance. The typical ohmmeter isn't good enough for the low voltage windings. These things need a four-wire milli-ohmmeter to get usable accuracy. You might be OK with a primary or high voltage winding that gets up into the 100's of ohms, but for a complete test you want to measure them all. The windings will generally not rise at the same rate or have the same final ending temperature, as power transformers do not try to distribute the windings together like OTs do. They're simply layer wound in chunks, and so the have different thermal resistances to the outside. One will be hotter than the rest, and there is generally a hot spot, usually the innermost winding, that runs 10C to 15C hotter than the outer winding. So you need to measure them all. You don't need to worry about polluting the test by the time you spend un-powering the transformer, disconnecting the leads, measuring all the resistances, and reconnecting. If this takes a few minutes, so be it. The very long thermal time constant works for you because it is just as slow for the thing to cool down as it is for it to rise. The time spent measuring is negligible compared to the time of the whole test. This may not be true for small transformers, though.

    Third, yes, as Trobbins mentions, you need to get the RMS current up to what it would be in an actual use. Rectification does involve RMS currents in the windings that are larger than the DC out of the rectifier/filter combination. For heaters, this can be ignored (mostly) because the heater load is usually resistive AC powered heater windings. For windings that drive rectifier/filters, you need to compensate by taking into account the DC loading AND the RMS transformer current to get that much DC.

    To find out how much transformer RMS current results from the DC output current, you need to do some math. Rectifier circuits with capacitor filters make the heating (RMS) current seen by the transformer winding be different from the DC current pulled out of the filter capacitor. This is worst for the full wave bridge rectifier set-up, not too bad for the full-wave center tap setup. You can either replicate the rectifier/filter/load you would see in an amp, or compute the transformer RMS current for a given DC load after rectifier/filter and use a resistor load to give the transformer AC the same loading as it would see with the rectifier/filter load. Both work. RMS is RMS for heating.

    3 Not allowed! Not allowed!
    Amazing!! Who would ever have guessed that someone who villified the evil rich people would begin happily accepting their millions in speaking fees!

    Oh, wait! That sounds familiar, somehow.

  18. #18
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,572
    Thumbs Up/Down
    Received: 1,732/18
    Given: 1,333/29
    Rep Power
    26
    FWIW , now knowing your core size, 114x95 mm EI lamination, stacked 50mm, "absolute maximum 100% perfect" VA capability is 361 VA ; Real World "good quality" commercial stuff (such as Drake, Dagnall, Hammond and equivalent) about 80% of that, so some 290VA .

    I wind transformers all day long using that lamination, buy it in 50kg and 100kg lots; we call it N° 155 following its US denomination (roughly 1.5" center leg width)

    It is fine for a standard 4 bottle 100W Tube amplifier ... or a 300W RMS SS amplifier.

    Not sure about your KT88 project.
    If two of them, it should easily cope; if four .... very unlikely.

    Sec 1: 356 VCT @ 600mA 213VA <-- this sounds about right for an unefficient 100W Tube amp , no way you can pull 150 or 200W RMS out of it; I wouldnīt expect more than 50% efficiency, given that SSamps are about 75% efficient ... and donīt have to waste power in heaters, screens or plain saturation voltage (about 60V) or even pure waste in high idle current "just to sound better"

    Sec 2: 6,3 VCT @ 7,5A <-- 48VA IF it adds up to your hungry filament needs, no complaints. But ... does it?

    Sec3: 98 V @ 150mA <-- 15VA No big deal.

    Sec 4: 12,6 V @ 1A <-- 12VA Same.

    Losses are more into copper DCR than actual magnetic ones, the coil under constant load heats up more (and faster) than core itself.

    As RG said, coil temperature can be accurately measured by measuring copper resistance increase ... which is huge.

    As of load testing, resistors are not a real world load ... since you amp power supply is not a resistor by far.

    IF you want realistic load testing, load filament winding with resistors to full rated current with resistors, that oneīs easy, BUT for the HV winding, ONLY realistic load is to connect it to a diode bridge, proper capacitors, and load resulting DC with resistors pulling same current as amplifier at full blast.

    Only other way is to actually build the amplifier and burn it in. Not kidding.

    2 Not allowed! Not allowed!
    Juan Manuel Fahey

  19. #19
    Senior Member nevetslab's Avatar
    Join Date
    Apr 2010
    Location
    Burbank, CA
    Posts
    1,689
    Thumbs Up/Down
    Received: 584/1
    Given: 1,376/1
    Rep Power
    11
    Quote Originally Posted by J M Fahey View Post
    IF you want realistic load testing, load filament winding with resistors to full rated current with resistors, that oneīs easy, BUT for the HV winding, ONLY realistic load is to connect it to a diode bridge, proper capacitors, and load resulting DC with resistors pulling same current as amplifier at full blast.

    Only other way is to actually build the amplifier and burn it in. Not kidding.
    Or, in lieu of the planned amplifier, substitute that with a similar amplifier that would be running the same planned power level. I'd set up the connections of your Transformer-Under-Test with quick Disconnects, or even a well-isolated terminal strip so you can get the Kelvin 4-wire test clips onto the multiple pairs of windings. Obviously with AC mains disconnected. That's how I had characterized prototype power transformers for new products in the engineering lab. I also had the prototype xfmrs wound with K-type thermocouples to add to the temperature rise trend, but relied on the change of resistance of the copper in all primary/secondary windings. Also tracked the ambient temperature in the localized test zone, as it changes over time from the radiated heat, and is in the equation.

    2 Not allowed! Not allowed!
    Logic is an organized way of going wrong with confidence

  20. #20
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Finally my PT was coming. German quality made. EI 114 / 50mm stack. At first sight it looks acceptable built, but two remarks: the lamination looks thick, actually twice thicker than regular Hammond use,for instance, and the end bells are huge, there are more than twice deep than is necessary, which could be a problem for room accomodation on some chassis. Pictures below:

    Click image for larger version. 

Name:	20190404_191614.jpg 
Views:	16 
Size:	1.58 MB 
ID:	53222
    Click image for larger version. 

Name:	20190404_191808.jpg 
Views:	27 
Size:	1.64 MB 
ID:	53223
    Click image for larger version. 

Name:	20190404_191851.jpg 
Views:	26 
Size:	1.93 MB 
ID:	53224

    And my power jig: 610 ohm/240w and 0.9ohm/60w

    Click image for larger version. 

Name:	20190404_192252.jpg 
Views:	21 
Size:	2.61 MB 
ID:	53225
    Click image for larger version. 

Name:	20190404_192306.jpg 
Views:	20 
Size:	2.59 MB 
ID:	53226

    I didn't found my temp probe for my Fluke so have to improvise something else

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-04-2019 at 06:15 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  21. #21
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,762
    Thumbs Up/Down
    Received: 1,329/1
    Given: 780/2
    Rep Power
    4
    From the pictures it looks as if the core is stacked with alternating lamination pairs. Nothing wrong with that. What is the brand/ manufacturer?

    0 Not allowed! Not allowed!
    - Own Opinions Only -

  22. #22
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Quote Originally Posted by Helmholtz View Post
    From the pictures it looks as if the core is stacked with alternating lamination pairs. Nothing wrong with that. What is the brand/ manufacturer?
    Hi, It is a Tube Town transformer and think is one they made it "in house". It did not looks like Chinese rebranded one but more like craft one instead. Just judging by wires/leads they used stuff for professional electrotechnic application...just an example, It have a touch of professional electrotechnic workshop, if you know what it means...So I believe is a well built transformer judging by how it looks like. ...I have a problem with its specifications instead...

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-04-2019 at 08:30 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  23. #23
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,762
    Thumbs Up/Down
    Received: 1,329/1
    Given: 780/2
    Rep Power
    4
    The wire resistance method gives an equivalent average wire temperature along its length. For comparing to the (to be known) insulation class temperature limit, the internal hot-spot temperature needs to be known. Is there an established method to derive/estimate the hot spot temperature from the average for a given transformer size?

    I remember that we had to provide the certification lab with transformer samples having embedded thermo couples to get the safety approval.

    0 Not allowed! Not allowed!
    - Own Opinions Only -

  24. #24
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,762
    Thumbs Up/Down
    Received: 1,329/1
    Given: 780/2
    Rep Power
    4
    Quote Originally Posted by catalin gramada View Post
    Hi, It is a Tube Town transformer and think is one they made it "in house". It did not looks like Chinese rebranded one but more like craft one instead. Just judging by wires/leads they used stuff for professional electrotechnic application...just an example, It have a touch of professional electrotechnic workshop, if you know what it means...So I believe is a well built transformer judging by how it looks like. ...I have a problem with its specifications instead...
    The Tube Town specifications add up to 288.15W secondary power. I think that's feasible, especially with grain-oriented (as advertized) laminations and over-sized coil former. It is clearly advertized as a replacement for a 100W Marshall - not for a 200W 4xKT88 Major.

    1 Not allowed! Not allowed!
    - Own Opinions Only -

  25. #25
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Quote Originally Posted by Helmholtz View Post
    The Tube Town specifications add up to 288.15W secondary power. I think that's feasible, especially with grain-oriented (as advertized) laminations and over-sized coil former. It is clearly advertized as a replacement for a 100W Marshall - not for a 200W 4xKT88 Major.
    Clearly understood , less than didn't bought this PT as an replacement item but for its electrical specifications which should be in conformity with what is state. If it is proper suited for one or another application this is another story. In my understanding it should work on its rated specs, except the fact of overloading somehow.
    I made some custom powertransformers in a past at one workshop in my town. I don't remember to be asked from one technicians what I need it for? I specified my volts/currents demands an they dimensioned and made it by request. It is a first time I heard about this nuances like ..ok it is good enough if you use it like this or that....It is a simple mains transformer , for Christ, should no matter if I supplied a bulb , a power resistor or a power amp. As time I don't exceed its specs should be able to perform as same in terms of V/A
    If it will not work into a Marshall Major 200w will be because its voltages and current ratings didn't recommend it for those application and not because this transformer it was recommended by seller as an 100w Marshall replacement....just my 2c

    1 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-04-2019 at 09:48 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  26. #26
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Allright, I have some preliminary measurements. With 230V on the mains and 7A draw from heaters winding I got 366V no load/ 6.12V heaters, and with 610ohm load for 580mA get 352,5 V but heaters drop to 6 V. This is a damn good regulation, less than 5 per cent.Pretty unhappy by heather voltage but think I can fix it if have room to wind one or two turns over insulation. Further tests will follow. Thank you guys!

    I will let it toast for a while but still not have a proper temp probe to determine the moment when comes in balance with ambient so it have no sense to let it toast for hours without to be able to have a conclusion

    ...and to not forgot: It is absolutely quiet. I cannot heard it at all

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-04-2019 at 11:34 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  27. #27
    Senior Member trobbins's Avatar
    Join Date
    Jun 2010
    Location
    Australia
    Posts
    704
    Thumbs Up/Down
    Received: 71/0
    Given: 0/0
    Rep Power
    10
    Yeh I'd imagine a type test may somehow include embedded sensors, or a confident margin is used based on previous type tests of similar parts.

    With respect to 'full spec load' testing, you still end up having to assume (or make an educated guess) the designer's insulation system rating, and also rationalising your own operating margins, such as for environment ambient temp, and the 'average' music power you could be operating at given that would be more than idle power but less than continuous sinewave testing for x hours.

    If you don't want to include a rectifier and filter in to your test jig, you can use PSUD2 to estimate the rms winding current for a given DC output loading, based on your rectifier and filter part choice.

    2 Not allowed! Not allowed!

  28. #28
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Quote Originally Posted by trobbins View Post
    Yeh I'd imagine a type test may somehow include embedded sensors, or a confident margin is used based on previous type tests of similar parts.

    With respect to 'full spec load' testing, you still end up having to assume (or make an educated guess) the designer's insulation system rating, and also rationalising your own operating margins, such as for environment ambient temp, and the 'average' music power you could be operating at given that would be more than idle power but less than continuous sinewave testing for x hours.

    If you don't want to include a rectifier and filter in to your test jig, you can use PSUD2 to estimate the rms winding current for a given DC output loading, based on your rectifier and filter part choice.
    Thanks for You advices. I will but have not conditions for the moment. Further tests will follow.
    Btw. It is from almost one hour under loads and core temp raised insignificant, max 5 Celsius degree. It is same cold. But this is something relative.I still not have a temperature probe to perform a test.
    Till this point I.m happy with what I get.

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-05-2019 at 12:15 AM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  29. #29
    Old Timer J M Fahey's Avatar
    Join Date
    Oct 2007
    Location
    Buenos Aires, Argentina
    Posts
    11,572
    Thumbs Up/Down
    Received: 1,732/18
    Given: 1,333/29
    Rep Power
    26
    Cool results so far, but please remember:

    It is a simple mains transformer , for Christ, should no matter if I supplied a bulb , a power resistor or a power amp.
    is absolutely not realistic

    Filaments and resistors pull sinusoidal current from a sinusoidal voltage, and thatīs what you are measuring so far, but diodes feeding a *loaded* capacitor pass narrow peaks of very high current , absolutely different from the first case.

    RMS Loss/dissipation/Temperature rise (all related to each other) depend on the square of current.

    Consider a simple 50% duty ratio compared to continuous/full cycle load (resistive).

    Not mentioning any particular resistor, since value is same in all examples:

    * 1 A during 1 second (resistive load) means "X" power

    * 2A during 0.5 second means same average but twice RMS power

    * now consider that in typical amplifier power supplies capacitor charging duty cycle is around 10% to at most 20% (under very heavy load) and youīll see that loss situation is serious.

    From experience: 5% regulation with resistive load, and the only parameter manufacturers supply, and they swear by it because "I measured it at the factory test bench" translates to 15% , often 20% drop in amplifier voltage rails (rectified and capacitor filtered DC).

    Of course, hobbyists and diyers donīt care about cost and often overspec what they use, so sometimes they get better numbers, but thatīs a particular case, not widespread.

    So in a nutshell: in principle I expect youīll find it a VERY GOOD 100W Marshall transformer, that does not mean it will feed a Major, not even halfheartedly

    4 Not allowed! Not allowed!
    Juan Manuel Fahey

  30. #30
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Thanks JMF. So, to understand the application (meant a bridge/capacitor supply which should provide a certain DC current onto a load) determine an real operating current which is different from rms one. The abilities of a PT to sustain this type of current is a matter of transformer characteristics to can manage it in respect with "thermal stress". From this point a insulation classification became very important for transformer abilities.Over that should be an equivalent model which can translate this current regime (taking in consideration the load and capacitor battery)into a rms current one. I.m far out of math to can calculate but now all suggestions started to have more sense for me. The application (power supply) is not a rms one but have a rms equivalent which should be related by transformer characteristics. A true test should include the real PS application
    In this case measuring the rms current between transformer and rectifier and raised progressive the load in DC side monitoring permanently the tranny temperature till stabilization point should be pretty close, please ?

    Click image for larger version. 

Name:	20190405_151026.jpg 
Views:	16 
Size:	1.90 MB 
ID:	53245

    I can modify the value of load in DC side rising the current from transformer till 600 mA ( supposed my true rms dmm integrated the type of currents by PS application )
    After stabilization I can move the measurement after rectifier to see how much resources have in DC side
    My question for now is if my dmm is realy able to show true rms value no matter by current operation conditions? Or is just a rms when operate only with sinusoidal current ?

    For my understanding is more simple to conclude: a 300mA bridge power supply application request a 600mA RMS power transformer...for instance. But if rms value is only derivated from sinusoidal currents and not from whatever peaks current operations , things become more complicated to get it

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-05-2019 at 02:31 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  31. #31
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,138
    Thumbs Up/Down
    Received: 445/4
    Given: 255/0
    Rep Power
    16
    Quote Originally Posted by catalin gramada View Post
    For my understanding is more simple to conclude: a 300mA bridge power supply application request a 600mA RMS power transformer...for instance. But if rms value is only derivated from sinusoidal currents and not from whatever peaks current operations , things become more complicated to get it
    For a bridge rectifier/capacitor I usually assume I DC = 0.5 x I RMS transformer (as you have done above).
    Hammond says I DC = 0.62 x I RMS (Hammond Design Guide For Rectifier Use.pdf)
    If you post the transformer pri and sec winding resistances I'll run a simulation.

    1 Not allowed! Not allowed!

  32. #32
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    Quote Originally Posted by Dave H View Post
    For a bridge rectifier/capacitor I usually assume I DC = 0.5 x I RMS transformer (as you have done above).
    Hammond says I DC = 0.62 x I RMS (Hammond Design Guide For Rectifier Use.pdf)
    If you post the transformer pri and sec winding resistances I'll run a simulation.
    Thanks Dave
    I found 2.64 ohm into 230v primary with 15.52 ohm on HT sec(366v empty / 352.5v/577mA)

    0 Not allowed! Not allowed!
    Last edited by catalin gramada; 04-05-2019 at 04:02 PM.
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  33. #33
    Senior Member
    Join Date
    May 2006
    Location
    Cheshire, UK
    Posts
    2,138
    Thumbs Up/Down
    Received: 445/4
    Given: 255/0
    Rep Power
    16
    Which primary voltage tap are you using?
    And what is the capacitor value?

    0 Not allowed! Not allowed!

  34. #34
    Senior Member
    Join Date
    Mar 2008
    Posts
    725
    Thumbs Up/Down
    Received: 68/0
    Given: 178/0
    Rep Power
    12
    230v primary tap, 100uF cap

    0 Not allowed! Not allowed!
    "If it measures good and sounds bad, it is bad. If it measures bad and sounds good, you are measuring the wrong things."

  35. #35
    Senior Member
    Join Date
    Mar 2018
    Location
    Germany
    Posts
    1,762
    Thumbs Up/Down
    Received: 1,329/1
    Given: 780/2
    Rep Power
    4
    My question for now is if my dmm is realy able to show true rms value no matter by current operation conditions? Or is just a rms when operate only with sinusoidal current ?
    For a pure sine current you would not need a true rms meter. In principle a true rms ammeter should give the correct rms current. But even true rms meters have a crest factor limit, the crest factor (CF) being the ratio between peak and rms value. The AC current in this application is pulsed having a high crest factor of maybe 4 or higher. If this exceeds the CF limit of your meter, readings will be wrong.

    3 Not allowed! Not allowed!
    - Own Opinions Only -

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Testing power transistors
    By Mick Bailey in forum Music Electronics
    Replies: 14
    Last Post: 12-17-2015, 12:15 AM
  2. Stress testing the power amp
    By GainFreak in forum Build Your Amp
    Replies: 7
    Last Post: 02-19-2015, 12:42 AM
  3. Power transformers - OK to use 240VAC primaries with a 230VAC power outlet?
    By motzu in forum Maintenance, Troubleshooting & Repair
    Replies: 8
    Last Post: 08-11-2010, 09:54 AM
  4. power amp failure, testing methods
    By zbyszekskibinski in forum Debugging Your Build
    Replies: 7
    Last Post: 06-23-2009, 11:17 PM
  5. Testing power trans
    By davoux in forum Maintenance, Troubleshooting & Repair
    Replies: 7
    Last Post: 11-22-2006, 02:43 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •