Testing Power Transformer mA

Discussion in 'The Workbench' started by stickyfinger, Aug 15, 2019.

  1. stickyfinger

    stickyfinger Active Member

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    OK fellas I'm a bit confused. The 3 big marshall transformer builders use different specs.

    Merren, Classic Tone and Marstran all have different mA ratings for their JTM power transformers. 100 mA, 150 mA and 250 mA respectively.

    A power transformer that has less mA will sag under load, have less headroom and sound more brown.

    How do I test a original to get the specs? And who is getting it right??

    Why cant a manufacture get the specs right :mad:
     
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  2. john hammond

    john hammond Well-Known Member

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    - they're ALL getting it right.

    they changed them quite a bit during the build run

    What I'd do here is try and find out if theres any artists that you like that use these amps. find out which of the three variants they use and copy that one.
    the issue going blind into it is that the biggest one will punch like tyson, the smallest will sag the most..you gottsta establish what you want.

    I'd go the very biggest, unlike the later voltage doubled plexi ones , that headroom is just divine.
     
    Last edited: Aug 15, 2019
  3. john hammond

    john hammond Well-Known Member

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  4. john hammond

    john hammond Well-Known Member

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    largest powerx jtm45/100 + fuzz = THERE
     
  5. john hammond

    john hammond Well-Known Member

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    this...is hot damn!

     
  6. neikeel

    neikeel Well-Known Member

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    If it is a 45/100 you are talking about with a-43 Deake then 250mA is the right zone if you are talking about HT secondaries, but that is a bit low if you are speccing a 100w that does not sag.
    100mA is far too low, more like an18w secondary, the 150mA is a JTM45 spec (that is what an RS heavy duty PT from early 60s is rated at).
     
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  7. john hammond

    john hammond Well-Known Member

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  8. neikeel

    neikeel Well-Known Member

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    Yes Roe put a lot of legwork in there a lot of credit due.
    He did have a dedicated website for it.
    Nice to see on of my amps in the pics too!

    Edit:http://folk.ntnu.no/roef/JTM100.html
     
  9. stickyfinger

    stickyfinger Active Member

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    Sorry should have been more clear. Just a JTM 45 dual Kt66.

    I see the RS mains says 150mA on it. But what about a Drake as this is my main concern.
    Merren says on his site 100mA for his Drake1202-55/118. I'm going to assume he's correct for a Drake if he builds these him self.

    I built two Identical 68 JMP 50 lead spec amps. Both Iskra, Mustards, Merren output and choke. Everything exactly the same except the Power transformer was a Marstran 250 mA in one and Merren 100 mA in the other. The amps had the same 430 DC voltage. The tone change wasn't a parts tolerance difference.

    Marstan build was cleaner, had more headroom and overall a bit louder. Much preferred the Merren power build.
     
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  10. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    The 100mA transformer is a bit low which is why you perceive the aspects of less headroom, more compression and distortions.
    If you are using 430VDC and shooting for the 30-35W output with a bit dirtier sound then 100mA is about the place to be.
     
  11. neikeel

    neikeel Well-Known Member

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    It sounds like the Merren -118 PT has more sag so his JTM45 PT may be the same?
    Most people prefer up to 450v on the anodes of their KT66 in a JTM45 but also like it when it squishes , now whether that is purely down to the GZ34 or the PT I do not know, maybe a combination of the two. I had a 65 that squished far to early and it could not cope well with gigging.
    Both my current 64 (with RS HD PT and Deluxe OT and the 65 with the first Drakes (selectors on end bells) are remarkably similar in tone and feel. Both have B+ of the 440v which suits me perfectly, but then I really like the Marstran iron in my JMP50 2 in 1 so YMMV.
     
  12. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    The manufacturer rates their own replacement transformer. 3 manufacturers, 3 ratings, this would be normal / OK.

    1. Disconnect the center tap of the secondary from ground.
    2. Install current meter in series with the center tap, and connect the other lead of the meter to ground.
    Now the secondary center tap is connected to ground through the current meter.
    3.Set meter for "appropriate" amperage. The meter usually has separate jacks for measuring current, make sure the meter probes are inserted correctly for current measurements.
    4. Set meter for AC current.
    PT.png

    5. Push the amp (turn up volume to max) as high is it can go.
    6. Observe your meter readings when the current reaches maximum (it will not go any further than a certain point).
    When pushing the amp hard, it's going to max out.
    Then after that point, no matter how much more you push the amp, the current will not go higher.
    This is your actual secondary max.

    If an amp is biased cold, it's going to draw less current and the plate voltage will be higher.
    If the amp is biased hot, the current is higher and the plate voltage will drop lower.
    This is normal.
    So: your plate volts will change depending on the bias setting.
     
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  13. stickyfinger

    stickyfinger Active Member

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    I believe the Merren Drake is a universal 118-55 as it has both 425 Diode and 450 Gz34 taps.

    I'm concerned as I had a Drake 55 built for me as Merren didn't seem interested in making me one last time I talked to him. I had mine speced at 350 secondary and will make 450dc Gz34. But as you mentioned is the 100 mA after the Gz34 or at the secondary's on Merrens..

    I have a Merren here to test but looks like I will have to build the amp to test it.
     
  14. stickyfinger

    stickyfinger Active Member

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    Looks like I'm going to need to build the amp to test?

    I have a Merren power here. No amp built but want to test as I have a power transformer being custom built as we speak. I may have screwed up the specs and want to test my Merren Drake 55. My custom Drake 55 makes 350 secondary's speced at 100 mA.

    What do you think the mA difference will be at the secondary's 350ac compared to the gz34 450dc?? I may have time to correct my custom transformers specs.
     
    Last edited: Aug 15, 2019
  15. ampmadscientist

    ampmadscientist Well-Known Member

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    Start the reactor... Free Mars!
    To test the transformer there needs to be a load on it.
    The load could be a large resistor, or the amp circuit, etc.
    The transformer shop would use something like stove elements hooked together to make a high heat load.
    Or they string light bulbs together to make a load...
    I load the amp with a resistor and drive it with a sine wave till it peaks out. Then I can see what the power transformer limit is.
    I used a classic tone transformer for a 50 watt amp, and I could get about 128 ma.

    All I can think of is to make a transformer with taps on the secondary to raise or lower the voltage.
    The classic tone had 20% taps, but I would like 5% 10% 20% etc so I can adjust it better.
     
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  16. neikeel

    neikeel Well-Known Member

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    I am pretty sure Merren quotes his output as mA dc post rectification, so, as neat as AMS method is (basically subbing your ammeter into the HT fuse spot of a JTM45) that gives you the ac current. This is still useful when making comparisons.
    Question is: what are you trying to achieve with your build? Saggy or staying cleaner longer?
     
  17. stickyfinger

    stickyfinger Active Member

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    Historically accurate.

    I was thinking the Drake power, output and 3h choke all played the part of this era JTM 45 being a little dirtier.
     
  18. mickeydg5

    mickeydg5 Well-Known Member VIP Member

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    When Merren quotes this it means:
    1) The PT has HT secondary at 350VAC with rated typical current of 100mA
    2) That secondary voltage will approximately produce 450VDC at the B+ rail using a 5AR4/GZ34

    Again the whole HT supply system revolves around the typical high current of 100mA




    upload_2019-8-16_13-57-15.png
     

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