Power transformer help! JTM-45

Discussion in 'Building the Classics' started by Marcomel79, Jun 20, 2021.

  1. Marcomel79

    Marcomel79 Well-Known Member

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    35BA47E1-1D85-4228-B48A-5E8608D280CF.png 70A3129B-65D8-412C-8AE5-F5A1EE312772.png Hi everyone, im hoping to get some help here to choose a power transformer. I basically want to build a jtm45 clone from the tube depot kit, but wanna buy all the components little by little. They dont have the classic tone PT anymore, and they recommended a Hammond replacement. The thing is that from the schematic i can see that its not a match, but the person im talking to keep pushing for that particular PT. I can attach screenshots of the schematic and the PT transformer they wanna sell me. Its the first time tjat i try to build an amp so any help would be greatly appeciated
     
  2. Marcomel79

    Marcomel79 Well-Known Member

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    This is the PT i think is the perfect match for my build
    3B1F24BA-3239-473F-8D92-78EEC567D36B.png
     
  3. 351

    351 Member

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    I like it, you can run the 342vac with a tube rec at around 440v, give or take, and with the 312vac taps run 440vdc with a ss rectifier or 410vdc with a tube rec. Nice.
    Make sure its a lay down type.
     
    Last edited: Jun 20, 2021
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  4. Marcomel79

    Marcomel79 Well-Known Member

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    64330128-29F0-4C2F-83B3-B4A323388264.png
    Thank you for your answer. I will follow the tube depot ktm45 head build as ive never built an amp before and they have step by step instructions. In their version i have the option of using a ss or tube rectifier. Im just not sure what to do with the extra wires in the primary (i live in europe, 240V) and extra wires in the secondary high voltage. In their wiring diagram they only have 2 red and one red/yellow that goes to ground. Thats the picture of their wiring
     
  5. Ken Underwood

    Ken Underwood Well-Known Member VIP Member

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    You say that you have never built an amp before so i will ask the question, how proficient are you at soldering and if so then what type of solder and iron do you intend to use.

    This may seem as a very basic question but soldering is a basic function but a very important one, poorly done then you will have problems
     
  6. Marcomel79

    Marcomel79 Well-Known Member

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    Hi and thank you for your answer. I have no problem doing any guitar wiring and i also build pickups. I just have no experience with transformers. As i wrote on the post i found a transformer which is a perfect match for my build. I need to wire it for european 240V. For the secondary, i need 690v-150mA, 6,3v-5mA and 5v-3mA. The question is, what do i do with the wires im not using from the primary and secondary... you can see the pic of the data sheet here 2B9A2577-5093-4AB2-BFE8-954449FC4167.png
     
    Last edited: Jun 20, 2021
  7. neikeel

    neikeel Well-Known Member

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    https://www.marshallforum.com/threads/tubedepot-jtm45-conversion-to-a-jtm45.95381/As above that PT should work well. The OEM PT for a JTM45 would be 350-0-350vac HT @150mA. The heaters 3.15-0-3.15vac 6A and 5vac 3A.
    That PT is pretty close and gives you options.
    If you just want a guitar amp that you built of a Marshall flavour use the lower taps and SS rectifier as it is cheaper and simpler to build, from memory the Tube Depot preamp is not really true to JTM45 (I might be wrong).
    Read a builder from this forum’s experience here:


    If you want a true JTM45 then higher voltages and an efficient GZ34 are needed.
     
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  8. Marcomel79

    Marcomel79 Well-Known Member

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    Hello and thank you for your answer. Since this is my first build, it doesnt have to be a true JTM45, i just chose this cause it has step by step instructions. Kind of, learning by doing, it that makes sense... my question was, should i just cut and cap the wires i wont be using? The two 100v ones from the primary, and, the two 312.5v and the 50v one from the secondary...
    By the way, this amp does have a GZ34 rectifier. And yeah, they did say its their version of the JTM45, with an american twist, whatever that means. But for me, its the first step on understanding how an amp works...
     
    Last edited: Jun 20, 2021
  9. Travis398

    Travis398 Well-Known Member

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    Yes just cut and insulate unused wires. I leave the unused secondaries long and insulate incase I want to change voltage later. But not necessary if you aren't planning on changing taps.
     
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  10. Marcomel79

    Marcomel79 Well-Known Member

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    T
    Thank you! Much apprecieted!
     
  11. Pete Farrington

    Pete Farrington Well-Known Member

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    Yes, that one is much more suitable, the HT winding resistance is in correct range, that of the other one is far too low.

    But yeah, it would be a really good idea to get some experience by building a smaller scale project first, eg a nice Rangemaster treble boost pedal.
     
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  12. 351

    351 Member

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    If you have options, then keep those options.
    You can buy a tag board with lets say 5 or 6 tags on it, and attach it to the corner bolt of your power transformer. You can attach unused wires to that tag board, that way you don't cut them and regret it later on.
    Apart from the international power primary wires, I wouldn't cut any short, keep them and attach them to the tag board.
    Later on for example if you want to lower or raise the plate voltage of the amp, you can just run wires to or from that tag board, the tag board then becomes a junction point.
    I've seen unused wires just clipped and ' pigtailed' with no insulation on the tip on amps 50 years old, and this worked for 50 years with no problem.
     
  13. Marcomel79

    Marcomel79 Well-Known Member

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    Thank you for chiming in Pete, from ther posts ive read looks like you know a lot.
    Unfortunately patience is not my strongest side but im building the tube depot version with step by step instructions...also ive been watching Uncle Dougs videos religiously and ive learned a lot about schematics and components
     
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  14. Marcomel79

    Marcomel79 Well-Known Member

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    Yes, i will keep the secondary at the original lenght as also suggested by Travis39. You are absolutely right, its nice to have options!
     
  15. Pete Farrington

    Pete Farrington Well-Known Member

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    It would be a very good idea to build a light bulb limiter LBL before you get anywhere near test powering the amp up. If you screw something up it should kick in and limit mains current, thereby helping to reduce the risk of anything significant being damaged.
    An LBL needs incandescent or halogen type mains bulbs, low energy types are not suitable.
     
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  16. Pete Farrington

    Pete Farrington Well-Known Member

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    @Ken Underwood would it be possible for you to cast any light on the screen grid arrangement used in many JTM45,ie the 1k 2W shared feeding 470ohm 1W individual resistors; what prompted using that arrangement, rather than the individual only 470ohm that were used in the 5F6A Bassman.

    I really like effect that 1k shared resistor has on the tone and the control in the screen grid dissipation it provides.
    It’s regrettable that the RI models, and most of the kits and build guides, omit that resistor, as I think it creates a subtle but noticeable part of what I consider the character of a good JTM45.
     
    Last edited: Jun 20, 2021
  17. Marcomel79

    Marcomel79 Well-Known Member

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    :dude:
    Thats a very good idea. Ill be sure to do that! Its gonna take me a while to get there as im gonna build it little by little. Covid made money a bit tight...but that will also give me the time to study schematics and understand more about components. Again, thank you very much for all the help
     
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  18. Marcomel79

    Marcomel79 Well-Known Member

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    Just one more question Pete: i have found a company in England that sells PT for JTM45. That should save me a lot in shipping as i live in Norway...from the schematic of the amp i wanna build i see that the rectifier heaters need 5V- 3A. This PT im looking at is 5V-2A though. Will that work? Thank you in advance for taking the time to answer this newbie!
     
  19. Pete Farrington

    Pete Farrington Well-Known Member

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    Which rectifier type are you thinking of using? GZ34 is the standard type for a JTM45. It, and the 5V4, draws 2A.
    Of the commonly used valve rectifier types, only 5U4 draws 3A.
    So provided you’re not planning to use a 5U4, a 2A rated 5V winding is fine.

    Is that Modulus?
     
    Last edited: Jun 21, 2021
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  20. 351

    351 Member

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    If you're after a big voltage drop , the 5u4 tube rec is the way to go , 40-50 volts down from the SS value.
    The gz34 will drop around 15-20.
    The 5u4 draws 3A, the gz34, 2A.
    If your power transformer can supply 3A, then you can run a 5u4 and enjoy lower plate voltage ( lower plate voltage gets more gain, but with less punch)
    If your power transformer can supply only 2A then a 5u4 is off the table.
    But, if you run a gz34 off a 3A load, the 5v filament voltage won't be 5v anymore, the tube will run a higher filament voltage, something that may affect longevity. ( if a 5v 3A tap is only drawing 2A it will do so and run a higher voltage, lets say 5.15 or 5.2?).
    The closer to 5v you can get the filament of the tube rec to run, the better.
    Why even consider this business of plate voltage?
    Because it matters quite a lot, tone-wise.
    All factors equal, if you run a 5u3 vs a gz34, the gz34 will be harder sounding amp, more punchy and a bit cleaner.
    me? I'd pick lower plate voltage any day.
     
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