Power transformer getting too hot,low plate voltage

Discussion in 'Amp Tech Center' started by J-Turn, Mar 25, 2020.

  1. J-Turn

    J-Turn TDPRI Member

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    I got a silvertone 4733 organ amp with plans to turn into a Marshall-ish 18watt. I thought I would get it up and running just to see what it sounded like before I started to mod it. Fired it up, and it sounds amazing! And loud!
    Played it for about 15 to 20 minutes and thought I may not even want to mod it at all. Then I noticed that the power transformer was crazy hot five minutes later after being turned off.

    This is a (two)EL 84 amplifier
    I have the schematic that says pin 7 should be 305v and pin 9 should be 300v. With the tubes in and running, my voltage is near 180-190v and the Power transformer is getting extremely hot. 140+f

    with the power tubes out, the power transformer stays cool and the voltage on pin 7 is 417v and pin 9is 413v rectifier tube measures pin3 (dc) 415 pin 7 (ac) 313.
    I have changed the filter caps already, and tried different el84’s and ez81
    Is my power transformer dying/on its way out? Any ideas?’s
     

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    Last edited: Mar 25, 2020
  2. dogmeat

    dogmeat Tele-Afflicted

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    shorted winding
     
  3. Dacious

    Dacious Poster Extraordinaire

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    Probably dead electrolytic caps, or a very drifted resistor, maybe one of the B+ dropping resistors to the preamp. Nothing for it but to start measuring. Your tubes-out test indicates it might be the bias or a grid resistor.
     
  4. Jon Snell

    Jon Snell Tele-Meister

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    If you remove the rectifier and all valves. Power it up and check the temperature. If it is getting hot, the transformer has shorted turns.
    If it is cool, check for leaky smoothing capacitors.
     
  5. tubelectron

    tubelectron Tele-Holic

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    Disconnect your power transformer from all its loads and power it : does it comes as hot as previoulsy in 5mn ?

    If yes, then @dogmeat is right - too bad, but :

    If no - I mean if your Xfo stays cool unloaded - there's a problem in your amp circuit that draw too much power from the transformer, or you have made a wrong connection somewhere with it ?

    Good luck ! ;)

    -tbln
     
    Last edited: Mar 26, 2020
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  6. Wally

    Wally Telefied Ad Free Member

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    Given that you have removed the power tubes and witnessed much different results, I am with tubelectron in thinking that the output section—with tubes installed—is pulling too much current.
     
  7. Wally

    Wally Telefied Ad Free Member

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    Did you notice redplating in the power tubes? Did you take quick bias numbers? Do you have a tube tester?
     
  8. J-Turn

    J-Turn TDPRI Member

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    Update: I powered it up with all tubes removed. Left it running for approximately 25 minutes. After five minutes measurements on the power tube pins were approx 3.2v
    I watch those numbers slowly decline until right before I turned it off they were approx 2.5v
    The power transformer stayed extremely cool the entire 25 minutes.

    I have change all of the electrolytic caps, expect the 4 black &reds
    I have changed all the tubes with known working tubes.

    I don’t believe I hooked anything up wrong because it works exactly the same as it did when I just simply powered it on without changing anything.
     

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  9. J-Turn

    J-Turn TDPRI Member

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    I have question (maybe dumb) in the pics you can see the brown wires coming off the el84 pin 4+5 going to the rectifier pins and then to the plastic plug that would take it (elsewhere??) to the organ.
    Could those wires be causing any problems, the fact that they are not hooked to anything?
     
  10. muchxs

    muchxs Doctor of Teleocity

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    I like it. It's like a brown Princeton on the super cheap.

    Too bad about the PT.

    PM me if you want to get out from under it.
     
  11. Wally

    Wally Telefied Ad Free Member

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    “After five minutes measurements on the power tube pins were approx 3.2v.”

    Which power tube pins? Pin 7 is the plate. You had measured 417vdc earlier on the plates? Ime, when taking voltage measurements, one needs to know all of the pin voltages.....voltage charts are valuable even if and when an amp is working properly much less when one has a problem.

    http://tdsl.duncanamps.com/show.php?des=EL84

    if the plates were giving measurements of 3.2vdc, then there definitely is a problem with the PT or the circuit in between the PT and the tube sockets.......but I don’t understand why there is more than a 400vdc difference from your first measurements without tubes??.
     
  12. J-Turn

    J-Turn TDPRI Member

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    Voltages for el84 pin 7
    was/is 417v with the rectifier tube in and all other tubes out upon start up

    Pin 7 330v upon start up will all tubes out.
    Pin 7 280v 4min running
    I can watch the the voltage drop approx .2 every second
    Pin7 233v 10min running
    Pin 7 193v 20min running
    Pin7 180v 25min running

    6ca4 pin1 and pin7 316 ac volts after 25min running

    I don’t know why the voltage was so low on my first test today. After 25min this time the voltage was 180 (but still dropping) when I turned it off.
    Power transformer feels like room temperature
     
  13. J-Turn

    J-Turn TDPRI Member

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    Could those yellow diodes (in the pic) and sd701 sd702 be bad?
     

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  14. Wally

    Wally Telefied Ad Free Member

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    Okay....lift the B+ connection from the rectifier tube...what happens?
     
  15. J-Turn

    J-Turn TDPRI Member

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    In the pic the resistor should be 6.8k
    But is reading 8.02k
    I don’t have a replacement 6.8k but could this be causing all my problems?
     

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  16. Jon Snell

    Jon Snell Tele-Meister

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    If you removed the valves and measure the AC voltage across pins 1 & 7. If the voltage is stable and not dropping, it is an issue after the EZ81. If it drops with time and the transformer runs hot, you need a new transformer.
    Nothing else to do about it.
     
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  17. Wally

    Wally Telefied Ad Free Member

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    ????? Did you do this?

    where is that resistor? Without knowledge of where that resistor is it is hard to know how any variance in its value would affect things.


    send it to Muchxs for sale or repair.......generous offer from him, imho.
     
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  18. J-Turn

    J-Turn TDPRI Member

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    Certainly is if I don’t want to fix, and have it for myself. This thing currently sounds amazing (no pun) lol
     

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  19. J-Turn

    J-Turn TDPRI Member

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    So then it doesn’t sound like the PT and maybe that 6.8k resistor that measures 8+k going to the rectifier!(?)
     
  20. Snfoilhat

    Snfoilhat Tele-Afflicted

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    If the voltage is dropping, especially with the added clue that heat builds up, that makes me think that current is rising. Most of the current an amp draws through its high voltage winding (other than in the special case of a short circuit), is through the power tubes. But to cover more bases, you could say that it's any part of the circuit that goes from the power supply to ground.

    That could include:

    #1 the cathodes of the power tubes, then work your way back up: cathodes -> plates&screens -> output transformer -> B+.
    #2 any kind of bleeder or voltage-balancing resistors associated with the power supply, especially the first/reservoir cap(s).
    #3 the cathodes of other tubes,

    This thing seems to have bias that starts out somewhere okay and then slowly creeps hotter until the power tubes are drawing way too much current. I would look there, taking more thorough measurements of the unloaded bias voltage, maybe replacing all the bias supply parts (if it's cathode-biased, that means the cathode resistor, the cathode bypass capacitor, the grid leak resistors). Then taking more thorough measurements of the amp while loaded.
     
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