Not another JTM45 build!

Discussion in 'Building the Classics' started by jessemhopkins, Jul 26, 2020.

  1. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    That 1K drops the screen voltage minutely. The screens need to always be lower than the plates.

    I don't think the absence of the 1K has anything to do with blowing fuses, or high voltage to.pin 6 of V2
     
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  2. jessemhopkins

    jessemhopkins Member

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    Certainly not the latter. My tube sockets are coming in tomorrow, I'll replace v2 socket and see if it helps. I don't know why, but there's something in the back of my head saying I saw a repair video where a faulty socket was causing excess current draw. Might be nothing. I'm still pretty green, but this build has helped cement A LOT of concepts for me.
     
  3. neikeel

    neikeel Well-Known Member

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    Colloquially known as the 'flying' screen resistor (it sits flying betwen the screen turret and the spare lug on V4 it is 1k 1w in series with the two 470k screen droppers.
    George felt the amps sounded more open and less compressed without it so omitted it. The early 50w with Mullard EL34s had no screen resistors at all. Sound benefit is marginal and you cannot run modern EL34s without them.

    Have you tried tube rolling in V2 once you get it to seat properly?
    The voltage will go down when the tube draws full current
     
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  4. jessemhopkins

    jessemhopkins Member

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    I’ve only tried 2 so far, I’ll go through my stash once I get the socket swapped and the tube seated.
     
  5. jessemhopkins

    jessemhopkins Member

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    I replaced the socket and the tubes now seat properly. Unfortunately I’m still getting the same results on V2 plate. I’ve tried 3 different 12AX7 and they all read exactly the same.

    :erk: Does this smiley look sufficiently depressed?
     
  6. neikeel

    neikeel Well-Known Member

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    Does the amp run?
     
  7. jessemhopkins

    jessemhopkins Member

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    Considering I intend to run this amp with KT66s and every time I try to use them the HT fuse blows, no, it does not run. I recognize the possibility that the set I have may be “duds” but I don’t feel I have sufficient evidence to accept that full stop, or to try to get CE to replace them.

    I am extremely appreciative of everyone’s time and input. This is nobody’s problem but my own, and I do not wish to come across as entitled to anyone’s time. So please do not feel any obligation or burden to keep helping me.

    And even though it’s not finished yet, let me say a hearty “thank you!”
     
  8. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    With all tubes out, does the amp blow any fuses? If no, great.

    Then with all tubes out can you get a useable range of voltage swing on your bias?

    If yes, then turn the amp off and put in only the power tubes, and set your bias.

    Does any fuse blow? If no, then the tubes are probably good and so is the power section.
     
  9. neikeel

    neikeel Well-Known Member

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    Happy to help until you are sorted. If we are hindering vs helping please tell us.
    The above advice is good
     
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  10. william vogel

    william vogel Well-Known Member

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    I usually buy a quad of matched output tubes for any amp that runs two output tubes. Therefore if I have question of a dud I can swap in a replacement and have a solution. Having a light bulb current limiter also allows you to be able to find the excessive current draw without ever blowing fuses because the bulb becomes a resistor and limits the current. You can easily find the “short” while powered up and running without the chance of damaging anything.
     
  11. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Are you sure you checked the resistance of that 10k dropping resistor that is connected to the V2 plate? That high voltage is coming from that spot. But I’m not sure what the voltage should be at that spot without opening up my JTM45 and checking it. Maybe @neikeel knows what the voltage should be right after that resistor.
     
  12. neikeel

    neikeel Well-Known Member

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    Well if you have 470v on plates (bit high) then you expect about 400v after the first 8k2 dropper (ie before that 10k) and about 320v on the north end of the resistor.
    Things that cause an isolated elevated voltage would be part of that circuit not drawing enough current as current draw drops the voltage. As your v1 voltages look normal it appears that the issue resides on v2, most likely the cathode follower part of the tone stack.
     
  13. jessemhopkins

    jessemhopkins Member

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    Certainly helpful.
     
  14. william vogel

    william vogel Well-Known Member

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    This weekend I’ll open up my 1986 and get you some voltage measurements.
     
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  15. jessemhopkins

    jessemhopkins Member

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    That’d be great. I have to put it away for a few days but I’ll have some more time on Monday.
     
  16. Chris-in-LA

    Chris-in-LA Well-Known Member

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    That makes more sense than having a bad tube socket.
     
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  17. william vogel

    william vogel Well-Known Member

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  18. neikeel

    neikeel Well-Known Member

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    Kudos to William for doing that, so, spurred on I opened up my original 65 JTM45 (Mullard GZ34, yellow print ECC83s and GEC KT66s. Best sounding JTM45 I have played (and therefore kept) amongst the many that have passed though my hands, either mine or worked on. Drake trannies (selectors on sides) and 354-114 choke.

    V1 pin 1 219v, pin 6 218v heaters at end of chain 3.08vac
    V2 pin1 183, pin 6 318v
    V3 pin 1 250v, pin 6 241
    V4 449 plates, 448 screens (1k flying and 470R on sockets
    V5 same

    On PT ac = 350-0-350vac exact
    On PT ac - 3.15vac
    On PT ac = 4.9vac

    So HT after rectifier 453v, 446, 369 and 318 after each dropper/decoupler node

    By contrast another amp almost the same except custom PT for 300-0-300 for my Mullard EL37s:

    181/177
    152/269
    210/196
    340 plates
    324 screens (1k on sockets)

    HT is 300-0-300 for 350vdc/344/311 and 268 in HT chain

    Hope that helps?
     
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  19. jessemhopkins

    jessemhopkins Member

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    Man, thank you @william vogel and @neikeel so much for doing that. Very kind of you. It will be massively helpful to have those as reference.
     

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