Dating my JMP50

Discussion in 'Let's Talk Vintage' started by obx351, Mar 22, 2015.

  1. chilipeppermaniac

    chilipeppermaniac Member

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    Thank you neikeel. Boy I am learning quite a bit as I am trying to make up for the lack of a thorough examination and repair by a local amp tech a good number of musicians in Baltimore has used for years. He did me no favors the 2 times I had the amp to him for correction of fuse blowing issues. I will keep all your advice in mind along with the similar advice of a few other guys who are good with amps and good with electronics who I have already had evaluations and suggestions the same and similar to your advice so far.

    I forget if it was you or one of those other guys who also suggested NOT using a 2 amp slow blow fuse in the mains but rather use the regular 2 Amp ADC one.
    What is your take on that?
     
  2. neikeel

    neikeel Well-Known Member

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    Use the correct fuse. In the US at 120v use 2A slo blo (T2A) for mains and 500mA slo boo for HT. nothing else. If they blow you have a problem.
    UF5408 are 3A rated at 1000v. You only need two.
     
  3. chilipeppermaniac

    chilipeppermaniac Member

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    Thanks Neikeel, I mentioned you to my friend at another forum and he says you 2 have spoken in the past and he speaks highly of your knowledge. His name is Ivan and he is from Australia.

    He also explained the difference in the UF's. He agreed in only 2 of the 5408 vs his way of using 4 of the 4007's

    After I get the wiring ( white wire to standby) sorted out, I would like to add the 1k5's and then install new .22uF snubbers to bring the amp back to a condition where I am like at ground zero where my amp was either playable or exhibiting a fuse blowing problem lurking somewhere as to be like a benchmark before I go to more mods like the UF5408.

    This will be where I test the components and look for cracked solder joints, incorrect readings off transformers, tube heaters and other pins, caps, resistors and other tell tale signs of why amp has been blowing fuses.

    I think it wise to start from there and only when amp is operating reliably, then we can approach the UF5408 and bias caps if they test bad. I mentioned to Ivan that you suggested replacing the bias caps. He agreed and suggested to swap them for BC type.
    I had to ask him to clarify what the differences are and why.

    That is it for now. Not sure if I have told you, but you probably can guess, I am very green at electronics, but do grasp pretty much of this stuff despite feeling like a 53 year old 1st grader.

    Oh and as for yellow wires etc for the 1k5 deal, I will have to inspect my tube sockets and get back to you on what wires my amp has going where. Then you can walk me through the procedure again then.

    Your assistance to this point has been greatly appreciated. Thanks
     
  4. chilipeppermaniac

    chilipeppermaniac Member

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    Thanks again Neikeel, Sticky seemed to be understanding me on the ohmites and you may be as well but also suggest using Welwyns and wiring them in like you instructed.

    These are indeed what I was asking about,

    1kOhmite25.jpg

    Not the brown snubber caps. These.


    don't they and the Welwyns do the same thing?
    Also, while I am at it, I can look at the wires to my sockets.
     
    Last edited: Feb 1, 2017
  5. chilipeppermaniac

    chilipeppermaniac Member

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    Ok, on my 1st socket closest to the Power transformer looking down on the amp turned upside down.

    Empty pin 6, then orange pin 5, yellow 4 , white pin3, Pin 2 has 2 red, then pin 1 and 8 have a bare jumper to ground, then pin 7 has 2 black

    Next tube socket has same, 6 empty, then 2 greens on 5, 2 yellow on 4, 1 red on 3, 2 red on 2, 1 and 8 with bare jumper to ground, then 2 black on pin7.

    Also, remember my amp has a voltage selector socket and an impedence selector socket unlike the other amp I pictured which likely only has the impedence socket since it was a US model.
     
    Last edited: Feb 1, 2017
  6. chilipeppermaniac

    chilipeppermaniac Member

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    nos-6ca7-mullard-oo-getter-pin.jpg I pulled up a xF2 site and it is nice to see the pin layout numbered on the tube
     
    Last edited: Feb 1, 2017

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