V1 preamp tube in a JTM-60

Discussion in 'Marshall Amps' started by Nilzon, Jun 6, 2009.

  1. Nilzon

    Nilzon New Member

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    Hi all!

    New to this forum and a newold Marshall owner. Just bought a JTM-60 that has some great clean tones. It needs new tubes so Ive ordered some, I was recommended a better tube for the V1 in the preamp section but which tube is it? Is it the one nearest the power tubes or on the other side.

    The manual or schematics doesnt reveal anything!

    Cheers from Stockholm/N
     
  2. solarburnDSL50

    solarburnDSL50 Well-Known Member

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    V1 is usually always the slot furthest from the power tubes. The PI slot in next to the power tubes.
     
  3. BrotherJ

    BrotherJ Member

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    Hi,
    I used to own a JTM615 previously. Look at the back of the amp, V1 is the 12ax7 on the far right.
     
  4. Nilzon

    Nilzon New Member

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    OK Thanks, theres no marking of the tubes at all seen from the back.
     
  5. MartyStrat54

    MartyStrat54 Well-Known Member

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    Hey welcome aboard. No matter what, all of your small tubes are 12AX7's. These are high gain dual triodes. V1 like SolarBurn has stated is the tube farthest away from the power tubes. It should have a polished metal shield covering it. To remove the shield, carefully twist the metal cover in either direction until the tabs on the cover match up to two slots in the chassis. The cover will now come off.

    You will get the most gain change in V1 and V2. If you are changing the power tubes, the power tubes need to be biased. The JTM-60 is moderately difficult to bias and if you haven't done it, I would recommend taking it to a tube shop.

    Also, try this thread for info on preamp tubes:

    http://www.marshallforum.com/marshall-amps/3824-preamp-tubes-who-what-when-where-why-how.html

    Marty
     
  6. MartyStrat54

    MartyStrat54 Well-Known Member

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    Sorry, I need to ask you something? Are your preamp tubes in a straight row, or are they in an odd arrangement? The preamp tubes on my TSL602 are not in a straight row, they are in a one-two-one pattern. Is that what you have with the JTM-60?

    Marty
     
  7. solarburnDSL50

    solarburnDSL50 Well-Known Member

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    Sounds like they are a single line by how he asks in the OP. At least that's the impression I got.
     
  8. BrotherJ

    BrotherJ Member

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    Yep, they are in a single row (And horizontal)

    Looking from the back of the amp & going from left to right are the two EL34 power tubes, then the four 12ax7's V4(PI) V3, V2 & V1.

    The V1 preamp tube on the far right doesn't have any metal shield cover on it.
     
  9. MartyStrat54

    MartyStrat54 Well-Known Member

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    Look at your preamp tubes. Does V2, V3 and V4 have a rubber grommet (ring) attached to the chassis between the tube and the chassis? If so, is there one of the rubber grommets on V1? If not, the metal shield is missing and this can be verified by inspecting the area of the chassis around V1. You should see two small cut-outs in the chassis. The cut-outs accept the lugs on the shield. The shield is important in that it cuts down on RF radiation which in layman's terms means noise. Since V1 is the first gain tube, it is important to keep it quiet as possible. Your amp came out in 1996. I see no reason why it would not have a shield on V1. If you determine that the shield is missing, I would try and locate a shield at a TV shop (old Ma and Pa type), or order one from your music store through Marshall.

    Marty
     
  10. BrotherJ

    BrotherJ Member

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    Hey Marty,

    These ones were the series made cheap to compete with the low priced tube amps at the time, to my knowledge there was no rubber grommets for any of the 12ax7s or metal shield cans on the V1 tube of any of the units I had seen (Then again it could be just the units that made thier way to Australia.

    I really do miss the JTM615 i had, the clean sound was great
     
  11. gitarnik

    gitarnik New Member

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    hi.. i got a question - just to be sure. amp needs to be rebiased only when I change power tubes, not when i want to change preamp tubes, or PI tube? do i have to look for preamp tubes or PI tube with some special parameters, or just any ecc83 or 12ax7 will work in preamp section or PI slot? sorry for that stupid question but i need to be sure - i don't want to damage the amp ;)
     
  12. gitarnik

    gitarnik New Member

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    and i have one more problem. i've noticed that one power tube (the second one, counting from left to right) is cold, it don't glare;) it really makes me wonder because the amp is working ;)
     
  13. MartyStrat54

    MartyStrat54 Well-Known Member

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    For a long time people (including myself) used a matched (balanced) tube for the PI slot. However, you do not need to do this. Any good 12AX7 will work in the PI slot. All of your preamp tubes should normally be 12AX7's. They can all be interchanged in V1, V2, V3 and V4.

    As far as your power tube? Swap the tube around and see if it still does what you described, or if it is now affecting the good tube. If it stays with the "cold" tube, then it sounds like it's time for a power tube replacement before other problems are created.

    Marty
     
  14. gitarnik

    gitarnik New Member

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    the faulty tube acts the same in the first slot - it don't glare, the shiny one works properly in the second slot.. so i have to buy some new tubes and find a guy who will rebias this amp. Thank You very much for help:)
     
  15. MartyStrat54

    MartyStrat54 Well-Known Member

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    Better to get it done right and have the peace of mind that the amp will give you great service for at least two years (depending how much you use the amp and how hard you drive it).

    Marty
     
  16. gitarnik

    gitarnik New Member

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    ...so i decided to retube this amp by myself. i bought a new pair of electro-harmonix EL34's, and a bias tester. i've read that for EL34's bias should be set between 27,5mA and 46,5mA. i've began with putting the tester under the first tube (the one nearest to the transformer) - i've set the bias to 36mA. after some time i've put the tester under the second power tube - the bias was "jumping" from 31mA to 34mA - so i set it up once again around 36mA. when i've put it back together and played it for few minutes i noticed that the tubes are glowing bright orange, and they produce in my opinion a lot of heat. should i back off the bias pot a bit? the amp sounds great but i'm worried about the heating problems and i don't want to change tubes again in two weeks or so;/ (sorry for my english ;))
     
  17. MartyStrat54

    MartyStrat54 Well-Known Member

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    Sad to say, but you set the bias way too hot. The bright orange plates are very hard on new tubes. It is also called "red-plating." The new tubes may be trashed, it's hard to say. I think the power tubes will need to be tested to see if they are still good and if they are, I suggest that a tech bias the amp.

    Marty
     
  18. gitarnik

    gitarnik New Member

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