JTM45 Clone, KT66 Power Tube Issues

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by Darkburst, Jul 6, 2008.

  1. Darkburst

    Darkburst Member

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    I recently picked up a used Rockytop JTM45 clone. It sounds great before it breaks up, but when I crank it there's some weird fizz/ghosting on the notes. Also, I noticed that the Valve Arts KT66 power tubes flash purple inside of them when it's cranked each time I hit a note. I've never seen that on any amp I've owned before. Bad power tubes? Poor bias? I'm weary of using it until I know what's going on. Any insight would be appreciated.
     
  2. RussB

    RussB low rent hobbyist Silver Supporting Member

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    All the power tube's I've ever seen have varying degree's of blue glow happening...it dances with the notes.
     
  3. JimmyR

    JimmyR Member

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    But flashing with notes doesn't sound too healthy! A blue glow is ok but not flashing.

    Unfortunately KT66s seem to be difficult to good examples of. I've tried two pairs of Valve Arts and both times experienced flashing/noise/extreme bias drift within weeks. The most reliable new production KT66 I can find is the JJ, but it's not a great example of a KT66. It works though!

    Check at Metroamp boards what they would suggest. If you don't mind it sounding more JMP-ish you could try JJ KT77s in it. You may have to change the bias resistor and grid stoppers but they sound great in my amps.
     
  4. Riscchip

    Riscchip Supporting Member

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  5. Mark Kane

    Mark Kane Silver Supporting Member

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    Does it have the power scaling option?
     
  6. Darkburst

    Darkburst Member

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    Yeah, I have the Valve Arts KT66's. I wouldn't mind trying some good 6L6's if anyone has recommendations. No power scaling on mine, though I have to tame this little beast with the Ultimate Attenuator. Just as a test I tried cranking my Splawn through the UA and didn't get any weird sound artifacts.
     
  7. Riscchip

    Riscchip Supporting Member

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    I like SED 6L6GCs. Nice tubes. I've also had success with TADs.
     
  8. GearHeadFred

    GearHeadFred Member

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    I run JJ El34s in my (Ceriatone) JTM-45 with very good results.
     
  9. hasserl

    hasserl Member

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    Interesting, you didn't mention the use of the UA in your first post. What impedance is this amp supposed to be? Is this amp designed to be run with the KT66's or with El34's? KT66's want to see about twice the load of EL34 or 6L6GC tubes. I'd start looking to be sure you were putting the right load on them before you start condemning the tubes.
     
  10. Darkburst

    Darkburst Member

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    I have amp set to 16ohm going into a 16ohm 4x12 cab loaded with Scumbacks. When I bought the amp it had the KT66's in it, the previous owner said amp was biased for them. I didn't think the UA would be a factor since I tried it with my 100w EL34 loaded Quickrod and it had no ghosting or fizziness.
     
  11. hasserl

    hasserl Member

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    I don't know that it is a factor. just looking at things that might be. The UA puts a 30 ohm fixed resistance on the output of the amp. It MIGHT be a factor here. I would try the amp without the UA to see if you have the same effect.
     
  12. phsyconoodler

    phsyconoodler Member

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    Any time you use an attenuator,the weak points of the amp usually show up.You are likely experiencing poor contact at one or both of your tube sockets.It's like a car's battery.When it's called on to peform in say,cold weather,it can spark and draw huge current through a dirty set of battery posts.Same thing with your socket connections.Retention them and clean the pins and try again.

    And,being a clone,I suggest reflowing all solder joints on the power tubes sockets too.
     
  13. Mooncusser

    Mooncusser Member

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    Tried a quad of VA KT-66's in my PLexi...............
    Did the EXACT same thing you just mentioned....TWICE.
    Alot of folks (professionals) say the VA's sound great when working, but are not reliable.
    Phillips walked me through the mod inch by inch to!
    Undid mod and currently use NOS SED Winged ~C~ 6L6GC's.
    Awesome effect in a Plexi circuit.
    Hope that helps. I was bummed to, but he SED's are stayin!
    Take care,
    Mass
     
  14. Darkburst

    Darkburst Member

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    Thanks for chiming in Mass. The more I read on current KT66's the more I realize that they're a crap shoot. Now I have to decide if I should do SED 6L6GC's or go for a more JTM50 sounds with SED EL34's.
     
  15. phsyconoodler

    phsyconoodler Member

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    Or maybe fix the real problem with the amp first?
     
  16. Darkburst

    Darkburst Member

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    I'm sending the amp off to Jesse. He'll fix whatever issues the amp may have and adjust the amp to accept either 6L6's or EL34's. He also told me that new KT66's are inconsistent and not the most reliable. I'm in a band now and can't worry that my power tubes will crap out every time we play. I've used SED Winged C EL34's in my previous amps (Bogner Shiva, Marshall 2555, Splawn Quickrod) and they've never crapped out on me.
     
  17. phsyconoodler

    phsyconoodler Member

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    The thing about JTM45's is they are set up for KT66's.The output transformer has a 6.6k impedance and EL34's like to see about 3.4k.
    It's a good idea to get better quality tubes for sure.
     
  18. hasserl

    hasserl Member

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    Note, just saw this in the Amps and Cabs forum. Seems there has been an issue with the output transformer wiring on some of these amps. http://www.thegearpage.net/board/showthread.php?t=387537&highlight=Rockytop

    If your amp is wired up incorrectly so the 16 ohm and 8 ohm outputs are crossed, than when you've got it set to what is marked 16 ohms, the amp is actually operating on the 8 ohm tap. Then you're running it into the UA which has about a 30 ohm resistance, which is double 2what it should be for 16, but is four times what it should be for 8. That would very likely cause a problem with the output tubes, sockets and possibly the output trans. Changing to EL34's may make the situation even worse, because they want to see even less impedance than KT66's.

    You may not want to mess with it, but I strongly suggest you print out that other thread and give it to your tech. He could trace out the wires and make sure it's wired up correctly.
     
  19. jiml

    jiml Supporting Member

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    My Rockytop is acting a little funny too, gonna get a hold of Jesse too..
     
  20. Darkburst

    Darkburst Member

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    The wires on mine have different colors:

    [​IMG]
     

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