Blew HT on my Valvestorm JTM45

Discussion in 'The Workbench' started by Rotorcraft230, Aug 2, 2020 at 9:22 AM.

  1. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    So My son was playing his JTM45 last night and after say 5-7 minutes it stopped producing sound. We saw that he had it on 8 ohms but the cab was 16 ohms. so we replaced the fuse selected 16 ohms on the amp. We placed the amp on the speaker cabinet with out the amp in in the cab and the fuse blew again and this time he saw a flash from one of the KT66 tubes. So upon further investigation I can see a very small black (smoke stain looking) say 1/2 mm in size on the inside of one of the KT66 tubes. I figure the power tube went bad maybe from using the amp on 8 ohms when it should have been 16 ohms. Since it blew twice I did go through a voltage check in the amp and found everyting in specs (no power tubes installed). We dont have any other KT66 tubes so I ordered some Gold Lion KT66.

    Would the miss ohm selection cause this or was the tube already on its way out. I would like to know if it was the ohm issue why did that happen.
    Thanks Mark
     
  2. Far Rider

    Far Rider Hendrixian VIP Member

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    You can run 8 into 16 (lower ohms into higher ohms), just not the reverse. How many hours did you have on the KT66? Did you check the Bias level?
     
  3. South Park

    South Park Well-Known Member

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    The speakers are power limiters for the power tubes . Check the bias
     
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  4. Far Rider

    Far Rider Hendrixian VIP Member

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    A good average Bias voltage would be around 36mv.
     
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  5. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    not many Just built the amp in March Id guess less than 10 hrs.
    I think I have them biased to 38 MV when I was voltage checking with out the power tubes in I did see -50v on the power tube sockets.

    I havent put the tubes back in Im scared of them.
     
  6. Travis398

    Travis398 Well-Known Member

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    I would guess just a bad tube, check bias. You may loose some power running it that way but shouldn't damage anything.

    I have heard it is hard on the tubes if you do the reverse, 16 into 8.
     
  7. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    FWIW they were Tung Sol KT66.
     
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  8. junk notes

    junk notes Well-Known Member

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    aww man, sorry to hear. :-(
     
  9. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    thats good to hear that 8 into 16 isnt that bad. Ill put the new tubes in bias them and report back.
     
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  10. Far Rider

    Far Rider Hendrixian VIP Member

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    I like Tung Sol tubes. I have them in my JTM45 build. For my JTM50, 2204 and HiWatt builds I went with Mullard.
     
  11. Travis398

    Travis398 Well-Known Member

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    I love this guy..........

     
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  12. william vogel

    william vogel Active Member

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    I would recommend that you order a matched quad. Then you’ll have a spare pair and usually having spares makes the tubes you’re using last forever.
     
  13. Guitar-Rocker

    Guitar-Rocker Well-Known Member

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    Funny how having a spare set usually always sets that tone, isn't it?
     
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  14. junk notes

    junk notes Well-Known Member

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    It might be an extra expense to order a matched set of power tubes from websites that offer the service, but you can expect to bias your tube amp within +/- 3mA, one way or the other. Makes a difference when sorting spares that one out of two (or four) would be +/- 10mA. ouch!
    Although, spares can be had in operating range if you have a valve tester.
    At one time I had to temporarily used a spare power tube, which was all I had at the time. It registered good, but a lower range, and was far off the mA from the rest of the quad.
    I am not recommending a matched set unless it is the same price as any other tube. *A perfectly matched set might sound sterile to your ears as it did mine. When they read just a couple mA's from one another I perceive the amps tone to be a fatter and comprehensive, more so if the amplifier was biased a tad hot or cold. YMMV
     
  15. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    Ok so I got some new tubes. I have it set up with a Variac, dim bulb tester and an amp clamp meter. With only the heaters running no standby yet the heaters look good and pulling .4A at 120vac Shut everything off now both standby and power switches to on starting with the Variac on 0vac and slowly turn up the voltage the bulb starts to glow at around 85vacand the amps are .6A. Something Isn’t right. So when I got the OT I measured the primary windings and they were
    107.3 ohms
    And
    116.4 ohms
    I didn’t measure the secondary windings when I got it But I have measured them now with out the power tubes in and they read
    16 ohm tap 1.0 ohms
    8 ohm tap .8 ohms
    4 ohm tap .4 ohms

    these readings are from ground to the corresponding selection. Those seem awful low to me but I’m not really sure about that.
     
  16. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    Ok so I measured another OT even though it’s a different one it came out similar. So I hav the Variac on 90v and the amp was pulling 0.7A Plugged in a guitar and played, seemed good, slowly turned up the voltage to 120vac biased amp to 38mA. Everything seems normal. The bulb was glowing some though, that’s what threw me off. Is that normal? Some glow? Seems like there shouldn’t be any but I don’t know. Never had a problem where I had to go through all this. I didn’t like what happened on the first post.
     
  17. Gene Ballzz

    Gene Ballzz Well-Known Member

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    Tubes sometimes crap out and poop happens! It seems evident that the amp was built well enough for the fuse to have protected the transformers and other components in the amp, as intended. For peace of mind though, you may want to closely inspect and measure the screen resistors! It's not unusual for a shorting power tube to take out and/or stress a screen resistor at the same time as taking out the fuse.
    Just Sayin'
    Gene
     
  18. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    Thanks Gene for the input
    Screen resistors
    457 ohms and 458 ohms
    What has me is the fact that the dim bulb tester was glowing, not bright but on. I did check another amp with the dim bulb tester and it was glowing as well but you could barely see it.
    Can anyone explain this to me.
     
  19. Chris-in-LA

    Chris-in-LA Well-Known Member

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    Yes, the bulb will glow, there is current going through it. But a working amp is not pulling many watts, at least not enough wattage to fully light a 100 or 200 watt bulb.
     
  20. Rotorcraft230

    Rotorcraft230 Well-Known Member Premium Member

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    B281A7A7-5532-4CFC-8031-5813268A0581.jpeg 3DC4F085-6D27-444F-820C-D6FE608AC259.jpeg Just for fun here are my voltage numbers.
     

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