Music Man HD130 Power Tube Question

Discussion in 'Amps/Cabs Tech Corner: Amplifier, Cab & Speakers' started by zach5150, Jan 26, 2006.


  1. zach5150

    zach5150 Guest

    Hi,
    I have a guy bringing me a Music Man HD130 head that I'm going to try and fix for him. He said it burned out a power tube....someone tried to repair it, and when he retubed it, it ran for a short while, then burned up another tube. He has a brand new matched quad for it, but naturally he doesnt want to install them till the problem is found and repaired.
    Anybody have any suggestions on where I should start looking for something that is burning up power tubes? I havent' even looked at the schematics, or have the amp in my possesion yet, but I just thought I'd put a feeler out to see if anyone had some suggestions.


    FWIW, my level of "expertise" is that I've been an electronics technician for nearly 15 years...starting off in high school at a TV and radio repair shop....then as an Electronics Tech in the Navy for 5 years, and for the last 5 years or so, I've been working as an Equipment Engineering Technician on Semiconductor equipment. So, I know my way around High Voltage, meters, schematics, etc....but, I've only gotten serious w/ tube audio within the last year. I'm starting my first amp build as we speak, and I've done some repairs on my old amps (and friends) as well as biasing etc for the last few years, but I'm certainly no guru like some of the guys here.....Anyway...just wanted to let you know my background so you'd be able to respond appropriately.
    Thanks
    Zach
     
  2. Fuchsaudio

    Fuchsaudio Supporting Member

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    If you have the solid state driver (only tubes in the amp are power tubes) you might have a shorted driver transistor. The TIP50 or TCG-198 works fine as a replacement. Change the IC to be safe too (a 1458 I think it was). If you have the tube driver, the MM's had some caps and resistors in the bias supply that tended to drift or weaken with age, making the amps run hot. Look for resistors that turned the surrounding pc board brownish...(normal is green). Music Man schematics are online. A quick search gets you all you'd need. Drop me a note if you get lost in there. While a little odd in some ways, it's not usually anything major that goes wrong with them.
     
  3. zach5150

    zach5150 Guest

    Andy,
    Thanks for the reply.....cool, that'll give something to get me going. The owner is bringing the schematics so I havent looked online yet...but, i've got a few places bookmarked just in case. I believe the model he's bringing me is the full SS driver section. If I find weak/unstable resistors in the PS section, would you recommend upgrading the wattage on them so they'll be more stable in the future?
    Thanks again,
    Zach
     
  4. Shea

    Shea Member

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    Hey Andy - do you think the brand of tube matters? Are there some brands of new manufacture EL34s that can't handle the plate voltage in an HD130?

    Shea
     
  5. jbltwin1

    jbltwin1 Member

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    Can't speak for Andy(an obvious pro) but JJ's seem to hold up the best in MY MM's. I use a 65-210 when space is at a premium-a twin otherwise. The E34L's hold up the best. I will say though that biasing is important with 700v on the plates. I run them about 22-23mA per tube @700v and they hold up. The old crap stated by MM of 1/2v at point Y-which relates to 50mA per pair- is just a litle warm. It also doesn't allow for one tube being hotter than the other. You can't tell if one is warmer when you measure PER PAIR>
     
  6. Fuchsaudio

    Fuchsaudio Supporting Member

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    I would up the wattage of any of the two watt resistors that ran really hot in those amps. Take them up to 5-W and raise above board a bit. Use silicone (not hot melt glue, as it never holds...) to anchor the part . The bias filter caps should also be changed, as they are mounted directly near those hot resistors, and have thus spent their life "baking" from the resistors next to them. The JJ's,real Winged-C Svetlanas and the Sovtek EL-34 (tall thin ones) seem to hold up well in those amps.
     
  7. zach5150

    zach5150 Guest

    Andy (and everybody else posting too!)
    thanks a ton for your guidance. I got the amp from him last night so i'm gonna try and take a peek at it over the next day or two. He gave me a new set of tubes he bought...Groove Tubes....so, not sure what factory they came from..hopefully, they're up to the task!
     

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