HXDA suddenly failed - bias adjust reads 0 mV

Discussion in 'Amplifiers' started by shinksma, Nov 24, 2018.

  1. shinksma

    shinksma What? I get a title?

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    Hi all, especially @LSchefman , perhaps...

    A couple of days ago I was jamming away, my HXDA made a weird high-hum sound for a few seconds, then cut out.

    I checked out the tubes on the back, and one of the power tubes wasn't glowing.

    Oh, OK, need to buy new tubes, first time I've had to do that, so I ordered some (not NOS, just regular "new" russian made) Mullard EL34s.

    Got them last night, swapped them in, went to bias them, and the bias voltage reads zero for both.

    I did go make sure my multimeter was reading properly by doing a bias test on my Archon, they came back good (34, 34, 34, and 35 mV).

    Testing with main pwr on, stby/pwr on, speaker plugged in, nothing plugged into guitar in jack, etc.

    The new tubes glow, so there is basic power to the amp - not a blown fuse.

    I looked briefly at the pre-amp tubes, they seem fine.

    So, what would be causing this? What generates the bias-voltage signal that I read on the back panel - i.e. what could have failed while main power to the power tubes themselves seems fine?

    I will continue to troubleshoot, maybe:

    the stby/on switch failed
    one of the pre-amp tubes is bad
    something more serious is dead

    But hints or tips of what else to check would be appreciated.

    I will also post this to the PTC section after a while, but thought I'd start here, since there has been essentially zero traffic there about HXDAs (just that one thread about manuals), whereas there is a fair bit here.
     
  2. WEDGE

    WEDGE Almost was a FG22 owner..

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    Did you maybe miss a fuse? That sounds odd.

    Edit......odder that the tubes glow and can’t bias them. Sound come out? I have had meter leads fail and give back readings, spray some contact cleaner in meter lead sockets, and check them by testing wall outlet voltage for lead continuity.
     
  3. Boogie

    Boogie SuperD

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    Grid resistor. You’ll find it across two of the leads on the power tube socket. They usually blow after a tube goes Chernobyl on you. A VOM will confirm the failure, but it’s usually pretty obvious. Any bad smells (besides you crapping your pants, like I would)?

    Hopefully that’s all. Good luck.
     
  4. shinksma

    shinksma What? I get a title?

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    Thanks @Boogie , I will check that out! (I presume I have to dis-assemble the combo amp more, removing the amp-section chassis from the cabinet?)

    No bad smells, that were obviously tied to the amp. The dogs have been gassy recently, though...

    :eek:
     
    matt3310 likes this.
  5. elvis

    elvis Hamfisted String Banger

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    The heaters make the tubes glow even if the plate supply is out. The filament supply is generally on its own transformer tap. There can be multiple fuses, so worth a second look.
     
  6. LSchefman

    LSchefman Hears Tones

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    I had that exact thing happen when a tube went bad on mine. There are fuses on the inside of the chassis that need to be replaced; they did their job and blew so the amp wasn't damaged. Pull the chassis and replace them.

    I highly doubt it’s anything serious.

    I didn’t know about them, so sent the amp to PRS. They replaced the fuses, no big deal. There really oughta be a manual with PRS high end amps (hint hint PRS).

    Side trip: That’s when I decided to go with NOS Siemens EL34s. New EL34s are prone to blowing. The NOS sound nicer, too.

    The tube that blew on my amp was one of the Winged Cs it came with. I tried a set of JJs in it, but didn’t care for them, and I’m glad I got the Siemens set. After trying them I got two more matched NOS pairs as spares.
     
    #6 LSchefman, Nov 25, 2018
    Last edited: Nov 25, 2018
    Sekunda and [email protected] like this.
  7. Shawn@PRS

    Staff Member Moderator

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    Sounds like the internal B+ fuse blew. If you replace it, you should be good to go.
     
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  8. alantig

    alantig Sassyless pants

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    If you guys used A+ fuses instead, he might not be in this predicament! #GradesMatter
     
  9. Shawn@PRS

    Staff Member Moderator

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    LOL
     
  10. shinksma

    shinksma What? I get a title?

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    OK, thanks for the continued updates!

    I have done nothing with the amp since swapping the tubes as noted above...I was either prepping for or away on a nice 8-day cruise in the Eastern Caribbean...

    And now that I'm back, all that reality that got pushed to the side is filling in any available spare time. Plus processing all the photos and GoPro video we took.

    Hopefully I'll be able to have a look this weekend!
     
    DreamTheaterRules likes this.

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